I can't help you with the gas mileage end of things, but speedwise...

Here's a nifty calculator. Have a look .

You'll need to know what your tranny gear ratios are (usually on a T5, Fifth is .7:1 or .9:1, and Fourth is 1:1), Tire Diameter in inches, and how high you'll be revving the old 261.

I think that it's safe to say that with a .7:1 OD, you'll be getting in the 20mpg range at the lowest. Probably closer to what my car gets (28-30)

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1946 Chevrolet 3600 - "Old Number Seven"

Cavalry's Here. Cavalry's a frightened guy with a rock, but it's here.