New member needing knowledge

[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.

 

[70GTO455CONV]

355 vs 390 rear

355 to my knowledge mean 3.55 turns of the drive shaft per 1 turn of the rear tires. A 455 motor has much more torque at a lower RPM so wouldn't this mean I would be spinning the motor at a lower rpm due to the higher gear which would put me closer to the peak torque which I would think the car would be faster. Now if I had a 350 motor that had a higher torque curve at a higher RPM I would want a lower gear (390 or above) to run the car closer to that higher RPM so I could get the most performance out of the car. I'm not to familiar with rear ends on cars I'm just thinking about all of this logically so please I'll take all the advice I can get on this subject.

 

[melsg5]

You are correct with your statment about number of turns of rear wheel versus driveshaft. The whole purpose of using a 3.9 versus a 3.55 is to increase torque applied to the rear wheels regardless of what RPM the engine is turning. People serious about drag racing go to gear sets of 4.11, 4.56, etc. They dont go down to 3.55. Peak torque occurs at a single specific point in the RPM range while the higher numerical value gearset is independant of RPM. You will get better gas mileage and higher top end speed with the 3.55. It all depends on what you are primarily doing with the car, cruising or drag racing. The 3.55 is the ideal midrange gearset, a little of both ends, good off the line and not too bad cruising on the highway.