how can some of you say tire size doesnt change gear ratio.if you have a 26'' tall tire with no od.and 4.10 gears the engine will be spinning at about 3500 rpm at 65mph.if put a set of 31'' tires the rpm will drop considerably.you can also "change"the ratio with different gears in the tranny.some trannys come with lower first gears than others.od.ratios are also different.i do agree that if you want to find out what gear ratio you have in the rear all you do is jack up the car mark tire and turn driveshaft.this does not change with tire size but going down the road the taller tire will change the actual ratio applied from the rearend.
Oh....and I do actually have something of value to add http://www.richmondgear.com/streetcalculators.html
Randy's Ring & Pinion RPM calculator. Have had it bookmarked for ages, it will answer all these questions, even has a calculator for tire size. 700R4 has a tall enough first in it I was able to pick up the tires in a 4400lb truck with a 2.73 rear in it - not often, I had to hit it really hard - but in a light car it should hardly matter unless you put pro-street meats on the back. If you beat on it a lot it's going to break pretty fast anyways, it's a 700R4 - Changing tire size doesn't change the gear ratio so much as it increases or decreases the number of revolutions the tire has to make to travel the same distance. So at the same RPM with a taller tire you'll be running a faster speed; with a shorter one, slower. Ideally you should gear the car to put the highway cruising RPM somewhere around the motor's torque peak, it's most efficient there - the torque helps overcome aero drag and rolling resistance. With a 700R4 a 3.73 is about right, running bigger 15" tires at any rate.
I believe some people are confusing "Rollout" with "Gear ratio". The other people, those with the ability to use logic, have it right.
Easy way to dispose of this: What is the gear ratio with 10 pinion teeth, 41 ring teeth, and no tire? It's still 4.10.
Why does it always end up as a food fight? By the way, for a 30 inch tire, 1000 times the overall gear ratio (trans ratio times rear gear) will be very close to the engine speed at 90 mph. Similarly (but harder to multiply), the speed at 60 mph will be the gear ratio times 667. Good to two digit accuracy, which is as close as I can read a tach anyway.
Once, I found a pair of tires that ACTUALLY DID change the gear ratio. I swear, I stood there and watched them jack up the car and swap out the gearset! I think the tires didn't do a good job because the rearend howled like crazy thereafter.
Not sure if it's been discussed here, but in drag racing we commonly look at "roll-out" or tire circumference. Roll-out can be used when determining a change in tire/gear ratio combo. I believe if you take the total rollout and divide by current gear ratio it gives you a base number. Then take that number and multiply by the new ratio and should give you the appropriate rollout size to achive the same performance.
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