Dave
At 09:19 AM 10/4/00 EDT, you wrote:
>OK, I've got 33" tires. Going from a 3.55 to 4.10. Which speedo pinion gear
>(how many teeth) do I need to compensate? The formula I have is this, not
sure
>if it is right: # of teeth = (265 x tire diameter in inches) / axle ratio
The formula that I have seen is (cannot vouch for it).
# of teeth = (263 x Axle Ratio) / Tire Diameter (round to nearest
integer)
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The method that I've seen involves using three step formula process.
In order to use it, you would need to know the tire diameter and the proper
number of teeth of the speedo pinion gear for those tires.
In your case, the 235/75 is about 28" outer diameter the # of teeth on the
speedo gear
is 33 (according to what you sent). For a 33" tire and 4.10, you would do
the following:
Step 1. Find the tire revs/mile for the old tire and for the new ones.
* Tire revs/mi. = (63,360 in./mi.)/tire circumference (Formula 1)
Old tire revs/mi. = 63,360/(3.1416 x 28) = 720.29
New tire revs/mi. = 63,360/(3.1416 x 33) = 611.15
Step 2. Find error factor either by calculation or by measured mile method.
* Error factor = (new tire revs/mi. x new axle ratio)/
(old tire revs/mi. x old axle ratio) (Formula 2)
Error = (611.15 x 4.10)/(720.29 x 3.55) = 0.9799 (speedometer is
slow)
NOTE: If error > 1, the speedometer is fast.
If error < 1, the speedometer is slow.
Step 3. Find required driven gear tooth count.
* New Drive Gear = Old Drive Gear x Error (Formula 3)
(teeth #) (teeth #)
New Drive Gear = 33 x 0.9799 = 32.3367 ===> 32 (nearest whole teeth)
You would see if the parts dept has a 32-teeth speedo pinion gear for your
tranny.
If it does not exist, you would have to compromise and use the closest one
to it.
Hope this helps.
Bob. Ont, Canada.
'97 Dakota CC, 5.2L, 4x2, 3.92 SG, auto., 4265 lb.
PB: 14.737 @ 91.75 9.364 @ 72.95
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