1972 C.K. 10 4x4 Power -vs- Gas Mileage

[Keith Wilks]

Due to the high fuel cost of late I have been thinking about my next engine / trans. replacement for my old 4x4 and how that affects the amount of fuel it will use. When I got the truck in the early 90's it had the original engine(350) and I replaced this with a new 350 crate engine in 1995. The power plant has always seemed under powered and I get approx. 6-8 m.p.g. using the Edelbrock 4 barrell qaudrajet which is like the one that originally came on it.

Does anyone have suggestions concerning an engine / trans./ carb. etc. upgrade that would increase horse power and save fuel? I don't know that this is even possible with this old 4 x 4 ? I have toyed with the idea of the 350 H.O. crate engine with an Edlebrock fuel injection carburetor, but don't know if this would improve anythng? There are so many other ways to go and I am sure someone has found something that works.
Thx,
K.

[1969 C/K CST]

4x4's for normal use really don't need to be overpowered. .02 I think your choice is a good one. The 383 truck motor has pretty much the same hp but has alot more torque. I don't know how the mileage would be affected but it may be an option. For mileage I like a manual trans. or an automatic w/ overdrive. Gearing will play a major roll in these combinations.

[toroflow]

Well, speaking from experience, I have a 1979 K20 Suburban 4x4, and its 400 engine was real tired (over 200K miles) and was gettin about 7 MPGs. I bought a good running used 305 from a 1987 El Camino (carb) for $200, put it in the Sub, and sold the used 400 engine for $300. So I made $100. AND, my fuel mileage went up to 13 MPGs on the road, and about 11 in town. Sure, it doesn't tow things very well, but who needs to blast up hills at 55 MPH while towing, when 40 MPH gets you to the same point, albeit a little later, and saves you money??

[Zoomad75]

I'd take a close look at your overall gearing and tire size in addtion to the engine. Even a fuel injected 350 is going to suck gas if you have deep gearing and tall tires. Right now you don't have an O/D so that is going to cost you some when cruising.

Low (numerical) gearing in the axles will net more fuel economy but at a trade off of power. If your tires are close to stock size changing to a lower numerical gearset wouldn't be that bad. Dropping from 3.73's to 3.42's would lower your cruise RPM's by quite a bit. Add a 700r4 in place of the TH350 and your cruise RPM's would drop even more with the smaller ratios.

Problem is that you are talking about a big heavy 4wd truck. That by itself is not commonly looked at as a high mileage vehicle. Look at it this way, a Fuel injected 350 truck from say 88-95 still barely managed to get 15 mpg on the highway. And thats with better areodynamics than our barn doors have. At most I'd say the best you could do by changing to a fuel injected engine, OD trans and small gear ratios (3.08 to 3.42's) and small stock size tires is 12-15 mpg on the highway. It will be a dog to drive and you would still have to keep your foot off the floor for maximum economy.

Heck even a new Silverado with a 5.3 does not have a highway rating of 20mpg, its less. The problem is everything is a trade off. A higher hp engine might give a little more economy since it is building more power at a lower RPM and running better, but that doesn't mean that you are going to get 10mpg better if you build a 300-400 hp engine. Add that to the mess with the gear ratios and tire sizes and you have got a big puzzle to work out.

What is the truck's main use? DD? Only see's street duty, no off-road? Towing? Weekend toy? Is it lifted or stock?

I'd say that your current engine is not worn out, but probably out of tune. I get barely 8mpg in my K5 with a 350, well tuned q-jet, 3.73's, no OD and 35" tires, so 6-8 mpg on yours if its stock (my assumption) is telling me something is out of tune. Edlebrock q-jets are good carbs, but it is still possible that the settings are too rich for what you need. Might be a good time to pull the plugs and look at them. if they are dry, but sooty black, the carb is running too rich. Not just at idle, but on the main meetering circuit as well. The carb might need to be leaned out some. Also look at the condition's of the plug wires and think about a switch to a HEI distributator to get rid of the stock points setup. You should be able to improve your mileage a good by a good bit. Lowering the maximum speed you drive helps a ton too. wind resistance builds faster, the faster you drive. I've noticed a big change when driving 75 vs 65.

