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tire height

  
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tire height

 
WScott WScott
New User | Posts: 40 | Joined: 03/12
Posted: 06/27/13
04:46 PM

i just put a set of new tires on my car, i went from 26" tall to 28" tall
I have 3.55 rear gears with the 26" tall tire,  the rpm is a little
lower with the 28" tall tires at say 60 mph. I know the rear gear set is still
3.55 but by putting the taller tires on would that be the same as changing the rear
gears to say 3.42 or 3.23. Would the performance difference be the same going
to 3.55 from 3.42 as going from 28" down to 26' tires or are these two different
animals altogether, just wondering  Wayne  

68scott385 68scott385
Guru | Posts: 1990 | Joined: 10/09
Posted: 06/28/13
05:26 AM

RPM = (168 x final gear ratio x mph) / tire radius

Use this formula to see how much of an rpm drop you'll get from the taller tires.


In one scenario I had 28" tires w/o od and 4.10 gears. I gained 5mph at the same rpm when I went to a 31" tire.

In another scenario I had 28" tires w/ od and 3.42 gears. I increased 100-200rpm at the same speed when I changed to 3.73 gears. At low speeds (35) it's 90rpm while at higher speeds (75) it's 195rpm difference.

It really helps if you can plug this stuff into a spread sheet to see what changes do to the rpm & speed.  
68scott385 68scott385 68scott385

Dave632 Dave632
Addict | Posts: 2222 | Joined: 07/08
Posted: 06/28/13
02:57 PM

Yes I agree, it is the same as changing gear ratios when changing tire size.
There is some advantage to the taller tire in relation to tire contact patch.
That is why a tall slick has an advantage over a shorter one.  
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pepsi1 pepsi1
Guru | Posts: 1718 | Joined: 09/11
Posted: 06/28/13
03:56 PM

Yeah as mentioned you will only drop to a 3:42:1. If you have a Posi unit,and  tires with the 28" it will be almost impossible to determine the difference. The real and tested way is to use your Tach. I would use that way and set the speed at 60MPH. Then just compare the differences. If your engines cam is coming into it's power range you probably won't notice the difference.
  If you really made a big tire like to 30"-32" I would really be concerned about
the difference in the RPM drop.If you have an O/D trans sometime the O/D will make it's last O/D into the higher gear at about that speed.
  But when all else fails go out and have fun and enjoy your Ride.

Bob