Those Non-OEM 240 Wagon Tires "May" be Costing You An Additional $40/Year or More.

I say "may" because I'm not sure if the math I did was the correct formula, or if the [OEM Tire] information I was initially given is correct, so bear this in mind. Recently I was involved in posting and answering some tire questions. I became interested in whether there was any loss in gas mileage if I went with a non OEM tire size. Here is one aspect of that mileage/tire size issue.

First, as many of you know, the OEM tire size for 240 Wagons is a 185/R14. That is a European metric size. The 240 Sedan takes a 185/70R14. But this was a source of some confusion, as many tire dealers who took a quick look at their OEM sizing charts only saw, or only had, the Sedan size listed. And you cannot imagine how many tire dealers were so quick to argue with me that there is no such tire size as a 185/R14, and that I needed to provide them with the missing number. But, when I whispered the word Volvo 240 wagon, suddenly they got quiet, and said, "Ah, ... Well, ... Uh, ... Yeah, it looks like your right about that size ... Gee, I never heard of that one before." (yeah, right!).

But the Wagon's aspect ratio is also different and is missing from the number size. Actually, it is not missing, but this is common in European [metric] tire sizing, or at least it was, and I was told that size, and sizing system, is no longer being used (?).

But as long as you bought that OEM size for the Wagon the aspect ratio was never an issue. However, what if you can't find it, or do not like the tire choices you have (like Michelin Rain Force, MX4 ... UGH!!). Well, then you need to know that OEM aspect ratio in order to get a replacement tire size that is as close to the overall diameter as your OEM tire, or you need to know the overall diameter of the OEM tires. Unless, of course, your tire dealer swears to God that he knows the answer. Now, that is what was very difficult for me to find out. I attribute this mostly to the laziness and ignorance of the tire sellers. If a tire dealer does not stock what you need or cannot get it, then they don't want to bother spending much time answering any other tire question you may have. The tire dealers I spoke to provided very little information, and in the end most of them were in disagreement with what the 185/R14's true aspect ratio was, includingVolvo dealers. Furthermore, I was told that any size could very well vary from one Manufacturer to another. But if European countries follow any manufacturing standards like we do in the US, then they were just blowing smoke up my (bleep bleep). Personally I discount that statement - their statement.

I was told by many tire dealers that the aspect ratio was 75, was 78, was 80, and one even said, "Aw, what does it matter, there all close enough!" But most tire dealer employees said, "I swear to God, I am not lying to you, I know the correct answer to this question." The problem was that this is what most of them said. So how was I, a non tire expert, to know who was right and who was wrong?

At any rate. Michelin and Cooper both make a 185/R14. They both list the overall diameter as 25.6" for that tire size. In revolutions per mile, it takes a 195/75 tire that is 1/10" less in overall diameter (at 25.5"), than a 25.6" tire, 2 ½ times more revolutions per mile. In terms of gas mileage that 1/10" is insignificant. But what if that was a whole inch less. Well, that is what this posting is about.

Someone at this site provided me with the tele # for a distributor of Gislaved tires. The original OEM tire manufacturer for the 240 Wagons. I called them and spoke to one of their salesmen. He told me that Gislaved listed their 185/R14 as having a 196 mm section width, and a 662 mm overall diameter (So, why didn't they just call it a 196/R14?).

I converted the overall diameter to inches, by multiplying by 0.04, which gave me 26.48 inches, or 26.5 rounded off. Now here is what I am not sure of. I figured that if 1/10th of an inch caused a tire to rotate 2 ½ times more per mile, then a 1" difference must be ten times that or 25 more revolutions per mile. Here is where the gas mileage calculations came in.

Consumer Reports and the EPA tested the 240 wagon at about 30 miles/gallon highway, or 15 city. Remember, these are the upper and lower limits. But they both had similar findings. So I used those findings to make my calculations. Okay, at 30 miles/gal/hwy., a wagon can go 474 miles on a tank of 15.8 gals. I also assume those wagons tested had OEM tires of 26.5" diameters on them. If those OEM tires spin 25.5 times less/mile, then they are turning 787.5 times per mile. That is 23,625 times for 30 miles/gal/hwy, and 354,375 for a 15 gal/tankful. By contrast, a 195/75 with an overall diameter of 25.5" turns 815 times/mile, or 24,450 times per 30 miles/gal/hwy, and 366, 750 revs per/15 gal tank.

