I have a 1979 GMC Heavy Half Short Bed Pickemup Truck.
It is in original pristine condition, and is my daily driver. I rebuilt the 350 engine with factory parts, put a ported intake manifold and a tuned Holly (not the big double pumper) on it.
I love the truck, but I only get about 12 mpg in it with the TH350 tranny. It doesn’t have a low geared rear end at all, and considering that I run out of gears at about 25mph, I think that 12 mpg is really good.
I was thinking about buying a 700R4 transmission and adapting it to my truck. It is three inches longer, so I would have to modify the crankshaft to compensate. I would also have to move the monting bracket a bit behind from where it is now.
SO
Is this just way too much crap to go through for not much gain? Some people were telling me that I could get in the neighborhood of 25mpg. I was like, yeah, whatever. Then someone was telling me that I would get about 12mpg, and that is what I get now. So both of my neighbors don’t know shit, so I must turn to you guys.
Disclaimer: I know next to nothing about your particular application.
My advice is to put a new rear end in it if you are running out of gears at 25mph. Do you know what the current final drive is?
If you wanted to go more, you could put a new gearset in it. I would think that parts for the TH350, which is incredibly common, would be a better bet than changing the entire transmission. You could probably pull a new gearset entirely out of some other TH350 application and it would drop right in.
If you did decide to swap the tranny, you mention modifying the crankshaft. Do you mean the driveshaft? Building a new driveshaft is a pretty common thing to do when swapping trannies, but usually isn’t the big deal. Do the bellhousings match up? Do you need an adapter plate? Does one exist or would you need to fab one?
My advice: Try a new rear end. You may be able to find a junkyard one. If not, I would think an entirely brand-new final drive would run you less than $800. If that doesn’t improve things, swap out the guts in your existing transmission.
Wouldn’t help. Top gear would still be 1/1, which is what has most effect on mileage. If you don’t do any heavy hauling, the rear gear change to a lower numerical ratio would help, but it would hurt your pulling ability in low. It’s pretty easy to figure out what rear gears you have in back by jacking up a rear wheel and counting wheel turns versus driveshaft turns. I’d go with the OD trans, myself, assuming you don’t want a new truck. Swapping out the carb for an injector system (cheapest would be OEM TBI) would be the other big thingyou could do for mileage.
Driveshaft. Damn. Duh. I am trying to do some Excel stuff here at work and I goofed.
Yeah, I thought about the rear end but not as much as I probably should have. The transmission uses the same yoke and is a direct bolt up. I didn’t really consider the rear end too much because every truck with the same transmission I have ever driven hits third at 25mph, half ton through 1 ton. I was just putting two and two together, but I really am not a automatic tranny guy.
My reasoning was keep the truck from reving at 2800/3200 going down the highway maybe the gas mileage would go up.
When you compare a tranny switch (new transmounts, custom driveshaft, new speedometer gear, endless dicking around with the linkage) with a rear end swap for a taller ratio at a fraction of the cost and hassle, it’s a no-brainer.
Definitely change out the differential gearing, unless you routinely haul loads close to maximum or pull large trailers. You probably have a gear ration of 4.10 or 4.11 back there. You can quite safely go down to the low 3.20’s if you want.
Besides, if you swap rear ends, and you don’t already have posi-traction, you can get that. You can lay down two black stripes instead of one. I know you didn’t buy that fat Holley for gas mileage.
The mounts and linkage is all the same really, there wouldn’t be any dicking around. I hadn’t thought of the speedometer gear, but wouldn’t it get jacked by swapping out the rear end anyway? How exactly does that work?
You are right about the Holley, but I drive in such a way that the back two barrels never open. I am driving a truck that has probably more horsepower than anything else out there on the road, but I am the one letting everyone else by…
Something I forgot to mention. I put a bigger cam in the engine to lower the torque range. Will this be a factor with the rear end swap?
Sorry about misleading people into thinking this was a transvestite from transexual, translvania thread…
Well, I just ran your vehicle through Cartest v4.5.
TH350 MPG
City: 10.3
Highway: 11.1
TH400 MPG
City: 11.2
Highway: 14.2
Approximately 2 MPG might pay for itself if you operate this vehicle a whole bunch.
Personally, I’d save that project until the tranny started going out, that way you get every dollar’s worth out of the old transmission.
Assumptions:
I pasted my vehicle build screen below. You’ll notice I used TH350 ratios except I slapped in a .7 for 4th gear when I simulated a 700R4.
▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀02:40:11p
CarTest - Car Acceleration Simulation, Version 4.5 ▐
Copyright (c) 1992-98 Patrick Glenn, All Rights Reserved ▐
▀▀▀▀▀▀▀▀▀ F1 Save Data ▀ F2 Clear All ▀ F3 Analyze ▀▀▀▀▀▀▀▀▀▀▐
↑ ↓ ◄┘ Select █ F10 Help █ Del Erase Entry █ Esc Exit ▐
▄▄▄▄▄▄▄▄▄ F4 Parameters ▄ F9 Calculator ▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▐
╔══════════════╦═════════════════════════════════════════════════════════════╗▐
║ EDIT/ANALYZE ║ CAR TITLE: FAKE SDMB PICKUP ║▐
╚══════════════╩═════════════════════════════════════════════════════════════╝▐
╔═══════════════ENGINE════════════════╗╔══════════════CHASSIS & BODY═════════╗▐
║ Disp: 5700 cc. Loc:FRONT Type:NORM ║║ Car test weight: 5000 lb. ║▐
║ Horsepower: 155 bhp @ 3600 rpm ║║ % weight on front wheels: 56 % ║▐
║ Torque: 225 lb-ft @ 2400 rpm ║║ Wheelbase: 107.0 in. ║▐
║ Comp Ratio: 9.1:1 Redline: 5500 rpm ║║ Tire section width: 235 mm. ║▐
║ Start rpm: 4700 with Clutch: BRAKE ║║ Tire circumf. (opt.): 6.56 ft. ║▐
╚═════════════════════════════════════╝║ Wheel diameter: 15.0 in. ║▐
╔═════════════DRIVETRAIN══════════════╗║ Tire profile: 75 % ║▐
║ Transmission: 4 speed AUTOMATIC ║║ Coefficient of drag: 0.450 ║▐
║ Gear Ratios 1st: 2.52:1 4th: 0.70:1 ║║ Frontal area (opt.): 25.20 sq.ft.║▐
║ 2nd: 1.52:1 5th: :1 ║║ Overall height: 63.2 in. ║▐
║ 3rd: 1.00:1 6th: :1 ║║ Overall width: 67.6 in. ║▐
║ Final drive ratio: 4.56:1 ║║ Ground clearance: 7.5 in. ║▐
║ Driving wheels: ALL ║╚═════════════════════════════════════╝▐
╚═════════════════════════════════════╝░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░▐
▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄█
The 700R4 is also electronically controlled, you will have to install a control module too. I installed one in a 79 Monte Carlo and was able to adapt the module from an 88 Impala to work. Many aftermarket companies make control modules trans swaps like you want to do. Jegs has a nice selection. This site describes the swap with a T350, it should help you too.
The only electronic control in the 700r4 is the Torque Converter Lockup. The OP didn’t mention if the th350’s got a lock up TC. THAT could create as much of an efficiency boost as the overdrive.
Budget wasn’t mentioned, nor was dudty load (do you tow?), but a gear vendors overdrive might be an option if money was no object.
What do you estimate the motor’s capabilities are? A stock built 700r4’s weakness is the torque in 3rd gear.
/me has a race converter stage 5 700r4 behind a Lingenfelter Superram’d 383 in my vette.
Somebody makes a bolt on unit for the back of trannys which basically adds gears to your transmission. IIRC, it’s set up in such a way that if you’ve got a 3-speed tranny, it acts like it’s a 6-speed. They have a variety of units, so you can pick from either performance or fuel economy. If you’re interested, I can dig up the magazine article on them, so you’ll know what the hell the damn thing’s called and can check it out for yourself.
This thread is beginning to remind me a conversation I had with a guy after the 73 oil embargo.
He decided his work truck was not getting good enough gas mileage. So he decided to work on it.
When he got done his Chevy P/U had a CAT turbo diesel engine, new trans, and a new diff.
His comment after I said this work must have cost a ton of money?
“Well figuring the increase in fuel mileage, all I have to do is drive it to the moon and back, and it will have paid for itself.”
An overdrive unit = $$$$ +lots of work
junkyard 700R4 = $$ +lots of work
Junkyard rear axle w/ different ratio =$ +some work
Rebuild your diff with different ratio = $$-$$$ +some work
Finding a compatible rear axle with a lower (numerical) ratio will give you the most bang for your buck.
Oh and back to the OP 25MPG on this truck? I doubt it. No matter what mods are done, I seriously doubt it.
I think Rick’s got the strongest bead on this problem. But af you considered a 2500-3000 beater econobox for when you need better mileage? Having a redundant vehicle really doesn’t cost that much more and give you a BUNCH of options if your truck gets sick.
And by $3000 beater, I mean you can get a 70,000 mile Saturn SL2 in excellent shape. You can find a BUNCH of good, low mileage cars, if you’re willing to give up age for beauty.
I have a little Ford Ranger with a 4 cylinder and a 5-speed stick. It gets 25 mpg in town provided that:
The outdoor temp is not below 40 or above 80.
The AC remains OFF.
I use pure gas. 10% ethanol blend results in a 10% reduction in mileage.
I use the truck primarily to commute in, as 75% of the commute is freeway.
Using the truck to deliver magazines —my part-time job–I only get a maximum of 22. (Most commonly just over 20)
I doubt that you could achieve 25 mpg in a V-8 full-sized truck.
The advice to spend your $ 2500-3500 you’d spend on a different trans. or a separate overdrive unit on another, more economical vehicle, is pretty sound.
