350 /454 mpg differences

[swb85]

Originally Posted by Baresteel10

Sorry for the hijack but, what octane are you guys running on with your higher compression motors?

10.4:1 static
8.3:1 dynamic
.039" quench pad
34* timing
runs on 91 octane all day long

[MylilBowTie]

My 86s 305 gets about 10 mpg.

[Kixwy2]

Do you guys think higher octane gas or a booster makes a big difference in mpg? I have a GM crate 350 (270 torque, 8.5:1 compression) with an HEI and a rebuilt q-jet, and I'm getting about 18mpg highway running 87 octane.

[MylilBowTie]

I doubt it would make any difference. Higher octane fuel burns slower and helps combat detonation.

[Captkaos]

anything above 87 octane on a crate motor is throwing money away. If it doesn't ping when you are driving it with 87, you will gain nothing from going to a higher less combustible gas.

[meatwagon83]

ok i know this is an ancient thread but i have to say ive seen 454s get 15mpg in mixed driving and ive seen 350s get 6mpg before, it just depends on the condition of the engine, engine combination (engine parts trans gears tire size etc) and the most important driver habits!!

[nheller76]

Yea I just built a 355/400 combo and my mpg is about 9-10 city. Don't know hwy yet.
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[kjc292]

well it being a 454 wouldnt it get worse gas mileage but tow the s**t out of a 350 since they both stock.... now if you have a stock 454 and who cares how much it costs 350, throw two carbs on it 350... well might be a different. story also how are you driving it..? now my father has a 77 single cab one ton dually with a 454 in it and accelerates as fast the truck will go without pressing on the gas(he had a 55 hard top with an ls 454 in the 70's built at baxters in delta in park portland, oregon, he had his fill of speed) and still complains about gas mileage. but on a side note, i asked if they would lump port a 292 head and they said no, its to risky, come on now. but anyways. it just depends.
but personally engines are built for what they're charging for gas at the time....

[meatwagon83]

Originally Posted by kjc292

well it being a 454 wouldnt it get worse gas mileage but tow the s**t out of a 350 since they both stock.... now if you have a stock 454 and who cares how much it costs 350, throw two carbs on it 350... well might be a different. story also how are you driving it..? now my father has a 77 single cab one ton dually with a 454 in it and accelerates as fast the truck will go without pressing on the gas(he had a 55 hard top with an ls 454 in the 70's built at baxters in delta in park portland, oregon, he had his fill of speed) and still complains about gas mileage. but on a side note, i asked if they would lump port a 292 head and they said no, its to risky, come on now. but anyways. it just depends.
but personally engines are built for what they're charging for gas at the time....

not necessarily, my uncle had a 87 k10 stone stock 350 sm465 and 513 gears, 6 mpg stone stock truck

[nheller76]

I think it really just boils down to one thing. 454 to 305 there is not much difference in mpg until someone gets behind the wheel. Personally I don't drive a 3/4 ton steel monster with a muscled up 355 for the mpg. But going through down town windows down at a stop light and a little kid points at my truck and starts talking about it to his dad and I can make him smile big when I romp on it. That is why I drive a square!
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[TwoFiftyShifter]

My 87 350 TBI got 12mpg in town, and 14 hwy at 60mph. 12mpg at 70mph.

That was a 3/4 ton 4 speed 4x4, 3.42 gears.

[meatwagon83]

Originally Posted by nheller76

I think it really just boils down to one thing. 454 to 305 there is not much difference in mpg until someone gets behind the wheel. Personally I don't drive a 3/4 ton steel monster with a muscled up 355 for the mpg. But going through down town windows down at a stop light and a little kid points at my truck and starts talking about it to his dad and I can make him smile big when I romp on it. That is why I drive a square!
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this is very true!

[Tx Firefighter]

I've had two duallys, both with TBI 454s. The one with overdrive got 10. The one I have now, without overdrive has never gotten over 9 no matter what, including 40 mile per hour back roads out on the country. I keep my stuff in full perfect tune, rebuilt throttle bodies and injectors, fresh ignition components, proper timing adjustment, and tires at 75 psi. That's as good as it gets for my stuff. And, I do not ever, ever drive hard. I'm very gentle on my trucks.
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[Pano]

Last I checked my 87 V2500 with a 350 TBI , TH400 trans and 4.10 gears was getting 11 mpg.
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[LynnJr]

My 1990 TBI 350 4x4 on a 700 mile to Ojai and back averaged 14 mpg.
My 1974 Carb 350 4x4 on the same run averaged 13 mpg and we got there and back an hour and a half sooner.
My dads 1972 1/2 ton 2 wheel drive 454 is getting 10 mpg.

