86 F150 4x4 302 FI 3:50 gears in the pumpkin 32x11.5x15 tires. With that info can anyone tell me if the dealer sold me the wrong gear for my speed sensor. The old one had 20 teeth and was black. The new one has 20 teeth and is orange. The speedo was correct before the old one wore out now with the new one it is 10 MPH faster. I called the dealer and told him this and he said that I had the correct gear. WTH???

 

The gear teeth are the same, so the ratio ought to be identical.

When were the tires added?

We looked at that at the Atoka G2G. If you go up in tire size, it DOES change the overall gear ratio back to front. Because the tires are a larger diameter, it throws everything off slightly.

It is also a logarithmic change - there is not much difference at 25 mph for example, but on my F100 with 33 inch swampers:
25 = 25

35 (on the speedo) = just under 40

55 = 60+

65 = 75 to 80 mph

The increased tire size means that you go farther with each revolution of the wheel. It also means reduced torque at the tire "Footprint", which was another point that came up.

I'm going to leave it to the brother who brought that one up to lay it out in a seperate thread....

A lot of great stuff was batted around!

 

My truck has 35x12.50's, upgrade from the origional 235/70's and it is 10MPH off, no way will you have it that far off with half the size jump, her Bronco II went from 205's to 31's and is 5MPH off. Are you sure the both have the same tooth count?

 

Yes both gears have 20 teeth the only thing I didnt check was to see if there was a difference in length of the gears or if the angle of the teeth were the same. Maybe the problem is with the dash cluster. mine has a tachometer so is there a formula that I can use comparing rpm to speed??? I was told by the dealer that you can check by going 60 mph for 1 mile and it should take you 60 seconds.

 

Try this:
http://www.miata.net/garage/tirecalc.html

It's a page with an online tire size speed calculator.

There is also this:
http://www.tciauto.com/tech_info/speedo_gears.htm

 

I wouldn't trust a local dealer around Memphis. I've had terrible luck with the ones I've tryed on parts. There right about 50% of the time.

 

Stock Tire - 235/70R15 ..........................Tire 1 - 35x12.50R15
Section Width: 9.25 in 235 mm .................Section Width: 12.51 in 318 mm
Rim Diameter: 15 in 381 mm .....................Rim Diameter: 15 in 381 mm
Rim Width Range: 6 - 8 in Rim ..................Width Range: 8.5 - 11 in
Overall Diameter: 27.95 in 709.93 mm ...Overall Diameter: 35.03 in 889.76 mm
Sidewall Height: 6.47 in 164.33 mm ..........Sidewall Height: 10.0 in 254 mm
Radius: 13.97 in 354.83 mm ....................Radius: 17.51 in 444.75 mm
Circumference: 87.80 in 2230.1 mm ...........Circumference: 110.0 in 2794 mm
Revs per Mile: 744.1 ...............................Revs per Mile: 593.7
Actual Speed: 60 mph 100 km/h .............Speedometer1: 47.8 mph 79.7 km/h

Speedometer Difference: - Speedometer Difference: 25.33% too slow
Diameter Difference: - Diameter Difference: 20.22%

This was taken from this web site http://www.1010tires.com/TireSizeCalculator.asp