Looking for better MPG (long)

[ChrisK10]

Alright folks, after a long hiatus I have got the fever again. I had been wanting to sell my 1971 Chevy C20 with a 454 and TH400 since I started college and haven't been driving it. I never really had any luck and I figure I should just keep it and make it the driver I would like it to become.

Right now with the 454, TH400, and 4.11 gears it is a real strain on fuel economy. It runs about 26-2800 RPM's at 55-60 mph (which is a lot higher than I'd like, especially for a 454). I'm guessing it gets 6-9 mpg. I would like my truck to be able to reach 15 mpg and still have decent power, which I figured would be a good goal.

I have been thinking about a few different options that I have in order to help increase my mileage and drivability. I could swap engines (I really want a 350 instead of a 454), swap gears (3.08's?), swap trannies (700R4?), and would most likely do a combination of something.

My dad knows a guy with an old cop car Caprice with the fuel-injected 350 and modified O/D transmission. I would think that if I could get this for a fair price it would solve a lot of my problems. If I could get this setup I would keep my 4.11 gears (which are posi) which would yield 2000 RPM with 30" tires in O/D going highway speed (about 62 mph). This setup is the one I am most for at the moment. Any ideas on possible mpg improvement for setup #1?

Another possibility that would be a more short term fix aimed more directly at lowering the RPM's at highway speed for the 454 would be to just swap out the 4.11 gears for 3.08's. Then, at 2000 RPM with 30" tires with a regular tranny in 3rd gear I could cruise at about 58 mph, which is much more reasonable for such an engine. However, I still feel it will be tough to increase the mileage of the 454 to anything over 10, so setup #2 isn't really a route I would like to go.

If I can't get the cop-car 350 and O/D tranny, the next best option that I would pursue is getting a 350 engine to swap out for the 454 (truck was originally a 350 anyways). If I kept the transmission (TH400) I would swap in 3.08 gears and I could then run at 2000 RPM with 30" tires traveling 58 mph, which is setup #3. Otherwise I could keep the gears and swap for an overdrive tranny which would give me 2000 RPM, 30" tires, and 62 mph, setup #4.

Right now I like (from best to least) setup #1, #3, #4, then #2. I am really looking to get rid of the 454. It's a good engine but it's just not what I want. A 350 would suit my needs much better it seems. I would really like to get the 350 and O/D tranny from the Caprice. It sounds like it would be a strong setup and should get decent mileage. If I can only find a regular 350 (my Dad really wants me to look into a TBI instead of a carb'ed engine) it may be easier to just deal with getting different rear end gears instead of messing with a different transmission. I've heard the O/D's aren't usually as strong either.

What would you guys suggest in order to gain the most fuel efficiency? Remember, I am mainly looking to get rid of the 454. Is 15 mpg attainable without having a super special engine setup?

Any advice would be greatly appreciated! Thanks guys and gals!

[RACINJASON22]

Wow I think you got a good head on your shoulders there. Planning is the key IMO and you have done alot of that already for yourself, Good job.

I'm partial to the SBC's myself and agree that your #1 choice if you can land it sounds the best to me. Keep us informed of your out come and as always we like pic's.

BTW 15mpg is attainable with out a special engine set-up IMO also. RJ22

[Ivan D.]

I ain't too familiar with the Chevy fuel injection systems, but I've heard that the TBI when in good shape runs strong, gets decent economy, and is pretty reliable too - I'd say the cop car setup sounds like the best choice for now. Something to look into when choosing the rear gears - is yer driving mainly city, or mainly freeway? 3.08 gears and 30" tires don't like stop-and-go traffic at all, with my stock 27" and 3.08s my Lincoln (302 MPFI/SEFI engine) was getting over 15mpg city consistently, now with the 29s I can't break the 12mpg no matter what I do - you'll be running 30s with a bigger engine and inferior fuel delivery and management system, so don't expect wonders. Now if yer daily driving is mainly on freeways or you do lots of long road trips a 350 SBC and 3.08 rear end should give you over 15mpg even with a 3spd tranny, just make sure the engine is in good condition and tuned right. Oh, and one thing that's generally neglected - change yer differential fluid to something synthetic, everyone I know who has done it swears by it.



