MTC_75vette

I just bought new rims and tires for my car and the new tire diameter is 25.7 inch vs my stock which was 26.5. The gear ratio is 3.70 with the 26.5 inch tire and to my understanding going to a shorter tire would bump my gear ratio up to a 3.82 gear. But when I was on the highway after putting on my new wheels instead of the rpm going up it seems to have went down. So did my gear ratio actually drop by going to a shorter tire? I am confused with the outcome....I thought a shorter tire would spin faster creating faster acceleration and higher rpm a highway speeds.

tnovot

Yep, that is correct.

flyeri

I built a spread sheet to compare tire sizes and gear ratios to compare different combinations. Plugging in tire sizes that gives heights of approximately those you stated and using a 1.0 final drive on the trans and a 3.70 rear ratio, 1700 rpm yields 36.3 mph with the original tires and 36.0 at 1750 rpm on the new tires. At 55 mph there is about 100 rpm difference.

7T1vette

Yes. Smaller tires do change the 'effective' gear ratio. But it also changes the accuracy of the speedometer. With the smaller tire, you don't go as far with the same number of tire revolutions; so the speedo says you're going 60, but you may be only going 57-58 mph.

However, If you really knew how fast you were actually going, you might have gotten there a bit faster, with that ratio change!

BKbroiler

Your "effective" overall ratio has changed but your rear end gear ratio is still 3.70.

MTC_75vette

Thanks for the help....so basically there is very minimal change with my new tire size. I thought the difference would be greater dropping an inch in tire size but no big deal. Thanks again for the help!

JohnRR

The change in tire size gives you about a 3% speed change. Your rear gears are probably 3.73 so the effective change would be equal to a 3.84 rear gear. So at 60 MPH you're really only going 58.2 MPH.

John

jb78L-82

Like everyone said above! When I went to 17 inch rims, I purposely kept the rear tire diameter the same as the 255/60/15's since with 3.70 gears I did not want an effectively higher numerical gear ratio-it's too much as it is for me. I run 255/45/17 ZR's in front with 255/50/17's in the rear.

I have a 2001 Pontiac Grand Prix that had 225/60/16's and I switched to 225/55/16's and the verified GPS speed went from 2,200 RPM @80 MPH to 2,500 RPM @ 80 MPH with the lower profile tire-Difference of 300 RPM.

Mike Ward

And the indication on your speedo will not have changed- only the error factor is different.

7T1vette

Say again??

C3 Stroker

....????..uh, yes, the speedo indication will change. Slightly higher number with the shorter tires.

Jims66

Maybe the older "taller" tires were bald and the newer "shorter" tires are actually the same size because they have full unused tread effectively keeping the RPMs and MPH the same. It could happen.......

Mike Ward

Sorry, nope. The speedo is driven off the transmission tailshaft. It has no idea what size tires are on the car- or if the car has no wheels at all and is on jackstands.

The ACTUAL speed of the car over the road will have changed with different size tires but not the indication. That's why I said 'only the error factor is different'.

C3 Stroker

I see your point, the actual car speed may be 57mph but the speedo will still indicate 60 with the shorter tires. But if you go an actual 60mph, your speedo will indicate 63......that's the change I'm referring to.

7T1vette

I believe I stated that it "changed the accuracy of the speedometer". That usually means that the "indication" is in error with the "actual speed". So, what's your point, Mike? That so many revolutions of the new [smaller] tire will provide the same numerical reading on the speedo as it did with the larger tire? That's true...but, so what, the OP still needs to know to adjust his speedo reading for the smaller tires.

Mike Ward

That's the point. Without verifying the accuracy of the speedo vs. actual ground speed via GPS or timed miles, it's hard to judge what's happened through a tire size change.

7T1vette

Point made...

TheSkunkWorks

Actually, it's a fairly easy to see how tire diameter affects speed for a given RPM/gear combo...

http://www.wallaceracing.com/gear-speed.php

...but remember, garbage in = garbage out.



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