Not sure if my car has aftermarket gears

[boosted_ford]

I suspect my car might have aftermarket gears simply because of the fact my speedo is off by 10mph. My car does have bigger tires than stock, but according to a calculator it would only throw it off by 2mph. I also get terrible gas mileage, (about half a tank used on a 30 mile drive) so thats another reason I suspect gears. I remember someone once posted up a certain formula to figure it out by knowing the tire size and what the rpms were in top gear. But I cant find it anymore so I made a video. My rear tire size is 275/40/17 and 245/45/17 in the front and in the video im doing 60mph while the speedo says 70. Im also an auto 96 gt. http://youtu.be/gmGLyANhBak

Ps I needed a trans rebuild, and new rear. The trans guy gave me a new posi rear. I asked him to figure out if I have gears and he didn't do it. Ive also changed fuel filter, new o2 sensors, sea foamed, new maf, and cleaned tb and I still get pretty bad gas mileage. And it had a tune up except oil it needs an oil change. Anything else I can possibly change to hopefully get a little better mpg? I just sit there and watch the gas gauge drop like it had a hole in the tank. The only mods it has is bbk intake and a flopro exhaust from the previous person

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[96gt4.6]

I don't think any mod could give you a 30 mile half a tank, i would start checking for a fuel leak somewhere. an easy way to check your gears is jack up the rear end and count how many times your tires turn per one full turn of the drive line and the amount the tires turn would be your gear ratio

 

[CC'S95GT]

Both tires have to spin together for that to work.
If they don't spin together then the rear ends needs rebuild too.