Auto Racing: Changing Gears, speed capability, fuel mileage

Question
I have a 79 Merc Cougar with a 351 5.8L engine.  I want to increase it's overall performance, especially torque and horsepower.  A mechanic has told me I should change the gears from 2:47 to 3:90.  What are the advantages/disadvantages to doing this?  I'm concerned my money could be better spent elsewhere. Thanks.

Answer
First off the chanage in gear ratio will have no effect whatsoever on torque or horsepower.  To do that you have to modify the engine which is expensive these days.

It will however enhance your acceleration, and detract from your fuel mileage. I am going to guess that your MPG figure will be reduced by about 25%  So if you are getting 15 MPG now you can expect  about 11.5 MPG after the gear change.  The other change will be in your top speed capability which will also be reduced by quite a lot.

For a street car ?  I wouldn't do it.

Dan Liddy
Sarasota, Florida