I'd take a good look at what you already have before dumping $3500 on a new crate engine, another 2 grand for an aftermarket EFI setup and a rebuilt 700r4 for anywhere from $500 to $1500. All that cost to get maybe 5 mpg better. At the cost of fuel in the mid $2 range, you would have to drive it a long time to even see any payback from all the parts you installed. If you got the coin to drop in it go for it, but if you sole goal is to save money at the pump it won't pay off. You'd be better off spending that money on a beater 3yr old Cavalier to drive to work and leave the truck for fun on the weekends and bad weather days.

[boundstaffpress]

Seems like a simple thing, but have you checked your tire pressure. Low tires, especially wide tires will dramatically effect MPG. Narrow tires, for whatever reason, yeild better milage. Same principal, something about contact patch and coeficient of friction.

A taller tire will act to reduce your rear end gearing, but will make it harder for you to get started. If you drive on the freeway a taller tire will help, but if you drive stop sign to stop sign, It only serves to eat more gas.

Rob Z. has some good advice. Build-ups never SAVE money. My wife can give you a long story about that.

If you do decide to change your setup, a smaller head, 180cc or smaller, will increase your low end torque and thus help get you started better. Has to do with velocity of air flow. Lower compression makes power lower in the RPM band, and will also improve in town economy, though minimally. And that brings you back to spending lots to save a little.

[1969 C/K CST]

I have a crate 350, 15x8 rallies, 31x10.50's, with stock ride height, 3sp manual, late model axles. My ratio is probably a 3.07.(not sure-something I need to look into) At 55mph spinning at 2500rpm. Mileage is aroung 13-15mpg. Not bad for a '69. My sacrifce is low end pull, meaning I'm pretty good with a clutch. I installed a heavy duty clutch to help out with take offs. I plan to swap to a granny 4sp to offset the high gearing. For now the 3sp is fine because I no longer plan to use the truck as I have in the past. I will be starting on a rebuild and making changes. My use then will be highway and light duty offroading. These truck are heavy and as stated above, a new one doesn't do much better.

[Zoomad75]

Gearing makes a dramatic difference. My K5 when I got it had 31" tires with the 3.73's. I swear I could smoke those poor tires in granny gear, but on the highway with the little tires and those gears it was pulling over 3 grand at 65. Switching the 33's slowed down my cruise RPM at a cost of the low end power. Now with the 35's I can *almost* use the granny gear in regular driving. Darn gear spacing is so darn wide on a Sm465 its not even funny. Still I like it better than an auto trans.

The 35's really are working like a poor man's OD right now for me. With the 465 I still have decent gearing off road with granny low and 4 low. So I got the best of both worlds. I'll probably switch to at least 4.10's or maybe 4.56's when I upgrade to a 14bolt in the rear. I don't expect it to get stellar mileage though....

Seeing and riding in a Frankenstien Suburban in DirtyLarry's garage right now might have me looking for a 6.5 Diesel though...I'm not normally a fan of 6.5's or 6.2's but this thing is bad a$$. With the banks setup it pulls harder than the bigblock in Larry's 78. I'm sure if one could keep thier foot out of it, it should probably get 18-20 mpg on the highway. Seat of the pants feel puts it similar to a stock early Duramax, but sounds like a nasty Cat or Cummins and blows black smoke like a coaltrain at WOT....

a 6.2 or 6.5 is a definate way to gain fuel economy. It wouldn't be cheap unless you got a doner truck to start with though.

[Keith Wilks]

Originally Posted by 1969 C/K CST

4x4's for normal use really don't need to be overpowered. .02 I think your choice is a good one. The 383 truck motor has pretty much the same hp but has alot more torque. I don't know how the mileage would be affected but it may be an option. For mileage I like a manual trans. or an automatic w/ overdrive. Gearing will play a major roll in these combinations.

Concerning being under powered, I think I may be asking too much from such a heavy truck and actually when it's in 4-wheel low it has more power than I need. It is only noticable when taking off or passing.
Thx,
K.

[Keith Wilks]

Originally Posted by toroflow

Well, speaking from experience, I have a 1979 K20 Suburban 4x4, and its 400 engine was real tired (over 200K miles) and was gettin about 7 MPGs. I bought a good running used 305 from a 1987 El Camino (carb) for $200, put it in the Sub, and sold the used 400 engine for $300. So I made $100. AND, my fuel mileage went up to 13 MPGs on the road, and about 11 in town. Sure, it doesn't tow things very well, but who needs to blast up hills at 55 MPH while towing, when 40 MPH gets you to the same point, albeit a little later, and saves you money??

Thx,
K.

