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.