So, I got help from Bank of Mom and got a new set of Cooper tires on my 84 240 yesterday.
I opted for the taller 195/70/14 instead of the 185/70/14's it had on it. I also went from BALD - maybe 1/16th or less of tread on one tire and maybe 3 or 4 /32nds of tread on the others up to 10/32nds of an inch on the new tires.

Anyway, I thought I wouldn't notice a difference, but I do. The car seems to accelerate more slowly and it's easier to lose speed on hills = more downshifting. Since the speedometer is driven off the transmission, not off the tires, it's basically reading the speed of the gears, not the speed relative to the road.

I think that would mean I wouldn't see any change in mileage based on my odometer. I got slightly better mileage, but it was all highway and it was about .75-1.0 mpg difference, so it could just be random variation.

Goatman - if you're reading this - my 1st gear no longer seems as excessively low.

All in all I think I would be happier with the stock 185/70/14's, if only because it wouldn't feel as slow as it really is :P

Or maybe it just feels slower because the old tires were barely keeping me on the road haha.

Happy Bricking

i wrote a very long detailed article about something related to this in a 4x4 forum that i was on. The BASICS are this: I proposed that while on a long all highway trip a taller tire WILL result in BETTER mpg (wont get into the math of it here, but trust me, it works)taller tire = less revolutions per mile. Your speedometer and odometer WILL be off, it will read SLOWER than with stock tires, but in this case NOT significantly. Yes it WILL accelerate SOMEWHAT slower but a 10 mm difference in height shouldn't make MUCH difference, but still some. Some of this make have to to with the tread itself, tread compound, tread design and rolling resistance. It DOES take MORE torque to turn a taller tire, and a taller tire will be heavier, resulting more torque required to turn the weight of the tire, this could be perceived as not being able to accelerate as quickly. ALso this would cause a DROP in MPG in stop/go driving, but improved on sustained even speed highway driving, which would balance it out. The taller the tire (vs. stock) the more the differences are.

does this answer your questions? if you like, I can go over the specific math of this with you. (i is a geek like that)
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-Chaos Manager-

I did the same with my Manual Trani '89. I too feel a bit of a sluggish off the line. Not enough to care and maybe my imagination. But I drive mostly Hwy on Level ground so I thought bringing the RPM's down a hair would help the engine and the mileage.
The Speedo is more accurate at 60MPH with the taller tires on my car.
--
'75 Jeep CJ5 345Hp ChevyPwrd, two motorcycles, '85 Pickup: The '89 Volvo is the newest vehicle I own. it wasn't Volvos safety , it was Longevity that sold me http://home.lyse.net/brox/TonyPage4.html

Hey yeah, I didn't reply... haven't been feeling well.

Anyhow:
The pressure is probably higher. 2 of the old tires were leaky and I wasn't keeping a close eye on them. I tried to keep them around 32-36 psi but eventually gave up. The paperwork says the new ones are at 35, but I didn't check it myself.

My speedo seems more accurate, according to the radar signs "Your speed is ___". Before my speedo always read higher than my actual speed by a few mph. Now it's dead on. This is the cable speedo, not the electronic speedo.

I know the revolutions per mile are only about 20 revs different, and that's out of like 640 per mile anyway - what I think makes the difference is the small increase in length of the "lever arm" from the center of the axle to the road.
By increasing the lever arm, I reduced my mechanical advantage my car had against the road. Extremely small tires are easy to spin with a lot of force, the opposite is true with extremely large tires. It's like upping my differential gears by a few .1's.

Anyway, eh mileage might be up a little. I think the engine is doing fewer revs at the same speed on the highway and I don't mind going 60 so much as I did before. Got to fix that OD....

Happy Bricking
--
1990 740 Turbo, on its way to stock specs, maybe beyond

Glad you're feeling better.

Yes, as you wrote, the lever arm is longer, but only 2% longer (per my calculations in my earlier message) -- do you really think that you can feel that change?
Since (per my earlier message) the tire pressure is not significantly different, it's probably the design of the tire or the thickness (and flexing) of the tire tread.

Use the tire size comparison site and compare the stock size 185/70-R14 with the size you bought AND with 195/75-R14.

The metric to watch is tire revolutions per mile. You should find that changing the diameter one size up (185 vs 195) while reducing the aspect ratio one size down (70 vs 75) provides a very small difference in revs per mile.

With the 195/70-R14 size the car is traveling faster than the speedo indicates, a very small amount but on the highway it will trip the radar used by trooper needing just one more ticket for the month. Been there done that.

Good Luck,

Bob

:>)

Is the air pressure different?

You went from a 185/70-14 to a 195/70-14 tire? The size alone shouldn't have created any change that you can notice:
195/70-14 is (195*.7*2 + 14*25.4) 628.6 mm tall;
185/70-14 is (185*.7*2 + 14*25.4) 614.6 mm tall;
although you effectly changed (i.e., made taller) your overall final gear ratio by 2%, thus reducing the torque to the rear wheels by that amount, but I cannot believe that any human could detect that difference by feel; you would really need delicate instrumentation to detect that difference.

Two more likely things come to mind:
1) the tires that you bought might have a vastly different construction design. Some tires are designed for more rolling resistance than other tires. And especially given your comment about your remaining tread (or lack of), it would have made your old tire much easier to roll than a new tire with full thickness tread -- thicker tread has to bend more, creating more heat (that's why racing tires used to be shaved*), which extracts kinetic energy from the car. Thus, you could be feeling the stiffness, or rolling resistance, of the new tires. And you could have reduced this effect if you opted for tires that maximize fuel economy.
[ * If you ever watched the movie, World's Fastest Indian, you'd learn why that motorcyclist shaved his tires' treads very thin to help break the speed record. Nowadays, it's no longer necessary, though.]

2) tire pressure. When you had your new tires installed, perhaps they didn't inflate your tires as much as they had been. Higher pressures makes it easier for a car to roll along, because the tire's sidewall and tread doesn't flex as much.

Good luck.

According to my tire size calculator, the 185/70 is 24.2 inches in diameter, the 195/70 is 24.7, for a difference of 1/2 inch. The new tire is 2.6% larger, so your speedo will read less than 2 mph slow. Not much of a difference.

The 70 is the percentage of the width. Wider tire equal taller tire.

Ron J

You have in effect geared your car up. To compare check this site out.

http://www.1010tires.com/TireSizeCalculator.asp?action=submit&reset=yes

Ron J