[Keith Wilks]

Originally Posted by toroflow

Well, speaking from experience, I have a 1979 K20 Suburban 4x4, and its 400 engine was real tired (over 200K miles) and was gettin about 7 MPGs. I bought a good running used 305 from a 1987 El Camino (carb) for $200, put it in the Sub, and sold the used 400 engine for $300. So I made $100. AND, my fuel mileage went up to 13 MPGs on the road, and about 11 in town. Sure, it doesn't tow things very well, but who needs to blast up hills at 55 MPH while towing, when 40 MPH gets you to the same point, albeit a little later, and saves you money??

Your trade off sound a lot cheaper than what I have in mind. I think I could take lessons from you on how to trade up and make money to boot.
Thx,
K.

[Keith Wilks]

Originally Posted by Zoomad75

I'd take a close look at your overall gearing and tire size in addtion to the engine. Even a fuel injected 350 is going to suck gas if you have deep gearing and tall tires. Right now you don't have an O/D so that is going to cost you some when cruising.

Low (numerical) gearing in the axles will net more fuel economy but at a trade off of power. If your tires are close to stock size changing to a lower numerical gearset wouldn't be that bad. Dropping from 3.73's to 3.42's would lower your cruise RPM's by quite a bit. Add a 700r4 in place of the TH350 and your cruise RPM's would drop even more with the smaller ratios.

Problem is that you are talking about a big heavy 4wd truck. That by itself is not commonly looked at as a high mileage vehicle. Look at it this way, a Fuel injected 350 truck from say 88-95 still barely managed to get 15 mpg on the highway. And thats with better areodynamics than our barn doors have. At most I'd say the best you could do by changing to a fuel injected engine, OD trans and small gear ratios (3.08 to 3.42's) and small stock size tires is 12-15 mpg on the highway. It will be a dog to drive and you would still have to keep your foot off the floor for maximum economy.

Heck even a new Silverado with a 5.3 does not have a highway rating of 20mpg, its less. The problem is everything is a trade off. A higher hp engine might give a little more economy since it is building more power at a lower RPM and running better, but that doesn't mean that you are going to get 10mpg better if you build a 300-400 hp engine. Add that to the mess with the gear ratios and tire sizes and you have got a big puzzle to work out.

What is the truck's main use? DD? Only see's street duty, no off-road? Towing? Weekend toy? Is it lifted or stock?

I'd say that your current engine is not worn out, but probably out of tune. I get barely 8mpg in my K5 with a 350, well tuned q-jet, 3.73's, no OD and 35" tires, so 6-8 mpg on yours if its stock (my assumption) is telling me something is out of tune. Edlebrock q-jets are good carbs, but it is still possible that the settings are too rich for what you need. Might be a good time to pull the plugs and look at them. if they are dry, but sooty black, the carb is running too rich. Not just at idle, but on the main meetering circuit as well. The carb might need to be leaned out some. Also look at the condition's of the plug wires and think about a switch to a HEI distributator to get rid of the stock points setup. You should be able to improve your mileage a good by a good bit. Lowering the maximum speed you drive helps a ton too. wind resistance builds faster, the faster you drive. I've noticed a big change when driving 75 vs 65.

I'd take a good look at what you already have before dumping $3500 on a new crate engine, another 2 grand for an aftermarket EFI setup and a rebuilt 700r4 for anywhere from $500 to $1500. All that cost to get maybe 5 mpg better. At the cost of fuel in the mid $2 range, you would have to drive it a long time to even see any payback from all the parts you installed. If you got the coin to drop in it go for it, but if you sole goal is to save money at the pump it won't pay off. You'd be better off spending that money on a beater 3yr old Cavalier to drive to work and leave the truck for fun on the weekends and bad weather days.

I am slowly learning that there are a lot of factors figuring into the overall equation here.
My truck is pretty much stock with a few changes for example I am running 16" tires (265's) instead of the 15" that originally came on it and I found an H.E.I. distributor and installed that. Most everything else is stock....350 engine, 350 transmission w/o over drive, 308 differetial, 2" lift. I use this truck everyday for mainly highway and in town, but occasionally I will off road some too. I certaily could slow down a little for a start as well as set the carburetor to up my mileage some........Your feed back is great and I will use it.
Thx,
K

[Zoomad75]

No problem Kieth. Here to help. Tradeoffs are everywhere when you start tinkering. Your setup is pretty conservitive. The tires (around 31" tall) and the 3.08 axle should put your cruise RPM at 65 right at 2100 rpm, which is pretty low, but right in line with what the newer stuff runs at with steeper gears and od trannys. A 700R4 would drop that Rpm by a few hundred at the same speed, but the cost to retrofit the 700 would be a little high considering you would have to change the t/case too. Again, the gain in mileage won't pay for the swap for a really long time..