Edit: my truck runs a freshly reconditioned 350 SBC with a TH350 and I believe 3.08 gears, so pretty much identical to yer option #3, however I'm not giving that as an example an use the Ford product instead is for the following reasons:

a) my "stock" Q-jet was way outta whack for what the engine requires, jets were right but metering rods were way too fat, so I had to keep the choke halfway closed pretty much all the time if I wanted to have some decent a/f ratio.

b) on my trip to school today (first time truck sees the street with the new carb & intake) my "new" Holley dumped about 1/4 gallon of gas in my engine and about 3/4 gallon all over my engine, real lucky I didn't catch on fire. I believe I managed to adjust it right in the afternoon tho, and on the way home it seem to work real nice and not spill gas no more.

c) my exhaust consist of long-tube headers and 12"-long turndowns, which is pretty bad when it comes to fuel economy and making power down low, I will be replacing those with a true dual system all the way to the axle but that's some $100 I don't have right now.

I'm sure tho that someone here with a good working stock setup (350 Q-jet engine, 400/350 tranny, 3.08 gears) will be willing to share their observations of fuel economy at freeway speeds, just give it some time.

[royo]

start with 308 gears . this will be the cheapest if you can find a doner 12 bolt rear with 308s . any kind of fuel inj . is going to need a computer to run also dont just give your big block away for a small block trade it for. a whole car or truck with parts that you need. a th700 is a good trade off on the400 because its got o/d and a 306 first gear but needs shift kit to make it live under a heavy foot keep it simple ps if you didnt live so far away i would put you in a 4 bolt 350 and 308 gears for the big block see ya

[cdowns]

i had a 300hp 350 with 411gears a 2004rtranny lowered with airdam to decrease air flo under the truck and got an honest 16-18 mpg at 80mph
synthetic oils properly maintained wheel bearings suspension and alignment and tire pressures all contribute to better fuel milage

[Chris27]

I have something very similar to your option #1. My 72 K5 4x4 has a 92' caprice TBI 350 and 700R4 tranny. With 4:11's and 33's I'm spinning around 2000 Rpm at 65mph. I average around 15mpg. I'm also turning a transfer case and a front diff, you with 2wd you might do better. I believe option #1 would achieve your goals.

[Russell]

Ah, just stab a turbo diesel in there, and enjoy 24 - 25 mpg

[Ivan D.]

Originally Posted by Russell

Ah, just stab a turbo diesel in there, and enjoy 24 - 25 mpg

You know Chris, this fella here brings up an excellent point!

[ChrisK10]

Originally Posted by Ivan D.

You know Chris, this fella here brings up an excellent point!

I'd be fun to have a diesel but I'm really lookin' for a 350.

[Bowtie001]

my 84 with a Qjet 350 and th350 and 2.73 gears got 19mpg. I had a lockup converter. If you've never driven one, you need to get one immediately. It's nearly fourth gear! Synthetic in the rear is a no brainer. I have heard caps are good for aero, but I don't know for sure but I had one. I know sliding back windows are.

Pump your tires up a whole lot. Don't ever use your brakes unless you absolutely have to, drive like there is an egg under your foot. You'd be surprised how much driving style affects mpg. I have a crazy friend who drives like he's at Daytona. In his 02 maxima he gets 17 mpg hwy when he drives like himself. Driving normally he gets 25 or so. Build in a cold air intake, that's good for a few points. An overdrive helps a bunch, but they can get pricey. If you can drive a stick it'll most certainly get better mileage. Here is an easy one - get the tallest tires that will fit your wheels.

Lots of stuff you can do. You just have to play with it. If the carb is running well the tbi won't give you any appreciable mpg highway - idle will be better though. I wouldn't want to go through the trouble of modifying a stock injection system for a modified engine though.

Good luck!

[martinkh]

How 'bout some info on that lockup for the TH350?

