Some of the Buick Roadmaster's had 2.56 rear axles with an overdrive transmission. (There transmission's overdrive ratio is nearly identical to the 518's ratio) These cars had 350 engines and weighed about 4200 lbs. I think this gearing put them at around 1300 rpm at 60 mph. I know a guy who is a Roadmaster fanatic. He thinks the 2.56 was a little to much gear. He likes the 2.72(?) or 2.94(?)axle ratio's better.
It is very true that gearing the car so it will obtain best fuel economy WILL make the transmission shift in and out of overdrive more frequently. (particularly in the 40-60 mph range) When I'm driving a vehicle with real stiff gears, I manually shift the transmission into third gear until I'm on the highway, then shift into overdrive.
If an automatic transmission is making the engine lug, the shift points on the transmission should probably be adjusted. (or manually downshift it)
If you want to run real stiff gears a lock up torque converter will be necessary. Otherwise, the torque converter will slip more at low rpms. I believe some higher stall, lock up converters are available. These should help with both performace and economy.
Getting the idle mixture set lean will be critcal to achieving good fuel economy at real low rpms. The following article on how to tune a carb for economy is worth a read, even though the carb is on a chevy the same priciples would apply on a Mopar.
http://www.chevyhiperformance.com/tech/e...eage/index.htmlPersonally, I'd try a 2.76 to 3.23 ratio depending what I had on hand at the time. I'd guess a 2.92 with a higher stall, lock up converter would be best, but that's a guess, since I've never built anything like an Imperial. A 3.23 would certainly help performance, but would probably hurt highway fuel economy just a bit. It might help around town mileage though?
I'm quite interested in building a C-body as a cruiser / summertime driver. When you get this done, I'd be very interested in the results you achieve.