The difference for a 195/75 tires is that it gets an overall 12, 375 revs less per tankful (subtract the lower tankful from the higher one). Dividing the revs/mile (815) of a 195/75 tire, into the 12,375 revs/tankful difference gives a loss of 1 mile per gallon of gas, or 15 miles per tankful. At a price of $1.70 a gallon, that comes out to a loss of $44.20 each year, if you use one tankful of gas weekly, for hwy mileage, or $88.40 per year, for city driving. In three years of city driving that is about the cost of four good tires, or one nice portable Emglo air compressor. Darn! Figure an even higher cost if your gas costs more than $1.70/Gal, and if you use more than one tankful/weekly. The only compensation is if the Gislaved's don't wear as well. But that is another matter entirely.

Sincerely,

Robert

PS. Sorry for any type o's or incoherency.

If we assume that a 185R14 is in reality a 185/78R14, The closest tire in diameter is a 205/70R14.

Coincidentally, that's what I run on my '84 245.

Have you ever had you speedometer calibrated with those 205/70R14's? It would be interesting to see if there is any perceivable difference from the 185R14's (or "185/78R14's").

I have checked the odometer on highway mileposts. It read exactly 1% low over 100 miles. I'm pretty sure, statistically speaking, that that is an insignificant factor. I can also say that the tires (General Ameri G4S) offer a nice combination of decent ride and tight handling. The only downside is that steering is a wee bit stiff at low speeds. But my rack could be going at 250k, also.

Robert, the 245 didn't all come with Gislaveds. My '89 came with 185R14 Michelin MX tires. Got over 100,000 kms. with them. Now running 195/75R14.

Kevin

Minor changes in tire diameter don't affect gas mileage much

because the increase in rpm is matched by a decrease in torque

requirement. Big changes in tire width will make a difference

though. Also your mileage needs to be calculated on actual

miles, not the odometer indicated miles because changing sizes

WILL affect that (although it could increase or cancel out the

odometer error....) I find one of my odometers is about 2.5%

off or one mile in 40. Another is about 15% off, giving about

7 miles for 6 driven. (The speedo on this one is about right were

the first one is 20% fast so if it indicates 60 I am going 50.

It is a little scary driving where the speed limit is 75 mph.....)

Yeah, that makes sense that not just Gislaved would make tires for Volvos. That's how it is for all the other vehicle mfrs. Wonder if their OEm size was the same? Btw, I run the 195/7514's on my wagon too. Thanks for the answer. Rob

In the time it took you to figure that out, and then type it, I made almost $500! Lost $40 or more on mileage (one year), made $460 (one hour).

Ha ha.

(Just giving you a hard time).

Jeez, it takes me amonth to make $500. Rob

Don't feel bad, my job is practically in the toilet. I have to get all I can out of it before I have another Y2K (50% employment).

Robert,

For what it is worth. My 86 245DL came with 195/70/14 tires from the factory. A common size with lots of tire options. No problem finding a quality tire for a wagon in that size.

I have no doubt it did. Guess Ive been victimized by my local Volvo dealer. It just exhausts me at how much misinformation they lay on me. Rob

So....I guess if you are on a tight budget you can spend an extra $20 per tire to get that funky size and hope they last long enough to make it up...

Just ribbing you....I won't poke fun at your math, but you left out some performance issues. I would NOT switch to a taller tire even if it as $100 a year...unless you came over and changed my gear ratios....on second thought...

He he he. Good one Paul.

Hehe.. how about those of us who run with less air in the tires... and also...I go for how they handle and look...

He he he ... how about those of us who don't? Put all the gas you want in my friend - I got stock in those oil companies. Btw, I find it odd you buy tires for a Volvo based on how they look, yet drive a square car. He he he

Please know first that my intent here is not to flame you. I think I get the jist of your point, but I beleive some corrections to your rationale may be possible.

First, a minor point:

"...Gislaved listed their 185/R14 as having a 196 mm section width, and a 662 mm overall diameter..."