[suxtobu65]

My 88 cc dually with a 496/400 averages 12 mpg city.. Made a 600 mile trip south at 65mph and averaged 15.5mpg towing an empty 16ft flatbed.

My 65c10 with 305/400 gets 12 mpg. But that has alot to do with the th400 trans. I was getting 15 with the th350 trans.

[tucsonjwt]

The key question is 1/2 ton or 3/4 ton. I have had 3 squares all 3/4 ton. They all got 9-11 city, 13-14 highway. The best of the lot was a 73 454/TH400 which would occasionally get 12 city, and was very fast. 1/2 tons might get better gas mileage with careful driving.

[Jensen Handcrafted]

I realize this thread is quite dated, but perhaps I can resurrect some conversation here. I am a recent convert, now Chevy owner. I have a 1966 Carryall/Suburban. The one owner prior to me, made an engine swap & trans swap to a 454/TH400. It is a bone stock '73 454 w/ Q-Jet setup. It is an absolute hog when it comes to fuel consumption, averaging only 6mpg across (8) tanks, with very little highway use. Looking for input on mods to improve this situation from quick and easy to time consuming and expensive and everything in between.

Thanks in advance for any and all input.

[Palf70Step]

Proably 2 things to check,

1. what rear gears do you have (I am guessing 3:73 or lower)
2. The other key thing is how accurate is the speedo/odmeter. If the changed trannys and didn't change speedometer gears, than you are off on your readings.

BY far the best way & cheapest to me would be to change your rear gears to a 3.08 or 2.76 if she is just a cruiser. Next would be an OD transmissions. With both your probably looking at 1000-1500 if you have someone else do the work.

If you have a 3:73 or 4:11 rear, that is about normal MPG for a 454. You can tweak/change carb and intake manifold, but if the gears are that low 6-8 is about normal.

[Jensen Handcrafted]

Thanks for the quick input, Bill. Looking forward to getting into this project and improved mileage is my first order of business. Cheers!

[tucsonjwt]

My best ever 83 C20 454/SM 465 was 11 city, 13 hwy. I had a 76 C20 with 350/350 TH which never got more than 9 mpg, city or hwy. I had a 73 C20 with 454/TH400 which got 11-12 city all the time. I used that 73 C20 to tow a storage trailer (about 6500 lbs.) from Michigan to AZ and got 6 on the hwy, but I once got 17 on that trip when I was traveling down hill for most of the tankful(I think that heavy trailer was giving me a free ride downhill.)

A stock 350 vs. stock 454 is maybe a 2 mpg difference in a 1/2 ton. Once you compare 3/4 ton pickups the difference disappears because you are pulling and extra 1000 lbs. in vehicle weight. I think you could have a 6 cylinder in a 3/4 ton and it would make no mpg difference.

[Dead Parrot]

Jensen - What is your plan for the 66? Gentle cruiser, track truck, tow rig? Neither the 454 or the TH400 are considered fuel economy items. If you are after a gentle cruiser + fuel economy, might be better to find someone building a tow rig and sell them the 454/TH400 setup and install a 350 + a 700r4 or similar overdrive transmission.

Did the PO do a rear end swap to match the engine and transmission? If not, current back end might be light for the current setup. Your 66 could have came from the factory with an inline 6 mated to a 2 speed powerglide.

[Jensen Handcrafted]

Originally Posted by Dead Parrot

Jensen - What is your plan for the 66? Gentle cruiser, track truck, tow rig? Neither the 454 or the TH400 are considered fuel economy items. If you are after a gentle cruiser + fuel economy, might be better to find someone building a tow rig and sell them the 454/TH400 setup and install a 350 + a 700r4 or similar overdrive transmission.

Did the PO do a rear end swap to match the engine and transmission? If not, current back end might be light for the current setup. Your 66 could have came from the factory with an inline 6 mated to a 2 speed powerglide.

Correct on the factory set up of the inline 6 and 2sp. I am looking to go the gentle cruiser route, but with a little punch to go to on an as needed basis

[meatwagon83]

Originally Posted by Pyrotechnic

I think mileage has to do with how efficient a motor is running for that particular application. I think a 454 that has been worked over with raised compression, improved cylinder head flow, a larger but not overly radical cam, and a free flowing exhaust could actually get the same or better mileage than a low compression smog 350.

Mine has a quadrajet on a performer intake, a comp he260 cam and a good bump in compression with headers and flowmasters, 13 mpg on the highway, th400 3.73 gears and 235/85-16 tires