M.P.H. off???

[PolandDogs]

So I have had my car almost a year (08 GT500) I bought it used, when I drive it I drive the speed limit going to and from town and just in general, I have always noticed a line of traffic ganging up behind me. At first I just thought it was me but, when I was on the way to All Ford Nationals last weekend they had road construction along with speed traps. I set my speedo at exactly 60 mph and it shot me at 51 mph. It said this 4 times up and back. My car is lowered on Ford Racing springs which wouldn't have nothing to do with this along with factory tire size's. Stock motor and to my knowledge has never been flashed. Although, I do know there is a ford racing rear differential cover on it. Now what are the chances that the previous owner might have changed the gears? Would this have anything to do with my mph being off this much? Could anybody tell me what mph in a certain gear that i should be running rpm's? Along with why is my speedo off this much?

Thanks Matt

 

[GT500RedStripe]

If you have a smart phone, check your speed with a GPS app like Commander Lite. Your gears and tire rotations per mile are adjustable with an SCT tuner. Should be an easy fix for you or your mechanic.

 

[PolandDogs]

Ok yea I will try that and I plan on buying a power package here soon with an sct X4 thank you

 

[PolandDogs]

Still do you think that my rear end gears have been changed? Why is my speedometer off this much? Anyway to tell if I have different gears without pulling the differential off?

 

[Catmonkey]

It's either the gears or tires or both that are different. 9 mph off is quite a bit. I'm guessing the previous owner installed a set of 3.73s in the rear end. That tire size is on the rear?

You could tape a string to the driveshaft and turn the rear wheel a full revolution and see how many wraps you get. 3 3/4 wraps would be 3.73s

 

[GT500RedStripe]

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[PolandDogs]

It's either the gears or tires or both that are different. 9 mph off is quite a bit. I'm guessing the previous owner installed a set of 3.73s in the rear end. That tire size is on the rear?

You could tape a string to the driveshaft and turn the rear wheel a full revolution and see how many wraps you get. 3 3/4 wraps would be 3.73s

Yea they are the factor tire sizes on the front and back, actually still factory Goodyear tires. Yea thats the way I been thinking it had to be the gears I will check it on my next day off and see what I can come up with thank you!

 

[shelby2]

I would say gears, that far off you might even have 4.10s . :burnout:

 

[PolandDogs]

I would say gears, that far off you might even have 4.10s . :burnout:

ok thank ya....wow i was hoping it wouldnt be 4.10s i was kinda hoping it would be 3.73 but i havent had time to check on anything yet