Mr. Gasket 8703 Hot Rod Calculator - Manual (Page 41)

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tIRe RAtIo
Calculating effects of Changing Tire sizes
your daily commuter has four-wheel drive, and you want some
extra ground clearance for those occasional off-highway excur-
sions on the way home from work. However, before you make
the switch to a taller tire, you want to know what the effects will
be to your final-drive ratio and even more importantly, to your
speedometer as you don’t want to draw any unnecessary atten-
tion while you are cruising down the highway.
In this example, your current tires are LT235/75R15’s. your local
tire store informed you that your current Sport Truck T/A tires
have a diameter of 28.9 inches, whereas the mud tires you are
looking to upgrade to have a tire diameter of 33 inches. your
four-wheeler currently has a final-drive ratio of 3.08.
Calculate the equivalent drive ratio (that is, the drive ratio that
will provide you with similar performance and responsiveness)
and the effect to your speedometer by going to a larger tire
diameter.
KeysTroKe disPlay

0.
1. Enter current final-drive ratio:

Gear
raTio
3.08
2. Enter Old (current) and New Tire Diameters:

(Old Tire Dia)
Tireo
siZe
28.9
in

(New Tire Dia)
Tiren
siZe
33.
in
3. Calculate the effect to your final-drive (D-EFF) and the equiv-
alent final-drive (D-EQV):

Tire
raTio
1.142

(Final-Drive Ratio)
d-eff
raTio
2.697**
 *
(Equivalent Drive Ratio)
d-eQV
raTio
3.517
(cont'd)
UG8703E-B-inside.indd 39 3/3/2010 3:03:59 PM