[Excaliber]

3.77 rear gear with an automatic is a VERY low rear end ratio. Heck thats "low" for even a stick shift! I'm surprised your ONLY turning 4500 rpm in the 100-110 mph range. I bet it's higher rpm than that, unless you got some VERY TALL tires!

Auto trannys are NOT a one to one ratio in high gear. A stick shift is (assuming no OVER DRIVE unit). Expect 10-20% "slip" due to the torgue converter. Therefore an Auto Trans will turning 10-20% more rpm than the same car with a stick.

Typical Ford or GM rear ratio with an auto would be 2.73, 3.0 would be "low". Same Ford or GM cars with a stick might be 3.3, 3.5 would be "low". Anything "shorter" than 3.5 rear ratio is getting into "serious" performance numbers, and THATS with a stick shift! I know, we ALL run low rear end numbers, but hey, we drive Cobras, we are NOT NORMAL.

Auto trans have a LOT of torque multiplication due to the higher engine rpm from the "slip" in the converter (stall speed is an example of this). Because of this "multiplication" you can run a "tall" gear ratio (2.73) that would be "unacceptable" with a stick shift. To high a gear ratio with a stick (2.73) would result in a burned clutch plate, sluggish accelleration, tough to get under way on a hill, etc. A 3.7 gear ratio with a stick, just like an auto trans WILL LIMIT TOP SPEED at "normal" rpm ranges (normal being defined as 5,000 or so). You can go "faster" if you spin the heck out of the motor, 7,000 rpm and like that!!

Modern Auto Trans have a "lock up" torque converter, thus the "slip" factor is eliminated, but it only locks up under light throttle and cruising speed conditions. Some (like the AOD) also have a 4th gear, an "over drive". 3rd gear would be one to one ratio, 4th (OD) might be .6 or .7 to one ratio. Add the "lock up" feature and you get LOW rpm at cruising speed.

Want to save wear and tear and get decent mileage? Go for a higher rear ratio, opt for an AOD auto trans or put in a stick shift.

22 mpg with a 3.7 rear ratio and a standard C6? Well,,,,,,that IS impressive, I've never seen it myself. 351W's are not known for great mileage under any conditions! I expect those numbers could only be achieved under "ideal" conditions and a low cruising speed. But, I could be wrong!

I'm running 3.55 gears with a stick and a 302. Top speed? 115-120 maybe. Engine rpm 5000-5500, stock motor. I could use 5th gear but the top speed won't change, only the engine rpm will drop. It takes horse power to "pull" that .8 over drive gear, and the stock 302 aint got it! But it DO accellerate nicely and it's the perfect gear for the short track I run! My best mileage cruising in 5th gear? About 18 with EFI. Real world mileage? 10 to 12.

Ernie

EDIT: The shift speed can be adjusted a number of ways. If your "deep" in the throttle and it's shifting THAT early, it clearly needs adjusting! It SHOULD shift around 4800-5000 with your foot buried in the carb.