If this is the case, the aspect ratio of the 185/R14 tire is 78. The math:

Overall diameter: 662mm/(25.4mm per inch) = 26.06"

Sidewall height: 26.06" - 14" = 12.06 (for both); 6.03" for "one" sidewall.

Tire width: 196mm/25.4 = 7.72"

Aspect ratio: 6.03"/7.72" * 100 = 78.14

"I converted the overall diameter to inches, by multiplying by 0.04..."

There aren't a whole lot of meausrements which have an "easy" or "exact" conversion between metric and English, but the linear measurement of distance is one of them. Per above, 25.4 mm per inch. This means 0.03937 inches per mm.

"...which gave me 26.48 inches, or 26.5 rounded off."

Per above, it's more like 26.06". For the sake of this discussion, let's call it 26".

"Now here is what I am not sure of. I figured that if 1/10th of an inch caused a tire to rotate 2 ½ times more per mile, then a 1" difference must be ten times that or 25 more revolutions per mile."

No, this isn't the case. Let's use some algebra to demonstrate.

Two assumptions:

First: All measurements are in inches.

Second: The diameter of the first tire is D(1).

The second tire is 0.1" smaller in diameter. Then the diameter of the second tire, D(2) can be calculated: D(2) = [D(1) - 0.1]

Remember the general equation for circumference:

c = (pi) * d

The circumference of the first tire: C(1) = (pi) * D(1)

The circumference of the second tire: C(2) = (pi) * D(2)

Substituting: C(2) = (pi) * [D(1) - 0.1]

Rearranging: C(2) = [(pi) * D(1)] - [(pi) * 0.1]

The third tire is 1" smaller in diameter. Then the diameter of the third tire, D(3) can be calculated: D(3) = [D(1) - 1]

Again, the circumference of the first tire: C(1) = (pi) * D(1)

The circumference of the third tire: C(3) = (pi) * D(3)

Substituting: C(3) = (pi) * [D(1) - 1]

Rearranging: C(3) = [(pi) * D(1)] - [(pi) * 1]

So for the second tire, the difference in circumference is acheived by subtracting one-tenth of "pi" from the circumference of the first tire, and for the third tire, the difference in circumference is acheived by subtracting "pi" from the circumference of the first tire. So the overall difference between the circumference of the second and third tires is not a linear multiple of 10.

"Here is where the gas mileage calculations came in."

"Consumer Reports and the EPA tested the 240 wagon at about 30 miles/gallon highway..."

Sheesh!! Is this the new 240 with the part-time electric motor?!? Just kidding.

"Okay, at 30 miles/gal/hwy., a wagon can go 474 miles on a tank of 15.8 gals."

Agreed.

"I also assume those wagons tested had OEM tires of 26.5" diameters on them. If those OEM tires spin 25.5 times less/mile, then they are turning 787.5 times per mile. That is 23,625 times for 30 miles/gal/hwy, and 354,375 for a 15 gal/tankful. By contrast, a 195/75 with an overall diameter of 25.5" turns 815 times/mile, or 24,450 times per 30 miles/gal/hwy, and 366,750 revs per/15 gal tank."

You kind of lost me in this paragraph. Here are my thoughts:

Back to the original 185/R14 tire... it's diameter is 26". This means:

Circumference = (pi) * 26" = 81.68". So this tire will travel 81.68" for each revolution (of the tire... and of the rear axle).

There are 63360" in a mile (5280' * 12 inches per foot = 63360"). This means the 185/R14 tire will turn:

63360 (inches per mile) / 81.68 (inches per revolution) = 775.71 (revs per mile). The rear axle also turns this same number of times (per mile).

In the case of a tire which is 1" smaller in diameter:

Circumference = (pi) * 25" = 78.54". So this tire will travel 78.54" for each revolution (of the tire... and, again, of the rear axle).

This tire (and the rear axle) will turn:

63360 (inches per mile) / 78.54 (inches per revolution) = 806.72 (revs per mile).

Let's assume that the "30 miles per gallon" figure is accurate. Then, for a given engine speed, transmission ratio, and differential ratio, the 185/R14 tire will yield:

30 (miles per gallon) * 775.71 (revs per mile) = 23271.3 (revs per gallon).

