Poor fuel mileage - 86 K10

[Orion88]

Well when I'm at WOT I don't care about mileage, cuz I only ever lay the hammer down during short bursts of road rage or for 10 seconds of fun. However I will let the engine idle for a few then recheck the plugs. Once again there would be no point in installing an A/F meter until I replace my rusty exhaust. I have a set of straight pipes and knock off flowmasters I took off my GTO that should fit just fine, I just have to find an affordable set of manifolds that flow better than my logs, then get some downpipes. I don't care for the hassle of headers personally.

[MikeB]

Originally Posted by Orion88

1405 - manual choke 600cfm.

I don't know if a properly jetted 1405 will ever get gas mileage as high as a 1406. Just look at the jetting charts.

And although the 3.08 is definitely way too tall for a 700R4 and 31" tires, I'm not sure a 3.73 would gain you 4 mpg like Tom saw with no OD. But you are definitely lugging the engine in OD. And 3.73s would make a huge difference in how well the truck performs.

Also, the unknown cam you have could be contributing to the problem. Increased valve overlap, which helps mid-high RPM power, will cause some of the incoming fuel to blow out the exhaust port at lower RPMs. Also, even 10 degrees more intake duration than stock will close the intake valve later, reducing cylinder pressure, efficiency, and torque at low-mid RPM. What is your manifold vacuum at idle?

[Orion88]

So would I be better off to keep an eye out for 3.42's instead of the 3.73's?

Ill pick up a vacuum gauge this weekend and see where my manifold vacuum is at.

[Liberty]

I get about 8mpg on the stepside with a 350 Crate, TH350 tranny, and Rochester Carb...
I WISH I could get 10.

[Orion88]

You can wish in one hand and crap in the other. Guess which one fills up first.

If you're not happy with your mileage try to find the issue like I am. Plenty of people are willing to help and worst case scenario nothing changes. Best case you end up with a better running engine and less money wasted on fuel.

[Liberty]

In my case everything is as it should be, just a classic GM truck, MPG was simply not on GM's mind when designing these pickups. Only thing left I could ever do is fix the locking torque converter on the TH350C. My point was low MPG is pretty common on these lol!

[Orion88]

I agree, mpg was not a concern in the mid-80s. But 8-10 seems too low to me, especially when there are people on this forum averaging 13-16. It just doesn't make sense to me and until I've tried everything to improve the situation I won't be satisfied.

[Liberty]

I've always figured my low mpg was caused by a combination of an aging engine, ethanol gasoline, 3 speed tranny, and an occasional heavy foot. Heaven forbid I turn the A/C on!

How the heck folks get 16 mpg in these trucks is a mystery to me. I guess if you granny it, or have fuel injection.

[MikeB]

I haven't driven my 82 C10 enough with its new 350 engine to know the exact fuel mileage, but my guess is 10-12 around the suburbs, hopefully 15-16 highway. And it's a light 2WD short bed with 2.73 axle. Are my goals too low?

I can say for certain that my 69 C10 LWB got approx 14 on the highway with a 350, 194/204 cam, 1406 carb, dual exhausts, TH-350, 3.08 axle, and 29" tires.

Wanna hear something funny? My 82 C10 with its original 250 six and 3 speed manual trans with 2.73 axle was EPA-rated at 24 highway! I don't know what the ratings were for the 305 and 350. As I recall, they came standard with 2.56 axles and TH350C lock-up transmissions back then. Talk about some low highway revs!

[rich weyand]

Originally Posted by Orion88

So would I be better off to keep an eye out for 3.42's instead of the 3.73's?

With a 700R4 and 31s, I would be looking for 3.73:1 or even better 4.11:1.

['76 Big Block]

My personal experiences with the two I've owned ('81 GMC step side with 355/350,Holley, headers, etc., and 3:08 gears, and now my '76, with 454/400, headers, and 3:73 gears) is that they seem to be "stuck" in the 10-12 mpg range around town, and occasionally will get 14-15 on the highway.
I say occasionally, because my experience has been that some people like to mess with an old truck when they see one, and want to show you up.
It's fun to "occasionally" show them that old doesn't necessarily mean slow.
Some guy at a red light yesterday wanted to show me how good his 5.7 Toyota tundra ran, so I had to pass him before we got to the next light (4 lanes----I'd never pass into oncoming traffic, especially in town)....

[Orion88]

The guy with the axles popped the cover off, and it has 3.42 gears, not 3.73. But for $250 for both axles with warn manual locking hubs I'd say it's a good deal. Hopefully the truck will run better with a lower ratio, even though I couldn't find 3.73s

[rich weyand]

Originally Posted by Orion88

The guy with the axles popped the cover off, and it has 3.42 gears, not 3.73. But for $250 for both axles with warn manual locking hubs I'd say it's a good deal. Hopefully the truck will run better with a lower ratio, even though I couldn't find 3.73s

That's a really good deal. Give him the money before he changes his mind.