Speedometer Gear Help, with 140mph Speedometer

[vdubn]

I thought that I had everything figured out so that my speedo would be pretty accurate. In the end, driving around, its reading about 15 mph faster than my actual road speed.

I am running 275-35-18 rear tires, and 3.73 rear end, and I installed a red speedo gear (forgot to count the number of teeth, but I think its 20 teeth).

I called the company I got the speedo from, and they have no idea what to run.

Does someone know, if I get a new gear, do I need to increase or decrease the number of teeth to slow the speedo down?

[FoxChassis]

You are missing one piece of the puzzle...number of teeth on the speedometer DRIVE gear (the gear inside the transmission, on the output shaft).

P275/35R18 = 789 RPM*

Speedometer Drive Gear Teeth x Tire RPM x Rear Axle Ratio / 1000 = Speedometer Driven Gear Teeth

7 x 789 x 3.73 / 1000 = 20.6
8 x 789 x 3.73 / 1000 = 23.5

*This is a calculated tire RPM, which may not necessarily be the actual RPM. Actual RPM is determined by the tire manufacturer.

[FoxChassis]

A "red" Ford Parts Dept manual transmission speedometer driven gear is 21T.

https://performanceparts.ford.com/do...UsageChart.pdf

[Dr. Matt]

More teeth on the driven gear = lower speed indicated on the speedometer. If you are reading high, get a gear with another tooth, or two.

[JKengineering]

The best way is to calculate what gear you need:
Drive gear teeth x axle ratio x tire revs per mile / 1000
7 x 3.73 x 846 / 1000
22.1 teeth
The red gear should be 21 teeth

You may have to change the drive gear to a 6 tooth if you can't get a 22 tooth driven gear.

Hope this helps, Mike

[4-barrel Mike]

LMR speedometer gear calculator: https://lmr.com/products/Mustang-Spe...ear-Calculator

Mike

[erratic50]

If you look in the opening for the speedometer drive, most of of the T5 7 tooth main shaft gears are yellow afaik.

[Ourobos]

https://lmr.com/item/LRS-17271D-K/83...ear-Kit-Manual

[vdubn]

Ourobos, that is exactly what I bought today. I don't know which drive gear I have on the output shaft, but I am guessing it must be the later 8 tooth gear, otherwise my speedo would read much less. I think the 23T driven gear should be compatible with the 8 tooth drive gear... if that is in fact whats in my tranny. I should have looked when I had the trans out of the car.

Either way, 2 teeth should slow the speedo down a good bit. From what I can see, that is the highest tooth count, 23, for any of the driven gears, so unless I change the drive gear, that is the best I can do for now.

[erratic50]

If you look in the opening for the speedometer drive, most of of the T5 7 tooth main shaft gears are yellow afaik.