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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
excursions 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 attention
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.
KEYSTROKES
1. Enter current final-drive ratio:
2. Enter old (current) and new tire diameters:
3. Calculate the effect to your final-drive (D-EFF) and the
equivalent final-drive (D-EQV):
(Final Drive Ratio)
(Equivalent Drive Ratio)
Repeated presses of [Tire Ratio] will toggle through the inputs and
(Old tire diameter) TIREo S 28.9 SIZE IN
(New tire diameter)
GEAR S 3.08 RATIO
TIREn S 33. SIZE IN
TIRE
1.142 RATIO
D-EFF 2.697 RATIO
D-EQV 3.517 RATIO
H
R
C
OT
OD
ALC
DISPLAY
0.
™ — 35
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