[Bowtie001]

The truck wasn't mine when it was installed. My pop put that together when I was somewhere around 14. This is what I think:
It was a normal TH350 with a lockup converter - I don't know which tranny it would have been out of, but it wasn't a modified transmission. They work on a hydraulic pressure switch - he had to buy a few to fine tune the lockup speed to around 50 (I would highly recommend more like 40 if you're doing this - it's roughly 1 psi to 1 mph in high gear). I don't think it was anything special - or if GM ever made a lockup 350.

Sorry I don't know more, but it sure did rock. Interior noise levels went down a bunch too. My dad rebuilds transmissions professionally - if you really want more info - I can find out, but lets not hijack this guys thread... pm me for more...

-Brian

[Russell]

Yep, the TH350C is infact a stock transmission that came in some of the later model trucks (thinkin 87 till the TH350 was discontinuted), the lockup works the same as a 700r4's, and uses the same torque converter as well.

[dmnall]

Screw it get a complete LS2 and T56 from an 05/06 GTO and get around 15 - 16 in Town and around 26 on the freeway @ 80. I am pretty sure our trucks versus the weight of the gto are about on par. Change the gearing to 3.42 with this combination and you can expect better mileage. My Gto on long trips averages anywhere between 28 - 30 @ 80 mph. My truck averages 6 - 8 at 80 and both are running premium (91).

Charlie

[ChrisK10]

Originally Posted by dmnall

Screw it get a complete LS2 and T56 from an 05/06 GTO and get around 15 - 16 in Town and around 26 on the freeway @ 80. I am pretty sure our trucks versus the weight of the gto are about on par. Change the gearing to 3.42 with this combination and you can expect better mileage. My Gto on long trips averages anywhere between 28 - 30 @ 80 mph. My truck averages 6 - 8 at 80 and both are running premium (91).

Charlie

I'm kind of on a limited budget here!

[ChrisK10]

About the lockup converters... can you install one on a TH400? Or is it just something for TH350's?

Thanks for all of you guys' ideas so far, they have been very insightful!

[Chris27]

Not sure if the TH400 ever had lock-up. My dad had a 80' Chev he bought brand new, it had the TH350 with lock-up. I'm not sure when they started.

[dmnall]

Originally Posted by ChrisK10

I'm kind of on a limited budget here!

I can understand that! You are not the only one on a budget as well. First thing I will do is get my 700r4 and put in my truck once I do the swap over from lwb truck to swb. I am just saving the money up and by the end of the year I will be purchasing the ls2, t56 trans, hpe s cam, fast intake, injector o rings, new wiring harness and then start getting the ls2 with roughly 500+ HP and m6 trans. Just looking at these parts 8K for engine, trans complete with hydraulics and computer. Then another 1200 - 1400 for cam, springs, pushrods , intake, gaskets and cai. Then bolt up headers and get the computer tuned to turn off vats, rear o2 sensors, cags (skip shift), and get 25+ on freeway@ 80.

Charlie

[70GMC396]

What about just optimizing the rear end / tire combo, going to a 600-650cfm (if you don't care about top end) and do some fine tuning. Wouldn't that be the cheapest, easiest and still get you in the 15-16 range?

[clay68c10]

With TBI on my 454 and 3.08 gears, I'm getting 10mpg driving in Dallas commuter stop & go traffic. That's up from 8 with a pretty well adjusted Edelbrock carb. I haven't put an EGR valve back into the equation yet, but I hear that will bring up the mileage also. If you're driving at some steady speeds you may do ok with TBI and not swapping engines.

[ChrisK10]

I'm pretty set on swapping out the 454 for a 350, mainly because that was what came originally in the truck. I know it will be hard to get more than 10-11 mpg out of that big block just about no matter what I do with it. Seeing 15 mpg is something I want and would allow me to drive the truck without going flat broke every 100 miles.

Right now I'm either going to go with a 350 engine, TH400 tranny, and 3.08 rear end gears, or a 350 engine, 700R4 tranny, same gears at 4.11. It just depends on the best combo that I can find in my area.

How much would a rebuilt 700R4 cost?

[tavenner13]

ive got a 454 with 3.08's in a 12 bolt and i still get only around 6-9 mpg if im lucky. im installing an aftermarket tach this week so ill let you know the #'s on my crusing RPM'S. ill measure the tires tomorrow theyre 275/60/15