Automatic gear's help

[dusty430]

Hello,

When I bought my 01 mustang Gt w/automatic. I seen that the diffrential cover was replaced. With this in mind, I was wondering if the gears have been swaped in my car. I have 20" rims in the back and 18" rims in the front. Tire size is 245/40ZR20 in the back and 245/40ZR18 in the front. I was driving at 75 mph at 1700 RPM however my speedo is off by 5 mph due to the tire size. I was really going 70mph at 1700 RPM. Any help would be highly appreciated because I want to put 4:10 in but I don't know what I have.

Thanks,
Dustin

 

[CanadaStang]

this should help:
http://www.allfordmustangs.com/Detailed/636.shtml

 

[dusty430]

I am still confused this is my results
Your Rear Gear Ratio is : 2.0035 (Toploader or C4)
Your Rear Gear Ratio for OverDrive is : 3.0053 (T5 or AOD)

I thought that stock was 3.13?
Why am I so low?

 

[streetstang03]

why such big wheels? only 245s on those things

 

[jeffnoel]

Hello,

I was driving at 75 mph at 1700 RPM however my speedo is off by 5 mph due to the tire size. I was really going 70mph at 1700 RPM.

Thanks,
Dustin

You've got it backwards a taller tire means your speedo is reading low not high, so when your speedo says 75 your actually doing 81 if your speedo was recalibrated for the 20s. But with a tire 2" taller than stock I wouldn't believe what your speedo says at all. The easiest way for you to check what gears you have is to raise the rear off the ground and rotate a tire 1 full turn and count how many time the driveshaft turns. What does the diff cover look like are you sure it's not stock?

 

[dusty430]

I used one of those on the side police scanners to tell the speed. It is always 5mph faster. The cover isn't black. Ill try what you said

 

[Cobra Jay]

The easiest way for you to check what gears you have is to raise the rear off the ground and rotate a tire 1 full turn and count how many time the driveshaft turns. What does the diff cover look like are you sure it's not stock?

The cover isn't black.

The covers aren't black from the factory, they come in a natural or zinc plated finish.