13 Isn't Bad, Considering...
MS: My 1971 Chevelle has a 350ci, 700R4 transmission, 12-bolt rearend carrying 3.31's, and 26" tires.The engine has Corvette aluminum heads, HEI with about 36 degrees all in by 2200 rpm, 9.8:1 compression ratio, high-rise dual-plane intake, 600cfm Edelbrock, Lunati roller cam (215/218 dur. @ .050; 268/276 adv. dur.; 489/.503 lift; 111 int. centerline; 115 lobe sep.; powerband of 1500-5500).
The transmission has a 2100rpm torque converter that automatically locks-up in overdrive, which kicks in at about 40-45 mph.
Currently, I make about 1800 rpm @ 60 mph, and get rotten gas mileage in town (13 mpg), but great gas mileage (about 20 mpg) on long highway trips at 70-80 mph & 2200- 2400 rpm.
No vacuum problems, and vacuum advance is adjusted out to the maximum. Do I need to get higher gears in the rear to improve economy? If so, what? 3.5's? 3.73's? Mathematically, 3.5's will give me about 1950 @ 60 mph, and 3.73's will give me 2070. Or is this just a timing curve problem? Is 2200rpm for full timing the right spot?
J_: Getting 13 mpg in town with a high power 350ci isn't bad. I drive an Olds 350ci with a mild cam and stock everything else and I get 13 mpg in town and 17 on the highway if I'm easy on it. My Cutlass has 2-series gears. I think that you're not going to get much better, especially since you already have an overdrive transmission (which I lack).
Archivist: Tom Wilson
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