85IROCTPI

If a car (with 16 inch wheels) has a rear end ratio of 3.73:1 and travels 1 mile, the wheels rotate X amount of times.

A. If the wheels are changed to 15" wheels and the ratio changed to 4.10:1 does X increase or decrease ?

B. If the wheels are changed to 20" wheels and the ratio is changed to 4.10:1 does X increase or decrease ??

Thanks

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Jester

If you change to a 15, 17, or 20 inch wheel, the total diameter doesn't neccesarily change at all. Say a given tire is 26" across total. it can be 15 of that in wheel, 11 in tire. OR, it can be 20" in wheel, 6" in tire. Since the diameter doesn;t change, niether does the circumference, therefore they turn the same # of times per distance traveled.

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85IROCTPI

That may be true but if you read the question the gear ratio was alto changed. how will this affect the rotations in a given distance ?

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www.thirdgenfbody.com - Leading the F-body into a 5th generation

85 IROC-Z
305 TPI
Bone Stock
Dead for the winter

"Their's nuthing like the sound of a small block chevy and a bass thumping sound system in the morning" - me


For those of you who do not know how to post pics, e-mail me your pics and your mb screen name and i'll post the pics on my site at www.thirdgenfbody.com/pics/yourname then post them on the message board for you, but please keep them small. I recently recived 7 diffrent emails from a unidentified user that crashed my comp - sharp85iroc@email.msn.com

AlkyIROC

Simple math.

Take the ratio multiply by the section width divide by 2540 multiply by 2 add the rim diameter.

An axample:

245/50R16

245x50/2540x2+16 = 25.64" tall tire

235/60R15

235x60/2540x2+15 = 26.1" tall tire

If that was the tire swap then your 3.73 gears would need to go lower, 4.10 being the next available set, to have the same revolutions per mile.

I don't know any sizes for 20" rims so lets try a 19" tire.

245/50R19 is a Goodyear tire

245x50/2540x2+19 = 28.64" tall tire.

Now you would definatly need the 4.10 gears just to bring the engine rpms back up into the same power range as the 26" tall tires were with 3.73 gears.

As you can see rim size has nothing to do with gear ratio until you know what size tire is going onto the rim. Going to a taller tire will mean the tire rotates fewer times per mile but also has the same effect as putting higher gears in the rear end. Like going from 3.73 to 3.45. That's why you usually see race cars with 4.56 or lower gears. They usually have tires 30" or taller.

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slohand

Is there a fromula for determing how much change tire size makes? My car has 2.73s in it with 255/50/16 tires. I am looking at going to 245/45/16 tires and was wondering how much differece it will make. By my calculations it looks like I'll end up with about a 3:08 ratio.

Comments?

Sonix

iTrader: (1)

Right click and save as
https://webdisk.ucalgary.ca/~jmknopp...eed%20calc.xls

I made this spreadsheet a while ago
change gears, tires, tranny, and see on a chart RPM vs speed, and what drive/driven gear you'll need.
I'll have to upload an updated version in a bit here...

5678TA

i like this site.. pretty much every calculator you need..

http://www.tciauto.com/Products/Tech...alculators.asp

slohand

Thanks guys. Looks like I end up with a 3.15 which is right about what I wanted.