Now, we don't know the gas mileage for the second (one-inch-shorter) tire yet. However, let's assume the same engine speed and driveline gearing per above. Then we can plug the second tire's 806.72 (revs per mile) into the equation above and work backwards:

23271.3 (revs per gallon) / 806.72 (revs per mile) = 28.85 (miles per gallon).

SO:

Let's assume the car is driven 15,000 miles per year.

For the 185/R14 tire:

15000 (miles per year) * 1.70 (dollars per gallon) / 30 (miles per gallon) = 850.00 (dollars per year).

For the 1" smaller tire:

15000 (miles per year) * 1.70 (dollars per gallon) / 28.85 (miles per gallon) = 883.88 (dollars per year).

So there is a difference of 38.33 (dollars per year), which roughly supports your original calculation.

But realistically, would anyone put 1" smaller tires on their car? In the case of the 0.1" smaller tires (which, to me, seems quite possible), the result is much less dramatic. I'll spare you all the math details, but the key points are that this 0.1" smaller-diameter tire will give a gas mileage of 29.89 (miles per gallon), resulting in a fuel cost of 853.13 (dollars per year). Clearly not much different from the 185/R14.

I realize I'm not really modeling everything exactly correctly... most people in the world pay more than $1.70/gallon; different tires have different coefficients of friction; different tires wear at faster/slower rates; some people don't check their tire pressure or rotate/balance their tires as often as they should. But I believe the point is made: Vary the overall diameter of your tire (from stock) by +/- 0.1" or +/- 0.2" and there won't be an appreciable difference in operating cost.

Say what? Whew! Boy that's the last time I attempt any math stuff.

Seriously, I disagree that a tire that is one full inch shorter, will not have to turn twenty-five more times per/mile. You said it was not linear as I thought, but then went on to come to about the same calculations as I did. Go figure. If a tire that is 1/10th of an inch shorter turns two and a half times more then the math is correct. I will double check this one. Actually, I got this information from a tire mfr. who gave me the number of revolutions per mile for the various sizes. It was never my intent to cover all aspects of this, but just one small point. Of course there are other factors.

Btw, Les I never said what the aspect ratio is. I still don't know. I was only figuring this all out from the overall diameter, which is all I wanted to know.

Les: "You kind of lost me in this paragraph. Here are my thoughts: "

Reply: Again, I got this information from a tire mfr. who gave me the number of revolutions per mile for the various sizes. I simply multiplied the revolutions for one mile times 30 miles (or how far CR and the EPA says it can go on a tankful of highway driving), then I multiplied that by 15 gallons (roughly what a tank holds - yes, I know it holds 15.8 gals). So that will give a total close number of revs for a tank of gas - mathematically. Then I compared the revs for the two tire sizes on a tankful and subtracted one from the other to see what the difference was. This is odd. I went thru this with a few of the tire mfrs tech guys and we all got the same answer - now you come along and say this is all wrong. Hey, I'm no Einstein, but I know what common sense is. I think your math formulas are wrong, but your numbers are right. And that's what you said to me too, essentially. So how are we both getting the same numbers and both using two different formulas. Betcha mine is quicker.

No flaming intended.

Rob

"Seriously, I disagree that a tire that is one full inch shorter, will not have to turn twenty-five more times per/mile."

Ok, let's thoroughly kill this thing by looking at an example. Let's say we have 5 tires. The diameter of: Tire(1) is 23 inches; Tire(2) is 24 inches; Tire(3) is 25 inches; Tire(4) is 26 inches; and Tire(5) is 27 inches. I have picked these sizes because each tire is one inch "shorter" than the next. For the sake of simplicity, I will refer to these tires as "T1", "T2", "T3", etc.

We will assume that a tire is in the shape of a circle. Based on the formula for circumference of a circle, which is:

circumference = (pi) * diameter, or c = (pi) * d,

The circumferences of the above tires are calculated as follows:

For T1: C1 = (pi) * D1

C1 = (pi) * 23; C1 = 72.26 inches.

For T2: C2 = (pi) * D2

C2 = (pi) * 24; C1 = 75.40 inches.

