Speedo discrepancy

[sounnder]

Was driving the cobra this weekend and verified the speedo is about 10 MPH off. The rear tires are slightly larger than stock, so I don't think tire size is the cause. If the rear gears have been changed does it require a change in the speedometer gear?

Any opinions are welcome and thanks!

[jstanley55]

Yes it does. Figure out your ratio and LMR has the gear for it. Really simple to swap it. If its thy far off I'd bet you have 3.73's or lower. Assuming it's clocking faster than it should.

[NV93Cobra]

It's going to be a percentage, not a specific value.

The Cobra came with 3.08

So, assuming you have stock tire height the following is a good rule of thumb.

65MPH actual

Speedo will read:

3.27 --> 69MPH
3.55 --> 75MPH
3.73 --> 79MPH
4.10 --> 87MPH
4.56 --> 96MPH

[sounnder]

Yep you guys are confirming my suspicion.

Is there an easy method to determine the ratio? The only way I know is to mark the drive shaft and tire and count the rotations in relation to each other.

That or use NV93Cobra's speedo readings.

Suggestions?

[NV93Cobra]

Based on your initial comment I'd guess you have 3.55.

Here's the speed table for common driving speeds.

25-->29
35-->40
45-->52
50-->58
55-->63
65-->75
70-->81

My old GT had stock 2.73 and I changed to 3.73. The math was basically 1.333 * mph = actual speed.

[sounnder]

Thanks much man! You rock.

[93Cobra#2771]

Be aware, also, that if the trans has been replaced or modified, sometimes the driven gear will be different than stock. If so, the above chart could be off slightly.

When ordering replacement driven gear, you need to know whether the drive gear is seven teeth or eight.