Rear end upgrade??

BlueDream said

No tach unfortunately, but I have good news. I WAS able to find the tag, It was hidden under the paint so I never noticed it before. I guess I really lucked out, I've got 3.21 gears in an 8 1/4 that came with a slant six. So when I get the 318 in, how much improvement would I feel in the butt dyno with a sure grip and 3.55 or 3.90 gears? or would 3.90's be too much? I've never had the car on the highway but I'd like to take trips with my grandpa in it so I need it to cruise good at 70 mph.

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You also have quite a bit of play in final drive ratio based on what wheels and tires you select. Bigger diameter tires = lower RPM and more MPG. Smaller diameter tires = higher RPM and more fun.

But if you want to cruise comfortably at 70 mph, be careful what you choose. I would go find a tire size calculator on the web and figure out what size tires you need to get your desired RPM at 70mph.

If that engine is screaming at 70mph you'll hate it, both in the driver's seat and at the pump.