For T3: C3 = (pi) * D3

C3 = (pi) * 25; C1 = 78.54 inches.

For T4: C4 = (pi) * D4

C4 = (pi) * 26; C1 = 81.68 inches.

For T5: C5 = (pi) * D5

C5 = (pi) * 27; C1 = 84.82 inches.

This means that T1 takes 72.26 inches per revolution; T2 takes 75.40 inches per revolution; etc.

Ok, based on the fact that there are 63360 inches per mile, here is how many revolutions each tire will make in one mile:

T1: 63360 (inches per mile) / 72.26 (inches per revolution) = 876.83 (revs per mile)

T2: 63360 / 75.40 = 840.32 (revs per mile)

T3: 63360 / 78.54 = 806.72 (revs per mile)

T4: 63360 / 81.68 = 775.71 (revs per mile)

T5: 63360 / 84.82 = 746.99 (revs per mile)

Now look at the differences:

T1 takes 36.51 more (revs per mile) than T2.

T2 takes 33.60 more (revs per mile) than T3.

T3 takes 31.01 more (revs per mile) than T4.

T4 takes 28.72 more (revs per mile) than T5.

There are two conclusions to be drawn from this tedious exercise.

The first conclusion is that, in general, a tire that is one full inch shorter, will NOT turn twenty-five more times per/mile. However, as you might guess, there is one case where the the "one-full-inch-shorter-tire" does turn 25 more times per mile. This is the case for a tire of 28.9073 inches, and one of 27.9073 inches. I was able to solve this question using the quadratic formula, although trial-and-error would surely have yielded the same result.

The second conclusion is that not only are the differences not the same, case-by-case, the differences aren't even linear. See how there is less difference between the two biggest tires (T4 & T5) than there is between the two smallest tires (T1 & T2)? To further illustrate this point, consider two sets of tires that differ in diameter by 1": One set has a 1" tire and a 2" tire, and the other set has a 100" tire and a 101" tire. Clearly, the difference in the number of revolutions per mile will be dramatically greater for the first set of tires. And the math above supports this position.

"You said it was not linear as I thought, but then went on to come to about the same calculations as I did. Go figure."

It's called coincidence. Coincidences are allowed.

"If a tire that is 1/10th of an inch shorter turns two and a half times more then the math is correct. I will double check this one. Actually, I got this information from a tire mfr. who gave me the number of revolutions per mile for the various sizes. It was never my intent to cover all aspects of this, but just one small point. Of course there are other factors."

Ok, fine. Remember, though, that tire reps and salesmen are not necessarily mathemeticians (for that matter, neither am I).

"Btw, Les I never said what the aspect ratio is. I still don't know. I was only figuring this all out from the overall diameter, which is all I wanted to know."

I never said you DID state the aspect ratio; I just calculated it based on the numbers you provided (662mm overall diameter; 196mm tread width)(or whatever). I just threw that one in for free.

"Reply: Again, I got this information from a tire mfr. who gave me the number of revolutions per mile for the various sizes. I simply multiplied the revolutions for one mile times 30 miles (or how far CR and the EPA says it can go on a tankful of highway driving), then I multiplied that by 15 gallons (roughly what a tank holds - yes, I know it holds 15.8 gals). So that will give a total close number of revs for a tank of gas - mathematically. Then I compared the revs for the two tire sizes on a tankful and subtracted one from the other to see what the difference was. This is odd. I went thru this with a few of the tire mfrs tech guys and we all got the same answer - now you come along and say this is all wrong."

Well, maybe I should have said that it didn't make sense to me - hey, wait, I think I did say that!

"Hey, I'm no Einstein, but I know what common sense is."

Well, as I understand the life of Professor Einstein, if you have common sense, you certainly are not "an Einstein". Or was it your intent here to say that because I approached this issue from a scientific standpoint, using - what? - MATH?? - that I have no common sense?

"I think your math formulas are wrong..."

Be specific. Which ones? If I have erred in my logic or use of any formulas, I'll be the first to pay homage to the tire reps and their little charts.

"...but your numbers are right."

What?!?

"And that's what you said to me too, essentially. So how are we both getting the same numbers and both using two different formulas."

I addressed this before. Coincidence.

