Re: Fuel efficiency...what MPG do you get?
 

84' C10
TBI 305 swap
TH350 w/ 2200 stall (really about 2400 in the truck)
3.08 gears

I drive it like I stole it. Last tank was about 9.6 MPG. Used to get about 11-12 when I had the 700R4.

Re: Fuel efficiency...what MPG do you get?
 

74 k5 in sig when it was an auto would average 15 highway and about 11-12 city trans didnt like driving slow niether did I :lol: so it stayed out of town for the most part all while pushing 33x12.5s on a 2.5 inch lift. Havent driven it enough as a stick to know if it got better or worse:devil:

Re: Fuel efficiency...what MPG do you get?
 

I have almost the same setup. 383 stroker, 3.73 gears 700R4 tranny (we just put in the lock up converter, it was without the lock up feature until now), 28" tires, dual exhaust, 650 CFM carb. Prior to adding the Lock up converter, I averaged 15 MPG on long trips.

I agree, with the idea of a LS series engine and a T56 tranny. Good horsepower and a lot of room to adjust.

Re: Fuel efficiency...what MPG do you get?
 

No speedo 83 6cyl 465 4 speed tall gears guessing good but man is it slow.
Guys lie about gas milage worse than they lie about you name it?i had an 02 gmc HD with 8.1 gas allison on interstate cruz at 65mph never got 12 had a 99 with the 5.3 got 16 and thats the best milage i ever got in a pickup.

Re: Fuel efficiency...what MPG do you get?
 

84 c10
350 700r4
about 11 mph in town
on a good day.

Re: Fuel efficiency...what MPG do you get?
 

The 84 I had a 305,350 trans,2:73 gears got 10-12 on the highway. I could watch the gas hand drop. SERIOUSLY. I think it was supposed to have a lockup converter on it but don't know for sure. But I don't have to worry about it any more the exwife got it and the 72 in my avatar in the divorce. Oh well it happens,,,,,,,

Re: Fuel efficiency...what MPG do you get?
 

81 c10 lwb old 305 new th350 no idea on gears and 75% of driving on highway at 55-65 mph

averaging about 12. i know its running rich and needs a tune up but since it fires up at 23 degrees with no choke im not touching it right now.

Re: Fuel efficiency...what MPG do you get?
 

'85 SWB C1500, 355 sbc, 3.73 axle ratio, TH700R4, 28" tires, Quadrajet


14.5 MPG, 25 miles round trip to and from work.

Re: Fuel efficiency...what MPG do you get?
 

i think there are alot of facters that need to be made in this. like driving habbits tire pressure timing and carb tune. i mean my 78 monte got 12 with a all stock 305 after some tuning and tune up and and turning the timing up some(6 to 12) it jumped to 18 on the highway. lot better then the 12. with the 350 and better pipes and no cat and tuning it leaner i can get 24 with ac on. but i like power and to go faster down the road so i get 20 most the time. also its all about rpm's and power. too high lots of fuel to low lots of fuel. need the sweet spot

Re: Fuel efficiency...what MPG do you get?
 

I agree but I try to keep tires at correct pressure, I ,ve tried timing all over and ran speed limit or A little over by 5mph. Still around 10-12 mpg. No lead foot when figured. The 72 was the same way but I expected it with it.

Re: Fuel efficiency...what MPG do you get?
 

on my old 86 c30 crew cab dually (truck in my profile picture) had a 454/turbo400/3.73 gears with the quadrajet and a recent tuneup the best I saw was 12mpg, but the truck usually got around 7-8mpg. :lol:

Truck went through lots of gas but it was worth it! :chevy:

Re: Fuel efficiency...what MPG do you get?
 

City driving with mine does suck, but on the highway, I have spent enough time in getting the RPMs to a point where they are just about right. I still can put down 15 MPG and that is at 70 to 75 MPH.

My son's 86 does not do as well as mine and he has a 350 with a 700 R4 and the same rear end gears. It is all in the mix of each vehicle and how they are built.

Re: Fuel efficiency...what MPG do you get?
 

'74 GMC 1/2 ton LWB, 2WD, 250-6, 3OTT, 3.73 rear end, Mighty Monojet (1bbl) with a recent rebuild, HEI, fully desmogged.

Highway, I get 17 if I keep it at 55. Have done as well as 20, but I was taking it super-easy on some relatively flat back-roads between Portland and Eugene.

In town, I get about 15.

I have no plans to change anything in my drivetrain. Would like to move up to a small 4bbl, or more likely, a 2bbl on a Clifford intake with a set of matching headers. Might be a small improvement in MPG, but mainly it'd just be more fun to cruise around in.

I love a column-shifted manual, and prefer back-roads to the interstate. I live in Western Oregon, so I coast down all hills, and we have a lot of them. And I drive a semi for a living, so I'm used to taking my sweet time going uphill, which I do in my pickup too (within reason).

