Hello everyone,

I was driving on the highway the other day and my speedometer was saying i was doing 75mph but the interstate construction zone machine said i was clocked doing 65.. Ive even noticed that when im driving i have others passing me because i was driving to slow in the city.. speed was 35mph i was only doing 25 but according to my speedometer im doing the right speed.

Anyways, i was on American Muscle website looking at Motive 3.90 gears and in the reccomandations area they have **Recommended Upgrade. Changing the rear axle gear ratio will throw off the calibration of your Mustangs speedometer**

I recently purchased the car, do i already have aftermarket gears on there?? Would that be the reason for my speedometer readings to be off?? Is there anyway I can tell if they are stock or aftermarket?? Thanks!

 

From the 75/65 difference it would appear you do have aftermarket rear-end gears--3.73:1 would be my guess:

75/65 * 3.27 = 3.77 (speedo is 15% higher than actual speed)

However that you have the same 10 mph difference at 35/25 (+40%) would indicate that the speedometer indicator needle has been misplaced on the shaft.

Gears would make the error the same percentage at all speeds, not the same absolute value--I.e. if the gears were 3.73s the error error at 25 mph (actual speed) would be 25 * 1.15 = 28.2 mph; not 35.

To test the indicator placement use the instrument cluster diagnostic mode (click here for more) to digitally display the vehicle speed on the odometer. It should be very close to the same value as indicated by the analog gauge--at all speeds.

If it is not the indicator is misplaced, if it is them you likely have after market gears.

 

Is there any way I can tell if the gears are aftermarket or stock?

 

You can jack up the back end and count driveshaft turns vs. wheel turns, or with a GPS get your car's speed and note the RPMs at some speed in 4th (40-50 works nicely, use 3rd for an auto) and enter the actual speed and RPMs into my "Gear Guesstimator", which is pretty accurate based on feedback I have received.

It looks like this, just enter the real vehicle speed and engine RPM at that speed, set the tire size as needed and click Calculate Axle Ratio.

 

yep... lol i don't know why i bother coming in threads when i see cliff is the last to respond.... he usually gets it right down to the last detail.

 

Thx cliff. ill do that and let u know the results.