"Betcha mine is quicker."

And that's fine. When dealing with data, I prefer to derive results using methods that have no biases, no preferences, no agendas. Math is always 100% correct. There are no opinions or "feelings" in math. Of course, people can take math and create results to look how ever they want... because people are full of those qualities (biases, preferences, & agendas). But I know that you know that.

"No flaming intended."

Ditto. But not Mega-dittos.

Les -

I found www.miata.net/garage/tirecalc.html to be a nifty aid to calculating at least the speedo error with non-stock rear tires.

I also found that older 240 sedans used 175/R14, but when checking the tire size sticker on the rear edge of RH door, my 84, 86 and 88 244's all show 185/70 R14 stock size. WOW I thought 205/70's were close but now I see I have a 4.5% speedo "undercount" .

I think the subject has been killed, I'm going to turn in.

Regards,

Bob

:>)

Hi Bob, how are you doing?

"I found www.miata.net/garage/tirecalc.html to be a nifty aid to calculating at least the speedo error with non-stock rear tires."

I agree, it's kinda fun to punch in numbers and see the decimal places change.

"...WOW I thought 205/70's were close but now I see I have a 4.5% speedo 'undercount'."

I don't know specifically about Volvos... but in addition, don't most mechanical speedometers have an error of about 5% also?

"I think the subject has been killed, I'm going to turn in."

No kidding! I thought I was up late, then I see that you posted your message at 3:17am (or whatever). "BC the Night Owl", huh?

Later.

Les -

Glad you liked that site. How about the subtle connection with this original post re: 25 decimal places where 2 is enuf?

1986 Volvo 240's, maybe others, have electronic speedos. Sensor on ring gear send pulses to speedo and Odo which then makes those units do. The odo is notorious for quitting anywhere from 90K miles to 180K. Repair is iffy, costs $200 in Houston shop, less at an Austin shop. Bring back the cable, puhleeze!

When on a road with mileposts I usually do a speedo check. Easy on Houston - Galveston run, 75 miles all flat. My error of 4.5% was OK, way inside the tolerances of my rudimentary equipment. Watch the wristwatch with sweep second hand, the speedo, and the mileposts. There are weak points.

Nice to hear from you,

Regards,

Bob

:>)

PS This really belongs over on OPINIONS.

Oh man! Let's see here. The difference in tire diameter will change the number of tire revolutions per mile. So far so good. Now for the headache. A smaller diameter tire will increase revolutions per mile, but the odometer won't know it. According to the odometer, this will actually INCEASE fuel economy:).

With a smaller diameter tire, the engine will not work as hard, so will use less gas. Migrane time.

The 185R14s probably cost more than an easier to find size, so there goes the $40. How about using the larger diameter tires on the back, and the others on the front? Best of both worlds...

$40.00 a year, so what.. I buy my tires to do the following: Handle the Road, provide a Good Ride, and Look Good ! Yokohama,AVS.dB,.205/60/15/VR

/// Robert, do you have a day job? Keep it.

Ok, you have a major error. The tires don't just roll. They must be driven by the engine, through a power train and rear end. This involves a set gear ratio including the diameter of the rear tires. When you change the tires that changed the energy necessary to move the car.

What is the power difference with the two different diameters? What is the gas consumption difference?

If the tires just rolled the computation would be ok.

10-4 big daddy.

185/R14 = aspect ratio of 82

Bruce near D.C.

"185/R14 = aspect ratio of 82."

Just out of curiosity: How did you come up with 82?

25.944881" overall diameter - right?

Robert

So what's the moral of the story...do you have to buy Gislaved's in order to get the"correct" size for the Wagon...I have a sedan 185-70-14.

Maybe you could contact Volvo North America and see what ther tech tie experts would have to say regarding the subject...A 240 wagon could be on my list of next Volvo purchases provided my sedan dies on me someday...Which seem kind of unlikely...

PS...I was the guy who got you the number for Gisalved...

Keep us posted on the tire size mystery.

Jack W.

Will do.

Did you figure in the loss of diameter through tire wear? Geez, your cheaper than my in-laws!

Nope. that's a whole other subject. I better be cheaper, I'm living on $6,000/yr.