67-72 3/4 ton with good gas mileage

[64fleetside]

The engine is simply too big & heavy & the truck(also big & heavy) has the aerodynamics of a brick, 20 mpg can't happen. I had a throttle body 91 454 3500, got 13 w/5 spd OD. Brian

[67ChevyRedneck]

Originally Posted by 67_C-30

Not that it matters or makes a difference, but your 2000 Z71 engine is a 325 cu in. (3.780" x 3.62)

But I agree that 20MPG with a BBC is very optimistic is a truck, even with EFI and OD. They get about 1/2 that in stock form even in newer model trucks.

I've been confused on that for quite some time. Some people say it's a 327 and some say its a 325. I recently went to chevrolets site and they have a 5300V8 listed as 327cu/5360cc and a 5300 H.O. V8 listed as 325cu/5360cc. I'm pretty sure my truck isn;t an "H.O." version so it's a 327??? I know these are listed for new trucks, but they started using the engine in 99...so....

Here's the site GM truck engine specs then click on the "engine" tab.

[mrein3]

I had a '71 Monte Carlo with a bone stock 350, 2bbl carb, and 273 rear end. That car consistantly got 20mpg and once even got 23. However that car is 1500 or so pounds lighter than my 4x4.

A member here who no longer posts claimed he got 20-23 in a sub with a small block but I refuse to believe it until I see he speedo corrected against a GPS or something to verify that the miles he is computing with are really miles.

I get an average of 9.5mpg in my 4x4. It had a 350 and 3-speed manual. Then the averge was 9.9 - 10.0. Best was 13.6.
After the swap to an auto (th350) I was getting an average of 9.5.
I haven't got enough data to say what the 383 is getting but I'm sure it is worse.

All things being equal a 700r4 will get you 30% on the highway. So if you drive the highway most of the time and get 10, you'll get 13 with the 700r4. Swapping your 373s to 307s will get you 20%. So your 10 will increase to 12. Again this is all other things being equal, which they NEVER are.

What it comes down to is what do these increases cost you and how does that compare to the cost of gas. If it costs you $1000 to swap in a 700r4 and you'll only save $100 per year on gas, the payback is 10 years down the road. What are the chances you'll be in the same vehicle 10 years down the road?

If it is a collector truck you may have it that long but then you have a 2nd car which probably means your pay back is even farther down the road because you don't drive it that much.

Modern "big blocks" with all the computer controls, overdrive, and engineers working EVERY day to get the best mpg, don't get 20. What makes you think you'll get 20 under the shade tree in your back yard?

If you want 20 mpg get a 1/2 ton, 2wd. Swap in a 350 with injection and a computer controlling it. Swap in a 700r4. Find a 273 rear axel out of a newer truck. Don't plan on using your truck as a truck. Only drive with the wind. These things MAY consistantly get you over 20. But don't count on it.

Or leave your big block alone. Buy a Chevy Cadaver or Ford Escort or Dodge Neon to drive every day and use your truck when you need it or want to cruise.

[mitchfab]

Originally Posted by Palf70Step

There is probably a way to get close to 20 out of a BB, but how much cash are you willing to spend. You are talking tranny, rear end changes, injection and exhaust...etc....So for 5 or 6 thousand...probably more your likely to save 20-30 dollars a week in gas (depending on how much you drive maybe a lot less). If you want to sink money into something for gas mileage, get another vehicle. To me the price differnce isn't worth it. I like my truck and for me to cget another vehicle to insure, licenses and maintain...plus the cost of the initial vehicle (or changes to mine) is not worth what benefits I will get from it. So I just pay the extra $30 a week in gas. Just my $.02.

Well I drive 50 miles to work and 50 back each day 6 days a week. Its alot a fun to drive a Old Chevy as a daily driver . Thats what I got in mind.We get some nice wrecks in at the ins.salvage pool here in Dallas .I would love to put a nice late model big block setup in this 3/4 ton 71 Im workin on.Hec I heard
gas was going up to 3 bucks by the end of 05. I hope its just tall boot Texas talk.

[mitchfab]

Originally Posted by mrein3

I had a '71 Monte Carlo with a bone stock 350, 2bbl carb, and 273 rear end. That car consistantly got 20mpg and once even got 23. However that car is 1500 or so pounds lighter than my 4x4.

A member here who no longer posts claimed he got 20-23 in a sub with a small block but I refuse to believe it until I see he speedo corrected against a GPS or something to verify that the miles he is computing with are really miles.

I get an average of 9.5mpg in my 4x4. It had a 350 and 3-speed manual. Then the averge was 9.9 - 10.0. Best was 13.6.
After the swap to an auto (th350) I was getting an average of 9.5.
I haven't got enough data to say what the 383 is getting but I'm sure it is worse.

