83 C20, 350 V8, CA, Fuel Economy

[mistaake]

What type of gas mileage would a properly running C20 with those specs get? I'm not sure what gear ratio or transmission, but I'm assuming 4:10 and TH400?

Could I regear for better fuel economy, especially on the highway? I guess going to something like 3:73, 3:42, 3:08, etc. if it's even possible, would help a lot. How would that affect driving performance and fuel economy?

 

[CSFJ]

I don't have any hard numbers on my truck. It's an '87 v(k)20 with the TBI 350, a 700R4 and 3.73 axles, and the fuel gauge still goes down pretty fast. I'm not sure how high you can go as far as re-gearing a 3/4 ton axle.

 

[fussfeld]

Here's an interesting read concerning RPM and mpg:

http://www.oldsmobility.com/old/carlife_apr67.htm

If it's a c-20 then you have a 14 bolt rear? (not sure) If so, i think the highest you can go is 3.23 (pretty sure).

There are so many variables. How much driving do you do per week?

 

[Slick_james187]

Id say a safe guess is around 12 or 13. Dont know about the trucks, but in suburbans 454s seem to get better mileage than the 350s. My 77 burb bone stock with ca smog gets about 15 at 50-55 mph with 4:10 gears

 

[HotRodPC]

In the full float rear ends, they jump from 3.73 to 3.21, so looking to hurt your towing capacity but reduce rpms and maybe improve mpg the rare hard to find 3.21 is your option in the 14b Full Float. If you have the 14b Semi Float which you probably do if it's small block truck, then the 3.42 is an option in the Semi Float. I sold the 14b Semi Float 3.21 out of my C20 and installed a 14b Semi Float 3.42 axle I had out of the 85 wrecked C20 out at my dads. If I keep the truck, I'm probably going to go back to a Full Floater that I have with a 3.73 ratio and be selling this Semi Float 3.42. Sadly, Cali is wayyyyy to far away from OK to make that happen for you.

FYI, when I had the 3.21's in the C20 with an OEM 454, on the freeway at 65mpg for a round trip to my dads at 140 miles, I got 13.2 mpg and that's the best I ever got and the odometer was accurate to a nats ass. Not sure what you get now, but there you go. 14 avg highway is probably your best case scenario with a carbed engine. I only got about 9 - 10 in the city with the 3.21's. City is probably not going to change. Highway might improve a little and the motor won't scream as bad, but you're also going to notice some struggling during towing with the 3.21's vs 4.10's.

 

[highdesertrange]

if you plan your trip right so you are always going downhill you might get 15. but seriously I would say 12 mas o menos. remember over 55mph is what kills your mpg. highdesertranger

 

[mistaake]

if you plan your trip right so you are always going downhill you might get 15. but seriously I would say 12 mas o menos. remember over 55mph is what kills your mpg. highdesertranger

While Earth is round, I don't think going downhill all the time is always possible. Only half the time

In the full float rear ends, they jump from 3.73 to 3.21, so looking to hurt your towing capacity but reduce rpms and maybe improve mpg the rare hard to find 3.21 is your option in the 14b Full Float. If you have the 14b Semi Float which you probably do if it's small block truck, then the 3.42 is an option in the Semi Float. I sold the 14b Semi Float 3.21 out of my C20 and installed a 14b Semi Float 3.42 axle I had out of the 85 wrecked C20 out at my dads. If I keep the truck, I'm probably going to go back to a Full Floater that I have with a 3.73 ratio and be selling this Semi Float 3.42. Sadly, Cali is wayyyyy to far away from OK to make that happen for you.

FYI, when I had the 3.21's in the C20 with an OEM 454, on the freeway at 65mpg for a round trip to my dads at 140 miles, I got 13.2 mpg and that's the best I ever got and the odometer was accurate to a nats ass. Not sure what you get now, but there you go. 14 avg highway is probably your best case scenario with a carbed engine. I only got about 9 - 10 in the city with the 3.21's. City is probably not going to change. Highway might improve a little and the motor won't scream as bad, but you're also going to notice some struggling during towing with the 3.21's vs 4.10's.

Yeah, the fuel used to make the road trip out there would nullify any possible fuel savings by fitting a different ratio rear end. Thanks though

I would be happy with 13-14MPG. It's better than my Yukon which gets 10-13.