Cruising the Cyber Highway Hot-Rod Internet
The home of Australian Hot Rodding on the Net
Home >  Builder >  Technical Specifications >  CHANGING TIRE HEIGHT TO LOWER RPM'S

Here's two formulas to help determine your current RPM's and MPH. You can also check your speedometer for correct calibration by plugging your RPM's for a given speed into the formula.


The formula can then be used to determine what the change in tire size will do to your RPM's.

MPH X DRIVE RATIO X 336
----------------------------------------  = RPM'S
TIRE DIAMETER
 

If you would like to know your speed at a particular RPM, the formula is:
 

RPM X TIRE DIAMETER
---------------------------------------  = MPH
DRIVE RATIO X 336
 

(In both cases, "336" is simply a constant to convert the tire diameter to the linear distance the tire travels)
 

Determining your Drive Ratio

The drive ratio may be located at the following locations depending on how old of a vehicle it is:
 
  • A newer vehicle will often times have it listed on a spec sheet glued to the glove box door
  • An older vehicle can have a tag located on the differential cover. On 4x4 it could be on the front axle.
  • Their are also stamping codes located on the passenger axle that can provide differential information.
If nothing is listed for the rear end then you can use the following method to determine the gear ratio:
  • Raise the rear end and place it on jack stands
  • place a chalk mark on the tire and the drive shaft
  • have someone count the revolutions of the drive shaft as you rotate the tire one complete turn
  • This will give you a close approximation of the gear ratio
If for example, the drive shaft turns just a little over three turns the you probably have a 3:08 gear ratio.

All this may seem like a lot of work but knowing  what the effect of a tire change will have on your engines working rpm can save it from excessive wear and tear and maybe even extend its life.
 
 

Here's a little quick reference table to show how the rpm's change with tire size. At 60MPH.

 
TIRE DIAMETER
DRIVE 
RATIO
22"23"24"25"26" 27"28"29"30"31"32"
2.7325022392229322012117203819661898183517751720
2.9026572542243623392249216520882016194918861827
3.0728132691257924762380229222102134206319961934
3.2129422814369625882489239723112232215720882022
3.3130332901278026692567247123832301222421532085
3.4031162980285627422636253924482364228522112142
3.5432443103297428552745264325492461237923022230
2.7334183269313330082892278526862593250724262350
3.9025743418327631453024291228082711262125362457
4.0937483545343632983171305429452843274826602577
4.5641793997383036773536340532833170306429652873
4.8844724277409929353784364435143392327931743074
5.1247924488430041293970382336863559344133303226
Last Updated: 20/12/2001. Copyright 1996-2004 by GMW Consulting Services Pty Ltd, all rights reserved.
All other trademarks, tradenames and/or service marks are the property of their respective holders.
Copyright 1996-2004 Hot-Rod Internet All Rights Reserved.
Please read our Disclaimer, Terms and Conditions and Privacy policy