If I have some long miles to go in a hurry, I'll take the family Corolla.

Or more likely, I'll just leave a lot earlier and drive my truck. An empty road, a deep mug of hot coffee, dawn breaking over the mountains, and a happy inline-six warbling under the hood... Man, that's the only way to fly!

Re: Fuel efficiency...what MPG do you get?
 

1990 Suburban 3/4 ton 4x4
25 mpg highway
Cummins 6BT diesel
Unfortunately, diesel is $1 more per gallon so there is little savings over a gas vehicle.

Re: Fuel efficiency...what MPG do you get?
 

The other thing that happens is you dont have to fudge the# much to realy improve milage. Another guy and i got work trucks same time his got way better milage. No rounding up or down. 200 miles 10 gals 20mpg actual 190 miles 10.8 gals 17.5 mpg

Re: Fuel efficiency...what MPG do you get?
 

81 3/4 ton 250 3OTT 3.42 16.5 8 ply snows, 17mpg at 45mph, 15 at 55.

Re: Fuel efficiency...what MPG do you get?
 

'87 GMC SWB R10 4.3 tbi, 700R4 don't know the gears. I'm getting 18 mpg on the highway. It goes from zero to 60 in about 3 1/2 years!

Re: Fuel efficiency...what MPG do you get?
 

lousy....
5 mpg's empty, around 3 with the camper & trailer on it...
'85 K30 crew
454/T400/205, 4:56's 39" swamps

Re: Fuel efficiency...what MPG do you get?
 

Wow, that's great! My 69 C10 had a fairly mild 383 (maybe 350 hp) with a 600 CFM carb, 3.08 axle, and 29" tires. Best it ever got on the highway was 12-13 mpg. Probably got more like 10 around town. Even with a very mild 350 and a Holley Economaster 450 carb, the truck got 14-15 hwy. Both engines had 2-1/4" dual exhausts and were in excellent tune.

No numbers on my 82 C10 with 250 six, but will post some when I get the 355 installed. Should be pretty good with a mild GM roller cam, Vortec heads, and a 2.73 axle. Maybe 16-17 highway?

I think the biggest problems with these trucks is wind resistance. And you can't tune that out or make up for it with taller gearing.

Re: Fuel efficiency...what MPG do you get?
 

No doubt! Since becoming a truck driver in the early 'nineties, I've tracked my fuel economy on every vehicle I've ever owned. It's a great way to keep track of how well your vehicle is running, and in a semi truck, it's the difference between revenue-miles and repair bills.

I had a guy at work claiming his 14L Detroit was making 7 mpg over 900 miles. I pulled his fuel receipts and logs, and showed him how to do math with decimal points. Turns out he made 5.2 mpg...

7mpg in a 80,000 lb truck would be unheard of. If the numbers seem outlandish, they probably are.

Re: Fuel efficiency...what MPG do you get?
 

The 86 K10 I had, had a 305, 4 speed and 3.08's. Ran 32's on it and would regularly get 15.5 MPG.

Re: Fuel efficiency...what MPG do you get?
 

1990 1 Ton Crew Cab 4x4 Cummins 4" lift 34" tires with a heavy foot 22mpg

Re: Fuel efficiency...what MPG do you get?
 

1978 1/2 ton with 5.7 diesel, T400, Gear Vendors overdrive, and 3:73 rear end yields me around 20 MPG on the highway and around 17 MPG in the city.

Re: Fuel efficiency...what MPG do you get?
 

Ok, I'll add mine. '73 included as I still owned it when this thread started.

--'73 K-20 Vortec headed 350, Summit 1103 cam, dual exhaust and headers. Stock size tires, no lift, 4.10 gears, turbo 350, np 205, plow gear installed, no dump, dump bed installed.
13mpg

--'86 C-30 Stock 454, stock size tires, 4.10 gears,turbo 400, headers into factory duals.
11 mpg.

Re: Fuel efficiency...what MPG do you get?
 

The tranny and rear end gear combinations is the whole trick to this. My Son's truck had 3.08 gears and when we put a 700 R4 in front, the mileage disappeared. The engine had to work too hard to keep up with the hills and traffic. We put a set of 3.73 gears behind the OD tranny and his mileage returned to a 14 to 15 MPG average.

Re: Fuel efficiency...what MPG do you get?
 

Agreed. Its a simple (ha!) formula that considers engine displacement, terrain, final-drive, rear-end gears, tire-size, fuel-type, load, ambient temps, driving style, overall tuning, recent arguments with one's significant other, prevalence of Damn Good Music/Attractive chicks with long legs/New Boot on the Right Foot...:metal:

You can wring some damn good numbers out of damn near anything, Squares included.

But, life is meant for living.

Do it your way. I sure do!