All things being equal a 700r4 will get you 30% on the highway. So if you drive the highway most of the time and get 10, you'll get 13 with the 700r4. Swapping your 373s to 307s will get you 20%. So your 10 will increase to 12. Again this is all other things being equal, which they NEVER are.

What it comes down to is what do these increases cost you and how does that compare to the cost of gas. If it costs you $1000 to swap in a 700r4 and you'll only save $100 per year on gas, the payback is 10 years down the road. What are the chances you'll be in the same vehicle 10 years down the road?

If it is a collector truck you may have it that long but then you have a 2nd car which probably means your pay back is even farther down the road because you don't drive it that much.

Modern "big blocks" with all the computer controls, overdrive, and engineers working EVERY day to get the best mpg, don't get 20. What makes you think you'll get 20 under the shade tree in your back yard?

If you want 20 mpg get a 1/2 ton, 2wd. Swap in a 350 with injection and a computer controlling it. Swap in a 700r4. Find a 273 rear axel out of a newer truck. Don't plan on using your truck as a truck. Only drive with the wind. These things MAY consistantly get you over 20. But don't count on it.

Or leave your big block alone. Buy a Chevy Cadaver or Ford Escort or Dodge Neon to drive every day and use your truck when you need it or want to cruise.

Thats about sums it up .I get 30 mpg with my wreck now,But it aint a truck!!
I aint sayin what it is but if you see a silver bullit streek by its me.I love the mpg but the tinted glass hides the shame.

[Maximum Overdrive]

Originally Posted by mrein3

A member here who no longer posts claimed he got 20-23 in a sub with a small block but I refuse to believe it until I see he speedo corrected against a GPS or something to verify that the miles he is computing with are really miles.

Ya I never believed that bs that he got that milage either.

[COBRO]

My 72 C-20 with the 402,4-speed and 3.54 gears got its best of 15mpg and that was being very light footed. Now its got a warm 454 with a 400 trans and a Gear Vendors OD and Im feeling lucky if it gets 12mpg. Last time I checked it got 9mpg with mostly town driving.:::CB

[junkyardjohn]

Originally Posted by mitchfab

Whats it going to take to get 20 miles to a gallon with a 402?

TRY DROPPING IT FROM A PLANE 20 MILES UP WITH THE MOTOR OFF.

[70 shorty]

My 71 Cutlass with a 350, quadrajet, and 2.somthing gears got right at 20 mpg with a turbo 350 trans, strictly highway driving. It had 200000 on it also, and still got that mpg. It got around 15 around town. I also doubt you could get comparable mileage in a BB truck. Too big of an engine in a truck that's too heavy.

[thecatseye66]

Originally Posted by junkyardjohn

TRY DROPPING IT FROM A PLANE 20 MILES UP WITH THE MOTOR OFF.

Yeah, and siphon out all but that last gallon before you let it drop!

[shelby987]

wow, mitchfab, I can't believe you are still asking this question. It is possible to get your 20mpg out of a bbc, as some have said, BUT the cost of getting there would probably make it cheaper to go buy a brand new duramax and purchase everything in the banks catalogue (or just save yourself the time and buy that sidewinder on trucks!).

In a truck you simply have too many moving parts (which means more inertia) which means less mpg, the "aerodynamics of a brick", too much weight, and likely too heavy of a right foot. My advice would be to drive the truck when you feel like having fun, and drive your 'silver bullit' when you feel like hiding your shame while saving a couple of bucks on gas.

If you have to have your cake and eat it too (drive your truck and make 20mpg or more) you need to let the big block idea go. You would need an aux. overdrive, and super tall gears....gears that you are likely not to find to put into your truck axles, even then, swapping to a 250....possibly a very basic 327 would be the way to go.

The only suggestion I have.....if you really... really have to have that big block, is to get a tranny with a creeper (like the sm465).....pull out your diff, weld up some ring gears to give you a 1:1 ratio in the rear (3.08 just won't be tall enough). The creeper (if you start facing down a hill) should allow you to use the torque of the engine to almost get moving, may need some of those friends (probably best if its the ones in tall boots you talk so highly of) to give you a push just the same. Slowly work your way through the gears, once you get on the highway, NEVER GET OFF......with a properly built engine with all the goodies, sequential injection, hei, you should hit 20mpg.....oh yeah, keep it under 45mph though!
best of luck!

[mitchfab]

Thanks Dudes its been a real convoy! Hats off to ya!

[farside847]

I wonder if rebuilding my 30yr old fuel recovery filter/system would help out the 'ol mpg.