Tire size vs. Gear ratio

[saabfan9]

how does the tire size affect cruising rpm's with a said gear ratio? looking at running a 15" tire and moving to a 3.55. i want to keep it civil as far as cruising highway speeds. HELP PLEASE!!!

 

[diymirage]

I think you need to rephrase your tire size
I'm sure the 15 inch you mentioned refers to RIM size, but we need to know your tire diameter

having said that, most people will tell you 3.55 gears are THE best intermediate gear which will allow you off the line acceleration and acceptable cruising RPM

I personally run 3.23s, but that was more luck then planning

 

[skep419]

http://vexer.com/automotive-tools/speed-rpm-calculator

 

[saabfan9]

I have not bought tires yet because of this. i was thinking something in the range of a 225/60. what do you guys think? right now it still has a 2.76. not racing just a driver

 

[desmo900]

Use this calculator from Reilly Motorsports (Alterktion):

https://www.reillymotorsports.com/store/pages.php?pageid=12

It lists all popular Mopar transmissions ratios.

 

[AJ/FormS]

From the tallest 15 that will fit in there,( about 28inches), to the shortest typical tire, (about 24.5), there is a 14% spread. Each successive gear swap between 2.76 and 3.55, is about 9%. So moving from 2.73s to 3.55s will increase your revs about 27%, and going from the small tire to the larger, will reduce the Rs about 14%. Adding 27 -14 = about 13 % increase in Rs overall.Here is the Formula;
(RPM x TC)/(1056 x R1 xR2) = Mph; where TC is tire circumference,R1 is rear gear, and R2 is tranny gear.

;with your 2.76s;and assuming a current tire size of 24.5 tall
So; (rpm x 77)/(1056 x 2.76 x 1.00) = 65mph,and rpm =2460(zero slip)

:with 3.55s and 28s(255/65-15)
So; (rpm x 88)/(1056 x 3.55 x1.00)=65mph.and rpm =2769(zero slip)

And to compare; 2769/2460 =plus 12.7%. Which compares pretty nicely to paragraph #1.

You didn't give your current tire size, so I assumed it to be a teensy 24.5 tall.Your results may therefore vary. The math is never wrong. It's just math.
A 24.5 tall tire, on a 14 rim,could be a 225/60, or a 205/65,or a 190/70, or a 180/75.

 

[saabfan9]

AWESOME...thanks for the help guys. just picked her up sunday i will post pictures of the restoration