gas mileage
Melting Slicks
Joined: Jun 2006
Posts: 2,071
Likes: 147
From: S Nevada
Fuel mileage is more related to the size of engine, the rear axle ratio, and whether it has an overdrive transmission and lockup on the tranny than anything else. Any carb in decent condition can be tuned to provide nearly the same mileage as if the car were computer controlled EFI. But, most carbs aren't tuned that well.
A SB C3 with O/D and 3.55 or lower rear gear should be able to get around 20 mpg at highway speeds.
A SB C3 with O/D and 3.55 or lower rear gear should be able to get around 20 mpg at highway speeds.
Melting Slicks
Joined: Mar 2008
Posts: 2,666
Likes: 289
From: Spring Park MN
My 80 Vette used to have the engine and trans in my signature in it. I spent a lot of time tuning with a DTT Wego III digital wide band AFR gauge and a vacuum gauge. I had cruise AFR set to 14.2 to 14.4. I run an adjustable vacuum advance with an adjustable limiter I made. Timing at cruise was 40 to 42 degrees. With 11.8 to 1 compression and the quench (squish) set at .034", it didn't like more than 42 degrees at cruise. The car had 3.07 gears so to cruise in OD with the converter locked I had to run over 75 MPH minimum, but it was better at 80 to 85. Cruising at an average of 80 MPH the car would get 22 MPG on the interstate.
Big power doesn't mean you have to get poor mileage. Most cars just aren't properly tuned to get good mileage. Power valves are usually tuned for idle vacuum which is way wrong. They need to be tuned for cruise to power transition. Primary main jets should be used to set cruise AFR and the power valve enrichner passages used for WOT AFR. The vacuum advance setup is important to have the best timing for cruise combustion efficiency. Best timing for cruise will depend on your engine build. I take much care in tuning my cars, but that's what I do.
My 75 has 4.11 gears. It may not get as good of mileage as the 80 did, but I don't expect poor mileage. It may do better as overall gearing will be 2.87 in OD. I won't have to drive as fast to be in a good cruise RPM for my engine.
Big power doesn't mean you have to get poor mileage. Most cars just aren't properly tuned to get good mileage. Power valves are usually tuned for idle vacuum which is way wrong. They need to be tuned for cruise to power transition. Primary main jets should be used to set cruise AFR and the power valve enrichner passages used for WOT AFR. The vacuum advance setup is important to have the best timing for cruise combustion efficiency. Best timing for cruise will depend on your engine build. I take much care in tuning my cars, but that's what I do.
My 75 has 4.11 gears. It may not get as good of mileage as the 80 did, but I don't expect poor mileage. It may do better as overall gearing will be 2.87 in OD. I won't have to drive as fast to be in a good cruise RPM for my engine.
Pro
Joined: May 2011
Posts: 722
Likes: 58
From: Karlsruhe (Germany)
My zz502 with the Demon Sixshooter used to get ~11mpg when driven around town and between towns. I have 3.70 rear and a TKO 600 with 0.68 OD.
Last week I did the first big trip on our Autobahn. Drove always around 70mph at 2000rpm without hard accelerations and reached incredible 20mpg.
My brother was bothered because his BMW 318i with a 2L EFI engine has nearly the same consumption under this conditions
Last week I did the first big trip on our Autobahn. Drove always around 70mph at 2000rpm without hard accelerations and reached incredible 20mpg.
My brother was bothered because his BMW 318i with a 2L EFI engine has nearly the same consumption under this conditions
Le Mans Master
Joined: Feb 2001
Posts: 6,317
Likes: 292
From: Ocala FL
You guys will hate me. My '68 427-390 hp would get 19 mpg on the highway at 55 with the 3.08 rear. With the 3.55 in it, I'd get 15 to 16.
I have had to replace my original carb that literally fell apart inside, the first replacement wouldn't idle right, and the second doesn't have the choke set right, and other issues getting back to running normally, so I haven't had the best conditions for checking gas. Too much sitting around idling, jumping on it, holding it in gear too long before shifting, and such playing around, and it has gotten about 10.5 the last two tanks. I'm surprised with what has gone on lately, it wasn't more like 8 mpg.
When I get things back to normal, I'll post again.
I have had to replace my original carb that literally fell apart inside, the first replacement wouldn't idle right, and the second doesn't have the choke set right, and other issues getting back to running normally, so I haven't had the best conditions for checking gas. Too much sitting around idling, jumping on it, holding it in gear too long before shifting, and such playing around, and it has gotten about 10.5 the last two tanks. I'm surprised with what has gone on lately, it wasn't more like 8 mpg.
When I get things back to normal, I'll post again.
Racer
Joined: May 2014
Posts: 420
Likes: 5
From: Springfield Missouri
Wow, zombie-thread....
In the 4 months I've had my car, with it's out-of-tune 350, stock QuadraJet needing rebuilt, 3-speed transmission and 3.08 gears I've been getting around 9 mpg, assuming my odometer/tripmeter are working correctly. That's mostly putting around town with a few trips out on the highway.
In the 4 months I've had my car, with it's out-of-tune 350, stock QuadraJet needing rebuilt, 3-speed transmission and 3.08 gears I've been getting around 9 mpg, assuming my odometer/tripmeter are working correctly. That's mostly putting around town with a few trips out on the highway.
Pro
Joined: May 2013
Posts: 672
Likes: 31
From: Rapid City South Dakota
I've got a similar setup, Mark Jones 496 but with a 5 speed tko with 0.64 overdrive and 3.90 rear diff gears, 14.2 mpg at 75 mph and 14.7 at 65 mph. I'm pretty happy with that.