Re: Fuel efficiency...what MPG do you get?
 

mine got about 8 when i got it with the stock engine, i just rebuilt the engine with a small cam and a decent bump in compression, im hopeing for 11 or 12, the truck is a c30 crewcab,(like you dont already know!) with a 454/th400/3;73s,it has too short of tires on it right now, i am running 225/70/16, i believe they are 29" tall, i would like something 31" tall

Re: Fuel efficiency...what MPG do you get?
 

I've tracked my mileage very carefully over the last 65,000 miles and have averaged 12.01 MPG. About 65% highway driving, not necessarily expressway speed. I usually drive 65 on the x-way. Occasionally tow a boat or haul a 2000 lb camper which drops it down to 10 MPG but still figures in the overall average.

1985 GMC HD 3/4 4WD, stock 350, 790 cfm edelbrock, no overdrive, 3:73 gears, manual trans, dual exhaust, no headers.

Re: Fuel efficiency...what MPG do you get?
 

790cfm carb on a stock 350? Why so large?

Re: Fuel efficiency...what MPG do you get?
 

Sorry for the late response.

Had to replace the original Q-jet shortly after installing a new GM crate motor about 12 years ago. This was the recommended carb for that app.

GM refers to this as an engine for commercial use (towing etc.). It has most of the torque at a fairly low rpm, seems to max out under 4000 rpm. Certainly doesn't bog so it's not overcarbed. Can't remember the original carb CFM but I believe it was similar to the Edelbrock.

Re: Fuel efficiency...what MPG do you get?
 

225/70-16 = 28.5" tall.

Re: Fuel efficiency...what MPG do you get?
 

Shortbed lowered 2/4, 350 with Comp 280H cam, Edelbrock intake & 600 carb, headers-n-Flowmasters(2.5"), TH350, 3.43 gears, 29x12.50 tire= 13.3mpg mostly cruising around between 45-60mph..

Re: Fuel efficiency...what MPG do you get?
 

I believe that your gear ratio and the 350 tranny is killing you. Your tires should be around 29" tall and when cruising at 60 MPH, you are at approximately 2300 to 2400 RPMs. That is not bad for the engine, but apparently you are missing the power curve somewhere. You may not be getting into the cam profile effectively.

Re: Fuel efficiency...what MPG do you get?
 

LOVE IT, sounds about right !!!

Re: Fuel efficiency...what MPG do you get?
 

82 Long Bed 2wd
350 crate motor
Edlebrock 600 carb and Edelbrock manifold
Headman headers, Thrush Turbo mufflers
33x12.5 15 tires (old and brittle Mickey Thompson ATs)
SM465 Trans
gearing unknown (but probably stock as speedo says 45 when you are doing 65)
Levelling/Lift kit (1 inch rear 3 inch front)

Recently, I got around 10 mpg on a mostly highway miles tank (using MapQuest to determine actual miles covered), and driving 65 mph

Switching to 235/75 15 Goodyear Wranglers this week as these weigh 33% less than existing tires and were a stock size for this truck. Thinking about removing the lift kit, as well, if it does not involve sourcing too many expensive suspension components.

I would like to get 15 mpg on long highway drives down to North Carolina and up to New Hampshire for motorcycle rallies. Am I dreaming?

This truck was a badly neglected old girl that I would like to bring back to good health and mostly stock condition and eventually pass along to my grandson. I have replaced the plugs, fluids, relocated a fuel line that was firmly hugging one of the right side header pipes (yikes!) and replaced the blown out glass pack mufflers.

I drive like a grandpa. Even on the bikes. (Well, most of the time, anyway)

Re: Fuel efficiency...what MPG do you get?
 

I borrowed my dad's 2014 C10 (do they still call them c10's?) 2wd, small v8, for a highway trip and got 17 mpg @70 mph with cruise on and tach about 1600 rpm's. Any square getting close to that is getting phenomenal mileage imo. My 76 GMC K15 with 350/th350 308's can knock down 11 mpg all day long. 65 mph is around 2500 rpm's on the K15.

Re: Fuel efficiency...what MPG do you get?
 

My rig: 1977 C/10 Step side, 454, TH400, 3.73 posi, P275/60R15 rears (28 inches) I get around 8 mpg in town and about 12 mpg freeway.

Re: Fuel efficiency...what MPG do you get?
 

My 1984 K5 completely stock with 31 inch BFG AT's and CCC and CA emissions gets around 14 MPG highway. My 1986 K10 SWB on 35's and with 4.10's gets around 10 MPG. Both 350's and 700r4's.

Re: Fuel efficiency...what MPG do you get?
 

I am on my first full tank since getting this short bed diesel (6.2) on the road. 700r4 with 3.08 gears. I will report my results once I fill up next.

Re: Fuel efficiency...what MPG do you get?
 

I'm lucky if I get 10 mpg with my 82 Stepside.
350 Crate with Carb
TH 350 Trans
unknown rear end, no emissions equipment.

The cat was just removed and carb just replaced so time will tell if my mpg goes up.