Tire dia affect on gear ratio

[Purple73Cuda]

I am planning on changing my F70x14 tires to 15" radial tires. Is there a formula to figure out how the gear ratio would change by installing a taller ( or shorter ) tire?

[Oldschool]

Here ya go Purple73,

This little calculator uses a familiar dash layout for you to work your calculations with.  Just enter the information and it does all the work.  Just change any of the variables to get a new answer....        


http://vexer.com/68rt/speed.html



.

[CUDA JAS]

I am planning on changing my F70x14 tires to 15" radial tires. Is there a formula to figure out how the gear ratio would change by installing a taller ( or shorter ) tire?

you can alos try one of the spreadsheets at this link.

http://www.moparts.org/Tech/tech/pages/speedo.html

Jas

[Purple73Cuda]

Very good info guys.
I guess what I am asking is, in your experience if I go from a F70x14 to a 275/60x15, do I need to change the 3.23s to 3.55s to have the same acceleration with the taller tires?

[MEK-Dangerfield]

An F70X14 is how tall?  Even if it's 225's, the 275/60X15 will be taller.

[Bullitt-]

I would think the extra traction would be more than make up for any possible loss of quickness by the larger diameter.

[Purple73Cuda]

The F70x14 is 25.85" tall and the 275/60x15 is 28". I am wondering if the 2"difference is going to make the car act like it has 2.76s or a gear less than 3.23s.
Yes the traction and the look is going to make a big difference in the car. I just don't want to lose too much quickness.

[Bullitt-]

I put your #s in this chart using 215-70-14 for tire size & the difference is not that much IMO in 1st gear about 2-mph change at any RPM

http://www.moparts.org/Tech/tech/img/AUTOMATICchart.xls

215-70-14
GEAR   GEAR RATIO   MPH/1000RPM   MPH AT RPM                                          
         1500   2000   2500   3000   3500   4000   4500   5000   5500   6000   6500   7000   7500   
1st   2.65   9.00   13   18   22   27   31   36   40   45   49   54   58   63   67   
2nd   1.57   15.20   23   30   38   46   53   61   68   76   84   91   99   106   114   
DRIVE   1.08   22.05   33   44   55   66   77   88   99   110   121   132   143   154   


275-60-15
GEAR   GEAR RATIO   MPH/1000RPM   MPH AT RPM                                          
         1500   2000   2500   3000   3500   4000   4500   5000   5500   6000   6500   7000   7500   
1st   2.65   9.74   15   19   24   29   34   39   44   49   54   58   63   68   73   
2nd   1.57   16.46   25   33   41   49   58   66   74   82   91   99   107   115   123   132   140
DRIVE   1.08   23.87   36   48   60   72   84   95   107   119   131   143   155   167   

[shadango]

Here ya go Purple73,

This little calculator uses a familiar dash layout for you to work your calculations with.  Just enter the information and it does all the work.  Just change any of the variables to get a new answer....        


http://vexer.com/68rt/speed.html



.

Hey, this link is COOOOL! 

I could play with this all day!  Just the tool I was looking for!  Easy enough for an idiot like me to use!

And it seems accurate, using my current gear ratio, tire size etc.....seems to match up with the tach I get now, etc.   AWESOME!