283 fuel milage

[gregg pellicer]

I have a recently completed 1950 GMC on an S-10 chassis. 283,fresh rebuild,flat top piston's , power pack head's ,600 cfm edelbrock carb . The truck has 3.73 gear's and a 350 tran's. Fuel milage is terrible 10 mpg avg. The engine is turning almost 3000 rpm at 60 mph. First I know I need to change gear ratio. My question is what kind of milage can I expect and what RPM should I be turning at cruising speed with this 283.
Gregg

[Foot Stomper]

Once you think you have the right RPM figure, enter your data in this excellent tool.
There's 4 fields of information so enter three to get the fourth.

http://www.advanced-ev.com/Calculato...ze/?ratio=3.08

I entered your supplied data and discovered you are running a tire that's 25.1 inches tall... 185/70R15?

Perhaps running a taller tire will give you the desired result. A 30" tall tire will drop your RPM by 500. Or, 3.07 will have the same RPM.

Anyway, I don't know the target RPM you want, but the tool above will surely help.

[mattfranklin]

Is this steady running at 55 mph? Flat road? No headwind? 10 mpg feels kind of low for that combination. Things should improve a little with break-in. How is your spark timing?

My gut says that once you get the rpms, spark timing, and air-fuel right, you should be at least 15.

[1project2many]

The 283 should be pretty good on fuel. 15-16 mpg no problem even with that rear. If you're sure the basics are good such as no vacuum leaks and timing set properly, then check vacuum on the highway at 55-60 and compare to carb Power Enrichment valve setting. There's a good chance the small engine is at low vaccum which is making the carb drop AFR to 10:1 - 12:1. Like Holley carbs, it's rare that an Edelbrock carb is right "out of the box."

[gregg pellicer]

The tire is27" tall. I started out with a 390 holley. I was only getting 7mpg got it to almost 9 mpg with jetting and timing tuneing. I switched to a 600 edelbrock and got it to 10 mpg . Also I have used those gear ratio/tire size calculator's. I honestly feel that 3000 rpm at 60 mph is to much.What I'm looking for is does someone have any real world experience with a 283 in one of these truck's that get's decent milage and what kind of rpm was it turning at cruise speed.
Gregg

[Coupeguy2001]

You should probably look at a trans with overdrive like a 200R4 or 700R4,
The final drive is .65:1.
That means that you are reducing your engine's RPM by 35%.
that will give you 5-7 MPG immediately.

I have a 350 smallblock,
3.42 rear,
4 speed with overdrive,
27" tall tires,
and I get around 23 mpg till the secondaries open, then it drops to around 15.
2000 RPM at 75mph.

[IndyBowtie]

Hey Gregg
I have a 48 truck with a 283. With a T5 and a 3:08 rear I got 16, with about 1800 rpm at 55. Now have switched to a 700 r 4 and still get about the same, and about the same r's.

[xaircav]

My '53 with 283, Edelbrock 500 CFM carb, 255/70-15 tires and 2.73 gears gets mileage in the mid teens and turns 2200 RPM at 70MPH.

[gregg pellicer]

Originally Posted by xaircav

My '53 with 283, Edelbrock 500 CFM carb, 255/70-15 tires and 2.73 gears gets mileage in the mid teens and turns 2200 RPM at 70MPH.

Thank you. What tran's are you using?

[xaircav]

Originally Posted by gregg pellicer

Thank you. What tran's are you using?

SM 465 so no overdrive.

[gregg pellicer]

Originally Posted by IndyBowtie

Hey Gregg
I have a 48 truck with a 283. With a T5 and a 3:08 rear I got 16, with about 1800 rpm at 55. Now have switched to a 700 r 4 and still get about the same, and about the same r's.

You are the first person to actually answer my question. I also posted this question on the HAMB.Everyone want's to talk about my tune .Thanks for your response.I have a 700 r4 that I am going to install first.I don't think that will get me all the way where I need to go but it's a start. I'm sure I will also end up changing the gear also.I have both a 350 and a 700 I went with the 350 because the rear was supposed to be a 3.08 .Somewhere along the way it got changed to a 3.73.

[1project2many]

I have a 302 in my '57 3200. It's the same stroke as 283 with a 4" bore. Power gets to the ground through an SM465 connected to 3.90 gears. With the current tires highway rpm is about 2700 rpm and it gets 15-17 highway mpg. I'm using a copy of the 350/327 cam with crossfire fuel injection.

Although a 283 can get good mileage, these trucks are aerodynamic like a brick. Increasing speed on the highway quickly increases load on the engine regardless of the gear ratio. It's really important with a short stroke engine to pay attention when selecting a cam. It's also important to make sure fuel is atomized well and that you do all you can to encourage complete combustion. To this end, intake heating combined with the highest compression you can run under a good set of heads makes a big difference.

When I built my 302 back in '92 I was trying to prove a theory that GM went the wrong way running long stroke engines like the 305 at lower highway rpm. I felt they should have built a short stroke 302 and increased RPM to get increased mileage and decreased emissions. I used 80's 305 heads with 58cc chambers and small valves and ports, the Crossfire manifold with long runners and small ports, and cast pistons which provide 9.2:1. I was rewarded with a truck that got slightly better mileage than the trucks I was working on at the Chevy dealership although it had a bunch more pep and looked much cooler.

Today I regularly drive a '99 K1500 Suburban which also has a 302. This truck gets near 20 mpg on the highway. This time around I used Vortec heads, the HT383 cam, and 6" rods. I've reduced quench to a minimum to get compression as high as possible with the flat top pistons but with the 64cc chambers compression ended up at about 8.9:1. This truck uses a 700R4 with a "medium stall" converter and 3.42 rear gears . With 245/75R16 tires RPM on the highway is in the 1550-1650 range.

HTH

[Foot Stomper]

Originally Posted by gregg pellicer

You are the first person to actually answer my question. I also posted this question on the HAMB.Everyone want's to talk about my tune .Thanks for your response.I have a 700 r4 that I am going to install first.I don't think that will get me all the way where I need to go but it's a start. I'm sure I will also end up changing the gear also.I have both a 350 and a 700 I went with the 350 because the rear was supposed to be a 3.08 .Somewhere along the way it got changed to a 3.73.

You're welcome. I think a lot of guys here who tried to help with their advise would say the same....Even though only one guy actually answered your question.

[Henry50]

I used to get 16 mpg with my 57 chevy car with the original 283 powerglide and 3.36 rear end.

[mr48chev]

Unless you intend to pull a trailer I'd swap the 3.70 rear for a 3.0 rear.

I never figured out the ratio in the first generation Camaro rear I had in my 48 when I first had the 283 and M21 Muncie in it but it cruised at 80 at 3000 rpm with N50 tires. At a more reasonable speed it pulled 16 to 18 with a 650 Holley that I had played with the jetting on pretty seriously. With what ever gears it had an the M21 it didn't like pulling a trailer very well though.

To me if you if your trans works great I'd just swap rear ends for one with a ratio that works better with the 283 and automatic for road driving. It will loose a bit of the stop light to stop light poke it has now but that might help gas mileage too.

I didn't get in on the question and answer session on the HAMB but how well it is tuned for road driving has a lot to do with gas mileage and if the tune is more for get up and go you might be loosing a bit of mileage there.