The lift actually hurts the economy in the long run as it exposes more of the chassis to the airflow, which causes drag. More drag means you have to use more energy to overcome it. Again, its a trade off, you want to be able to run off road time to time so you sacrifce a little. Is the lift causing a huge loss in MPG? Probably not, but if it costs a 1/2 mpg and you are only getting 8 at the most now, but that's 6%. It adds up.

You really can't change the trucks Areodyamics (besides lowering which doesn't sound like an option). So you have to maximize what you got. Tune that engine up to run as effeciantly as possible. Add a high flow exhaust (with or without headers). Keep the tires inflated up to spec. Watch your driving habits. Holeshot starts are fun, but every time those secondaries open up, the toilet flushes more fuel.

[Keith Wilks]

Originally Posted by Zoomad75

No problem Kieth. Here to help. Tradeoffs are everywhere when you start tinkering. Your setup is pretty conservitive. The tires (around 31" tall) and the 3.08 axle should put your cruise RPM at 65 right at 2100 rpm, which is pretty low, but right in line with what the newer stuff runs at with steeper gears and od trannys. A 700R4 would drop that Rpm by a few hundred at the same speed, but the cost to retrofit the 700 would be a little high considering you would have to change the t/case too. Again, the gain in mileage won't pay for the swap for a really long time..

The lift actually hurts the economy in the long run as it exposes more of the chassis to the airflow, which causes drag. More drag means you have to use more energy to overcome it. Again, its a trade off, you want to be able to run off road time to time so you sacrifce a little. Is the lift causing a huge loss in MPG? Probably not, but if it costs a 1/2 mpg and you are only getting 8 at the most now, but that's 6%. It adds up.

You really can't change the trucks Areodyamics (besides lowering which doesn't sound like an option). So you have to maximize what you got. Tune that engine up to run as effeciantly as possible. Add a high flow exhaust (with or without headers). Keep the tires inflated up to spec. Watch your driving habits. Holeshot starts are fun, but every time those secondaries open up, the toilet flushes more fuel.

Thanks again, I believe you are right about the endless number of trade offs when you start tinkering. I think I will do the minimum at this time to up mileage and do the rest as I am able.
K.

[Keith Wilks]

Originally Posted by 1969 C/K CST

I have a crate 350, 15x8 rallies, 31x10.50's, with stock ride height, 3sp manual, late model axles. My ratio is probably a 3.07.(not sure-something I need to look into) At 55mph spinning at 2500rpm. Mileage is aroung 13-15mpg. Not bad for a '69. My sacrifce is low end pull, meaning I'm pretty good with a clutch. I installed a heavy duty clutch to help out with take offs. I plan to swap to a granny 4sp to offset the high gearing. For now the 3sp is fine because I no longer plan to use the truck as I have in the past. I will be starting on a rebuild and making changes. My use then will be highway and light duty offroading. These truck are heavy and as stated above, a new one doesn't do much better.

Man 13- 15 m.p.g. sounds really good compared to my 6-8. I think my auto trransmission is is another one of those trade offs that demands more gasoline, but I enjoy the hands off shifting too much.
Thx,
K.

[GM72K10]

Here's my 2 cents. My 72 K10 has a stock 89 350, HEI, dual exhaust but no headers, rebuilt original 72 quadrajet, stock suspension, auto trans, 3:08 rears. LT235 15 radials. Gets about 12 or 13 MPG just running around. Now compared to my work truck, the 04 K3500, which only gets 9 MPG (if I'm lucky), that's great!

[Keith Wilks]

Originally Posted by GM72K10

Here's my 2 cents. My 72 K10 has a stock 89 350, HEI, dual exhaust but no headers, rebuilt original 72 quadrajet, stock suspension, auto trans, 3:08 rears. LT235 15 radials. Gets about 12 or 13 MPG just running around. Now compared to my work truck, the 04 K3500, which only gets 9 MPG (if I'm lucky), that's great!

That's really good mileage!! I wonder if your late model engine has anything to do with it and especially since you are using the stock Q-Jet Carb.?

Thanks for your input.

K.