Speedometer Gear, Please Help
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Senior User
Joined: Nov 2001
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Speedometer Gear, Please Help
Need help what are the calculations to determine speedometer gear teath count?
I need two different speedometer gears
1990 F-150, 4x2, E4OD, Straight Six, geared 3.73
Original Tires: P215/75-R15
Summer Tires: P235/75-R15
Winter Tires: 32x11.50-R15
Thanks.
I need two different speedometer gears
1990 F-150, 4x2, E4OD, Straight Six, geared 3.73
Original Tires: P215/75-R15
Summer Tires: P235/75-R15
Winter Tires: 32x11.50-R15
Thanks.
Posting Guru
Joined: Dec 1999
Posts: 1,903
Likes: 2
From: Copley, OH
The equation is:
Driven gear tooth count = (tire revs per mile X rear axle ratio X drive gear tooth count) / 1000
The original tires were 752 revs per mile
Your summer tires are 721 revs/mile
Your winter tires are 655 revs/per mile
(This is Bridgestone revs/mile data check your tire manufacturer for exact numbers, they may vary slightly but usually not enough to change tooth count.)
You need to get a tooth count off the drive gear on your tailshaft. It SHOULD be either a 7 or 8 tooth gear. You can get a tooth count byshining a flashlight in the hole and turning the driveshaft by hand or, assuming the original gearing was correct for the original tires, you can figure out which from the existing gear.
If the original speedo driven gear is 19T or 20 T you should have a 7T drive gear, if it's 21T the drive gear should be 8T and the speedo should be off slightly since Ford doesn't offer the 22T gear that would actually be needed. This leads me to believe that you probably have a 7T drive gear.
With a 7T drive your summer tires need a 19T driven gear and your witer tires need a 17T
With an 8T drive gear the summer tires would need a 21T gear and the winter tires a 20T
Part numbers and color codes:
17T C7SZ-A (green)
18T C7SZ-B (grey)
19T C7VY-A (tan)
20T C8SZ-B (orange)
21T D0OZ-B (purple)
Driven gear tooth count = (tire revs per mile X rear axle ratio X drive gear tooth count) / 1000
The original tires were 752 revs per mile
Your summer tires are 721 revs/mile
Your winter tires are 655 revs/per mile
(This is Bridgestone revs/mile data check your tire manufacturer for exact numbers, they may vary slightly but usually not enough to change tooth count.)
You need to get a tooth count off the drive gear on your tailshaft. It SHOULD be either a 7 or 8 tooth gear. You can get a tooth count byshining a flashlight in the hole and turning the driveshaft by hand or, assuming the original gearing was correct for the original tires, you can figure out which from the existing gear.
If the original speedo driven gear is 19T or 20 T you should have a 7T drive gear, if it's 21T the drive gear should be 8T and the speedo should be off slightly since Ford doesn't offer the 22T gear that would actually be needed. This leads me to believe that you probably have a 7T drive gear.
With a 7T drive your summer tires need a 19T driven gear and your witer tires need a 17T
With an 8T drive gear the summer tires would need a 21T gear and the winter tires a 20T
Part numbers and color codes:
17T C7SZ-A (green)
18T C7SZ-B (grey)
19T C7VY-A (tan)
20T C8SZ-B (orange)
21T D0OZ-B (purple)
Tuned
Joined: Aug 2003
Posts: 386
Likes: 28
From: Garnett, KS
Are stock speedo's off in calibration even with stock tires and gearing?? I had a cop behind me blaring his stupid police siren horn at me and raising his arms, but I was going forty (the speed limit) MPH!! Made me wonder if my speedo is off.......
Frank
Frank
Elder User
Joined: Aug 2002
Posts: 886
Likes: 0
From: Cincinnati
I went from a 19 to a 21 tooth in my 97 Ex to correct the speedometer in the 60 mph range. At 20 mph it was 12 mph higher and at 65 it was about 7 mph faster. Higher speeds are now correct but it is still 10 mph higher at lower speeds. Like someone put the needle on wrong. Now I drive 10% fewer miles!
