Ideas for fuel mileage - underdrive pulley? electric fan?

[da_raabi]

I'm looking to get a few extra mpgs out of the dooley. Right now I probably get about 5-7mpg, even running 50/50 city/highway.

I've already got an oversize open air filter (no housing etc), true dual exhaust (stock manifolds though), de-smogged, rebuilt carb (with some choke issues - don't ask...), rebuilt trans.

454/TH400/4.10s with 235/85r16 31" tires

I'm looking into an underdrive pulley setup and/or an electric fan setup. Does anyone have some real-world experience with these kinds of mods? Any mpg improvements?

I know an OD trans and/or fuel injection would help a lot, but thats EXPENSIVE. I'm just looking for some cheaper ways to gain some mpg for now.

 

[Crispy]

Depending on how much you drive you will either never see a cost savings or i will take years to realize it. If you drive a lot then your best bet is a commuter car. You should see a cost benefit within a few months. I know its probably not the answer you wanted to hear but I was in the same boat with my Cummins. I bit the bullet and bought a commuter, its half way to paying for itself already and that was from a January 2019 purchase date.

 

[shiftpro]

Another reason why I like propane. I realize this is not the budget fix you're seeking. But it's a cheap clean high octane fuel for high compression (efficiency)
engines.

Also either your carb is worn out or your ignition system is weak and/or your parking brake is stuck because you should be getting better 5-7. Oh I see you pretty low gears... so anything over 55 mph and the need;e is dropping pretty fast. Maybe taller tires..?

 

[yevgenievich]

Afr gauge and get existing set up tuned

 

[84 M1008]

The TH400 and 4.10's aren't helping much. I'd go with 3.73's before I'd give up the TH400 for an overdrive trans.

 

[Big Chip]

I’ve got the same setup except I have 3.73 gears. I got between 7-10 mpg (mostly closer to 7) doing around 75 mph for extended periods. I bought a 4L80e that will be going in, maybe as soon as next month but it’s more so I don’t have to listen to the old 454 screaming at me on the expressway. Hopefully I’ll be able to report cost and mileage sooner rather than later.

 

[4WDKC]

Make sure its tuned up
fix the choke issue
make sure brakes arent dragging.
follow another truck
slow down
make sure tire pressures are up
buy a cheap commuter
install a 5 speed
efi
propane injection and engine rebuild/head swap for needed compression.


they just arent good on gas... the gmt400 guys bitch and complain about the same mileage with efi..The efi engines have cam specs suited for a small block and low compression, betting yours does too.

 

[1987 GMC Jimmy]

I agree with them. That underdrive system will be a downgrade for you in the FL heat, and I don’t think there’d be a significant recuperation with an electric fan over a thermal clutch fan.

 

[HotRodPC]

I'd keep the 4.10s. Rather than do a gear swap, that's funds towards a 4L80-E Swap and then you have your cake and eat it too. Still have the low gear for low end grunt and towing. OD to get the rpms down for highway driving. Of course all the above too, underdrive pullies if you want to do the expense, but E fans aren't a bad options either. Those 2 suggestions will also free up some of the parasitic hp loss and put it to the rear wheels for a noticable difference.

 

[Wumbo]

Fix the choke first.

When I bought my truck the choke was shot and the PO had "played" with the carb to get it to run somewhat. Best I would get was 8 mpg.

Fixing the choke and adjusting the high idle brought me up to 12 mpg.

 

[da_raabi]

Fix the choke first.

When I bought my truck the choke was shot and the PO had "played" with the carb to get it to run somewhat. Best I would get was 8 mpg.

Fixing the choke and adjusting the high idle brought me up to 12 mpg.

Yeah I decided to rebuild the carb to be ethanol friendly, and it did not go well. This was my second carb, and the first one went just fine. That car runs like a raped ape. For some reason this one is just a giant pain in my ass. My plan with that is try doing some real tuning when I have a couple weeks off for the new baby. If I could get this beast up to 12mpg I'd be ecstatic.

The 4L80E was always on the table, except for the cost. I could justify it if it was a direct swap, but the need for a modified driveshaft, computer, etc. just makes it kinda unrealistic. I just rebuilt the TH400 too, and it shifts waaaaay too good to just let go. I'm pretty proud of that rebuild lol.

I almost bought a beat up 91 crew that still had the trans, linkages, d/s, etc. in it for parts. Even had the correct speedo with the digital input. Man I should have done that. I could have literally swapped everything I needed and still got my money out of the body. Oh well. Live and learn.

So it sounds like underdrive pulleys are off the table. I can't really find any for a BBC anyway, and definitely not with A/C.

I think my diff is having trouble, the truck clunks a bit when you apply the gas. Almost like there is play in the diff gears. It's not U-Joints because the driveshaft was rebuilt last year. So if I end up rebuilding the diff I might just drop in the 3.73s, since I'll be spending the money anyway.

Thanks for the tips guys. It's much appreciated.

 

[4WDKC]

Yeah I decided to rebuild the carb to be ethanol friendly, and it did not go well. This was my second carb, and the first one went just fine. That car runs like a raped ape. For some reason this one is just a giant pain in my ass. My plan with that is try doing some real tuning when I have a couple weeks off for the new baby. If I could get this beast up to 12mpg I'd be ecstatic.

The 4L80E was always on the table, except for the cost. I could justify it if it was a direct swap, but the need for a modified driveshaft, computer, etc. just makes it kinda unrealistic. I just rebuilt the TH400 too, and it shifts waaaaay too good to just let go. I'm pretty proud of that rebuild lol.

I almost bought a beat up 91 crew that still had the trans, linkages, d/s, etc. in it for parts. Even had the correct speedo with the digital input. Man I should have done that. I could have literally swapped everything I needed and still got my money out of the body. Oh well. Live and learn.

So it sounds like underdrive pulleys are off the table. I can't really find any for a BBC anyway, and definitely not with A/C.

I think my diff is having trouble, the truck clunks a bit when you apply the gas. Almost like there is play in the diff gears. It's not U-Joints because the driveshaft was rebuilt last year. So if I end up rebuilding the diff I might just drop in the 3.73s, since I'll be spending the money anyway.

Thanks for the tips guys. It's much appreciated.

that could be brakes moving in the drums also.

the ecm for the 4l80 can be a 0411 ecm from any vehicle send it and have the new bin file uploaded. The ecm can be bought from a junkyard for $35+ then the cost of the bin change. or pull one from an older diesel truck with a 4l80.

 

[HotRodPC]

Where stand alone controllers used to cost $1000 several years ago, they're out there now brand new for well under $400 now. You spend over $400 in most cases for a rear gear swap, then add more expesnse to add or toss the Th400.

 

[DoubleDingo]

3.73 is a good gear. I put some in my Cherokee to run a bigger tire. It turns about 3,000 rpms at 70 mph after lockup, which us with TH400's don't have.

 

[75Monza]

I did a bunch of stuff to my big blue that helped around 2.5 mpg, changing to an all terrain tread from mud tread, good working carb, pipe in fresh air for the big air cleaner (made a dual snorkel 14"x6"), trying anything to reduce air drag (hard to do with something as aerodynamic as a brick, lol), putting a tension spring on my lead foot, and buying the more expensive non-ethanol fuel...my motor sure likes that better.