I am looking for some input on gear ratio for my '72 cutlass . I am going to be upgrading the engine form a 350 to a healthy 455 soon(around 450 horse) . I am currently running around 3500-400 at 65 mph with a 3.73 ratio, th400 trans ,225-70-15 tires and ac. I would like to get the rpms lower on the highway with the 455 but not lose a lot of the bottom end punch. I am currently looking at 3.23 or 3.42 ratios just not sure which would be the better choice.

 

I'm happy with my 3.42 gears.

 

3.42 isn't much of a change from a 3.73. A 3.23 won't let ya down considering the low end grunt that 455 will have.

 

I'm a fan of the 3.23 limited slips, my 442's all have them.

 

3.42 from 3.73 wont result in the cruising rpm drop that you are looking to achieve. That gear change will only knock 200 rpm out of the cruise RPM.


Go with 3.23 gears & a tall tire (27" or taller) or 3.08 gears & a shorter tire (less than 27").


If it was me I would go with the 3.23 gears as they are very versatile.

 

No you aren't. You need to check your tach. That size tire with 3.73 gears and 1:1 final drive puts you just under 3000 RPM. If you still want to reduce it, that's fine; just know that you're not running 3500 - 4000 RPM at 65 mph.

That said, presumably the 455 will have a little more low-end grunt and thus doesn't need as much gear to still be fun. A 3.42 rear will give you 8.3% lower RPM. a 3.23 will give you 13.4% lower RPM than 3.73. Both will be noticeable. If you do a lot of highway driving, I'd consider 3.08 for 17.4% lower RPM. This all assumes the 455 is built for lower RPM and isn't a screaming race engine.

 

Thanks guys. I have decided on the 3.23 as it was the factory ratio for similar trans and engine setups. I very well could be turning less than my tach says , I have only had the tic toc tach in the car so I have no other readings to compare it too. I only do a few longer road trips to shows in a year so the 3.23 should be the right fit for my ride.

 

The Tic Toc Tac is known to read higher than actual RPM. The faster you drive the more out the tac is. They were calibrated to read high to keep warranty claims down on over revved engines.

 

3.23 at the most but 3.08 is fine