8 MPG?

MisterB said:

Are you talking about a gauge like this? This is the best I could get it adjusted a few weeks ago when I did it

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Do you have a more performance oriented cam? 15” is not great vacuum at idle. I pull a tad over 22” at idle.

My engine builder bro has stories about crate engines... one had the cam off a tooth on the timing chain. Even though they warranty the engines, if it's not running right and you take it apart and find a problem, it's not like they will pay you for your time.

Something I just thought about seeing the low vacuum at idle.... somethings up.

shiftpro said:

My engine builder bro has stories about crate engines... one had the cam off a tooth on the timing chain. Even though they warranty the engines, if it's not running right and you take it apart and find a problem, it's not like they will pay you for your time.

Something I just thought about seeing the low vacuum at idle.... somethings up.

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Lol. Or maybe timing just needs to be adjusted, or like the manual with the gauge says... If you're in a higher altitude climate (which I am), things can be a little off from what the gauge says is normal, and you can still be in the normal range

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Frankenchevy said:

Do you have a more performance oriented cam? 15” is not great vacuum at idle. I pull a tad over 22” at idle.

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I probably do, based off how it sounds and how everything else was built, but I don't know for sure. Crate motor was put in by the PO.

One thing I just thought of though. When I connected the gauge, I removed the fitting where it connects to the brake booster. Should I have done it closer to the carb?

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Arkansas_V8 said:

I get about 11mpg. V20 Suburban, 37's, 4.10 gears, TH400. That's a TBI, but with no 02, so runs a tad rich.

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That actually doesn't sound too bad with a heavy 20 burb, big tires and short gears.

MisterB said:

One thing I just thought of though. When I connected the gauge, I removed the fitting where it connects to the brake booster. Should I have done it closer to the carb?

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You want manifold vacuum.

gotyourgoat said:

That actually doesn't sound too bad with a heavy 20 burb, big tires and short gears.


You want manifold vacuum.

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Aside from the 02 she is kept in good running shape. Hitting the gas like I do often drops that drastically.

Keep my tires at the limit for good contact/wear. Keep my bearings greased, and fluids changed. This heavy girl will roll forever, that helps a ton.

Arkansas_V8 said:

Aside from the 02 she is kept in good running shape. Hitting the gas like I do often drops that drastically.

Keep my tires at the limit for good contact/wear. Keep my bearings greased, and fluids changed. This heavy girl will roll forever, that helps a ton.

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You probably don't want to hear it but if you had more than a yard of exhaust piping your mpg might increase a little bit.

gotyourgoat said:

You probably don't want to hear it but if you had more than a yard of exhaust piping your mpg might increase a little bit.

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I am willing to give up those extra MPG. Lol

shiftpro said:

A warmed up 350 with 4 barrel in a 2wd can get 20 mpg... I've done it.

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Tell us how? Please!

73c20jim said:

Tell us how? Please!

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9.5 actual compression, RV cam, deck the deck to level with TDC. Referring to a 2wd truck. Heavy Half... no pollution junk.

Long tube headers and dual muffs.
And also driving in the prairies with no mountains.
Don't forget to flip the air cleaner lid!

There was some basic porting on the heads too, unshrouding and open the bowls a bit. Polished exhaust ports and combustion chamber but that doesn't do anything for performance.

Nice truck.

I have a 1978 GMC 3/4 ton 4x4 32" BFG mud terrain, Pace 350 crate engine, headers, Elebrock intake and carb, no lock up TB350 transmission it use to get 8 MPG. The engine going down the highway was always running high rpms and would climb most roads without ever down shifting.

I replaced the transmission with a NV4500 and the transfer case with a NP205. The truck now will get 14-15 mpg. However it is geared too high to run in 5th gear and it does not have the proper gear spacing to really operate properly till I change axle gearing.

Getting rid of the full time 4x4 transfer case that always kept the front axle spinning must of helped on the MPG. Being I did not change anything on the engine going to a manual transmission with higher gears did greatly help the MPG. However when pulling a low boy full of stuff the MPG backs down to below 10 mpg.

The title on the truck says 3/4 ton but it is not a full floating axle.

I also run a vacuum gauge to keep track of the best fuel driving style. I hope this helps you.

MisterB said:

Are you talking about a gauge like this? This is the best I could get it adjusted a few weeks ago when I did it.

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Assuming your engine is stock, 15" is low. Should be closer to 20". Heck, even with a moderate performance cam, my 355 will make 18-19" at idle.

Have you checked for small vacuum leaks -- at both ends of vacuum hoses, EGR valve (if equipped), carb gasket, distributor canister, etc?

As for gas mileage, I haven't measured it on my 82 C10, but my old 69 C10 with a mild 350 would get 13-14 on a highway trip. It had a 3.08 axle and 29" tires, also 600 cfm Eddy 1406 carb, cast iron manifolds, and dual exhaust. I tuned that carb so it was right on the ragged edge of being too lean. Pretty easy to tune using the comprehensive owners manual and inexpensive jets, metering rods, and step-up springs.

Don't feel bad, I get 6 to 7 around town but that's all I expect with what I'm running : 468 ci,big solid lifter cam TH400, 4:56 gears, 31x16x15 tires.