12-27-2018, 01:08 PM
The low end performance of the engine is a big factor in happy overdrive gearing. My 2000 Viper has a .5 sixth gear and with a 3.07 ring and pinion. Sixth can be used at 60 mph quite easily. I have the same trans in a '72 Vette and a 3.55 final drive. When it had a 427 in it with a 246 @ 50 cam, 70 MPH was about the slowest it was happy in sixth. When it had a mild 350 with a 196 @ 50 cam it was easy to use sixth down to about 63 MPH. I have an 11.5 :1 compression 406 in a Camaro that required a slower advance curve when I installed a TKO600 in it. It had been fine with the 1:1 four speed but even with the slower curve, 2000 rpm is about as low as I take it in fifth.
After doing six overdrive conversions in muscle cars I've found that carb and ignition tuning can get very picky as you try to use numerically lower overall ratios especially with any kind of performance cam and compression. There are a lot of hills where I live and that makes tuning even more important in trying to avoid constant downshifting. If it's flat where you live it will be easier to use low rpm/OD combinations. Heat comes into play too. Adding OD to older cars can put them into a situation (high load/low rpm) that is a asking for detonation.
After doing six overdrive conversions in muscle cars I've found that carb and ignition tuning can get very picky as you try to use numerically lower overall ratios especially with any kind of performance cam and compression. There are a lot of hills where I live and that makes tuning even more important in trying to avoid constant downshifting. If it's flat where you live it will be easier to use low rpm/OD combinations. Heat comes into play too. Adding OD to older cars can put them into a situation (high load/low rpm) that is a asking for detonation.