Gas milage: 6cyl vs. 8cyl

[cruiserman]

My 68 C10 with 292 got 12MPG. My C2500 with 402 gets 10.

[Huck]

Both are basically air pumps. They take air in, mix with fuel, add a little spark and pump it out. There are dozens of factors affecting fuel usage on trucks. Rolling resistance (big tires = resistance to roll). Wind resistance (big mirrors=wind resistance). Engine size (big=more consumption). Transmission efficiency (more gears correctly used =better use of power). Rear end ratio (best fitted to usage ie 411-1 good for stump pulling or heavy trailer towing while 308 are fine for highway with 342 or 373 a happy compromise). Tune up efficiency (dirty carb can cost you 3-4mpg easy!!)--Old plugs, wires, points ignition===all tend to rob mpg.
And probably the easiest fix is to drop a big hammer on your right foot as driving habits are a major influence.
My 72 ---I installed a new crate 350, 5speed trans, 308 posi gears, HEI ignition, Edelbrock 1406 carb, slightly oversize tires---and could get 20mpg flat highway. Installed an edelbrock intake and actually lost 1-2 mpg--went back to stock small/large intake and regained my mpg. Went to hooker headers and lost 1-2mpg--went back to ram horns and regained.
Lesson learned is that not all modifications are an upgrade to performance. Expert engine rebuilders will confirm that heads and cams do more for engine performance (power and economy) then external crap but I'm cheap so I tend to do the simple and then measure the crap out of that. I have personally driven a 72 Blazer with 350, auto and tons of miles and got 16mpg which is not typical so tune em, tweek em, upgrade em, and keep your foot out of them unless necessary and you will improve your mpg. Huck

[UltraBlue71]

A 20 mpg 350 chevrolet!! especially with a AFB!!WOW that sounds kinda like a unicorn to me.... as in I've heard of em but never witnessed one personally! I do agree that a 1406 will do better on gas than a crappy Q-Jet, but a good Q-jet, which has smaller primaries to begin with, should do better on gas IMO, than a 1406 out of the box, and anything will do better than a Holley, short of having a short man sit on your inner fender pouring gas down your intake!

[leddzepp]

One word: negligible.

[vegaschevy]

If your asking the difference between the two you should probably get a different car

[fishingjred73]

68 c-10 250 6cyl 3spd 3.73 rear gears. The best it ever got was 13 mostly 10-11

[UltraBlue71]

AGREED go buy a S-10 with a 2.5 to drive during the week and fart around in your 67-72 truck if your worried about MPG's! I just wish mine to have AC (to make my MPG's even worse) man its getting hot around here!

[PMchose]

72 C-10 SWB Stepside - 454HO/Holley 750 - 8-10MPG or less if I'm in it

[berencam]

13-14 with 350, 3speed and factory od.

[gontek]

I get 10 on the highway. Once it was 5 mpg after getting caught in construction zones a few times. Averaging about 9 mpg now. Turning off the motor at red lights added me about 0.2 mpg. Getting a quiter exahust system would probably save me 0.5 mpg too.

but I love my BBBBRaaaaaaaaaaaggggggggghhhhhhhhh!



1970 350/400 3.73

[americanmusc1e]

Originally Posted by tim21391

402 big block/auto.. i get about 4.. that is because im 18 years old and i have a heavy right foot..

ME TOO

I got around 9 with my 350/350/3.08 and my foot in it

best with the 350/350/3.08 was 12.5

but it I put in a 700r4 in it and the motor blew (I think it had something to do with my lead foot) and I'm waiting on a 383 to see how the mileage tests go,

[Average Joe]

Before I had a 454 and TH400 in my 69 c10 LWB I had a 250 6 popper that I rebuilt with an Isky supercam. Best mileage I ever got was 16.5 mpg going 60-65 and hangin with the semi's on I-5 between Bakersfield and Redding CA. This was with 3 speed on the column and 3.73's. I never re checked that combo but I would guess 12-13 on a typical 2 lane mountain hwy. The 454 that is in it now averages 9.5 hwy with TH400 and same 3.73's. In town seams to be around 7. My dad owns a 70 c10 with a stock 307, 3speed manual, 3.73's and claims 15mpg. He is also 70 years old.

[Jim_PA]

Originally Posted by 67shortie

I can't get better than 15 on the freeway, which I think is pretty decent considering. Now, add wind,

Gotta ditch the holley.

[JameslovesJammie]

'65 GMC
283 2 barrel
3.07's
SM420
235 series tires

Best was 15 MPG combined city/highway
Average around 12.

Keep the RPM's down and don't romp the throttle. Ease into it and it can be done. Living in a rural area really helps. I can't imagine it would be nearly as easy in cities.

[highperf4x4]

'72 402 Big Block, TH400 9mpg
'90 4x4 built 350, 5spd 14mpg
'90 Ford Van l6cyl, C6 15mpg
'04 Grand Prix GT v6, OD 24mpg

[67shortie]

Originally Posted by Jim_PA

Gotta ditch the holley.

In my case, it did not matter if I had this Holley or the Q-jet it was about the same.

I have tried different jetting/rods, power pistons etc and could not squeeze better than 15 or so on the freeway. I have accepted the fact that these trucks are rolling bricks and will not get better mileage with a carb'ed engine

[67chevy3/4tontruck]

you should be ok with the right rear gear is a big help too

[basemodel67]

How about you 5.3 and 6.0 ls swappers? What kind of mileage are you getting? My 250/PG/3.73 Short Fleet got 12- but it was struggling to go 65 downhill, so I sold it. Wanting a serious 6.0 to replace it soon with an overdrive auto.

[rscam]

I have a 355 with edelbrock 600 and NV3500 with 3.08 gears. I got 12.25 the first tank and 12.95 mpg after I rebulit the dist. Driving about 50/50 city/hwy. I am going to work the carb some more and see if I can get into the 13's. I babied it too.

[arm51]

since were on topic i was wondering how good a milage that a 400 sb will get with an edelbrock 4 barrel q jet

[johny1i]

Since my fuel gauge is erratic, I top off the tank every time I hit the pump and keep up with my miles to make sure I don't run out. That being said, I consistently run 11 mpg--to a tee. GM crate 350, edelbrock carb, TH350 trans, 4.10 gears, 31" tires.

[Jacfourteen]

my 68 3/4 ton gmc had a 305 v6, then a 351 v6, with 4.10's and a four speed w/granny. It always got right on 12 mpg no matter on the hiway or in the city. waiting to see what it gets with the 355 sbc w/holley 670, edelbrock intake, 268 comp cam, headers, 10:1 comp, etc.

[Elephanthead]

They both get bad mileage. I would get better mileage if I drilled a hole in the gas tank and rolled the thing downhill. 292.