New member needing info

[70GTO455CONV]

I currently have a 70 Gto conv A/C car w/ a 72 YC 455 producing about 450hp at the motor. It has a M-20 4 speed with a 390 posi rear. The rear end was leaking and the car no longer spun both rear tires only one. I brought it to a very well spoke of shop to rebuilt the rear and he suggested I go with a 355 gear instead of the 390. He said the car would be faster off the line and faster while driving as well as get better gas milage on the highway. I understand I will get better milage but wouldn't the car be slower with a 355 gear?

 

[melsg5]

The top speed would be higher but the acceleration slower. If wheel spin is a problem it would be reduced but beyond the wheel spin, again it would be slower.

 

[cmcmcm]

I think a 3.55 would be a good move. Your 455 has plenty of torque (500+ ftlbs) and would have no problem with it. 3.90's will work better with a close ratio 4 speed with a 2.20 first gear.

 

[Zimgxp]

The difference between the two isn't really that much. Maybe 50 - 100 rpm at 60 mph.