So, I am in the midst of finally doing a lot of upgrades to my 1989 Trans Am. I have to have someone else do it thought as I am overseas with the military, but WILL be returning this summer for a short while and moving back in December (Whoo hoo!!!)

The Car - originally a 305TBI car converted to a carbed 350 from a 1970 Camaro SS. The heads are from a 1972 Corvette LT1. Also have headers and a custom exhaust. Completely uncertain of my current HP or torque, but its significantly more than the 170 the car originally had. I figure its probably in the area of 300hp, but I realize that guess could be really high. The car will be used mostly for street cruising/daily driver duties.

Now the questions:

1. I intend to change out my rear end from its 2.73 one-legger to a 3.42 positraction unit.

a. How will this change my gas mileage?
b. How will this change my speedometer reading?


2. I also am going to upgrade to 17inch wheels (staggered 17x9.5/17x11) or something along that line.

a. How large can the tires be? (I admittedly dont understand how to read tire sizes)
b. Can the size of the tire offset the speedo change? If so, what could I do to minimize that speedo difference?

Also redoing the interior, dash, console, etc....

Later I anticipate putting an ATI Procharger on it as my final goal is to have 500hp.

Thank you all for having such a great board, I wish I had been more active on it before now. I am sure I will have more questions, but I appreciate any and all input.


PDG

 

Well you are putting in a bigger engine along with lower gears so obviously the gas milage will go down. Just chaging the gears themself without replacing the engine the in town milage would probably be about the same or maybe improve slighty. On the highway it my drop a couple mpg.

The speedometer will read faster than what you are really going. You can change the speedometer gear to correct this problem.

Depends on the width of the rims and and backspacing.

Yes but if you put on bigger tire (larger diameter) to correct the speedometer it would be the same as having the 2.73 gears installed. The car would not be any faster. It would actually be slower because you would have more massive tires that require more power to turn. Just swap the gears and put on what ever tires you want (that will fit) and then change the speedometer gear so that it reads correctly.