Speedometer calibration help needed

[Sleeper]

I need some help with recalibrating my speedometer.
I did a diff gear change from 3.00 to 3.25 (I know please read sig) and my speedo is reading about 5 mph low. I have a 11 tooth (orange) drive gear and a 31tooth driven gear.
If I use the formula:
#of teeth on old gear x new gear ratio / old gear ratio =# of teeth on new gear
(31 x 3.25) / 3.00=
100.75/ 3.00 = 33.58
Since thay don't make a 33.58 tooth driven gear, what combo of driven and drive gears should I use?

Thanks
Paul

 

[MeanBuicks]

Perhaps this'll help...

Speedometer Gears.

 

[Sleeper]

Thanks Greg

Help???

Something ain't quite right here????
If I use the formula on the TCI page to calculate TR/M it does not match the Dunlop spec sheet figure for revs per mile of 899?

Tire Rev. per Mile = 20168 /Tire Diameter (inches)
873.07 = 20168/23.1

Plugging both numbers into the other formula...

# Driven Teeth=
# Drive Teeth X Axle Ratio X Tire Rev. per Mile/1001

11 x 3.25 x 873.07/1001=31.28 TCI

11 x 3.25 x 899/1001=32.11 Dunlop

According to the TCI #'s and auto calculator I have the right driven gear in it now but I know it is 5 mph low.

They also list a 35 tooth driven gear for the 200-4R tranny but everybody is telling me that it does not exist???

FWIW. Tire is a Dunlop 205/50ZR15 that has a section width of 8.4" and a tire dia of 23.1

Thanks
Paul

 

[MeanBuicks]

My old GM 200-4R parts book lists part number 25522477 as being a 35T-orange gear for a 1988 Caprice w/ 2.56 rear. That part number is probably no good anymore but perhaps you can find a cross reference.

The formula is figured as follows:

5280 ft/mi = 63360 in/mi
tire circumference = pi x diameter = 3.14159 x d

tire rev/mi = (rev/tire circumference in) x 63360 in/mi = (1/3.14159 x d) x 63360 in/mi = (63360/3.14159)/d = 20168/d

Either they have some creative math going or they forgot to inflate the tire when they checked the revs/mile.