1985 gmc swb 305/700R MPG

[68c10owner]

Since my fuel gauge isn't working I tend to just put in $10-15 every couple days but I decided to do a test to check the mpg. I filled up and drove it fairly normal. Some around town but mostly freeway with a couple heavy runs merging onto the freeway otherwise normal acceleration and cruising around 65-75mph. Truck has a E-qjet, fresh 700R with stock converter, 3.08's and 30" tires. Not ideal tire and gearing but that's how I got it. Just going fill up to fill up I'm getting 12.81 mpg. My old 83 c10 swb with the same engine,trans and gearing but with stock shorter tires was getting 14.5-15.8mpg. I've done a complete tune up but could probably use a fuel filter. Thinking my tire size/gearing is killing my mileage. I think I need to either put some gear in it or use shorter tires. Can't afford either one right now. Was thinking of goin with a stock 350 GM crate engine but worried the mileage might be worse. I would think it would be better since it would make more power and require less throttle to get moving but I don't know. Not really sure what I'm asking here, if anything. Mostly just thinking out loud. I would like to hear from other members with similar drivetrain as to what your MPG is.

[Matt Man]

You need to make sure your speedometer is accurate. I think your mpg should be better than that how good of shape is the carb in? Recently rebuilt? Bogging ect... My truck with no overdrive and 33's with 4x4 gets 10 in town and the best I have seen is 14.5 on the freeway, but I usually average 11-12mpg overall. I would check to see if you have a sticking caliper, air in the tires ect. Also you need to remember that the fuel they use in the winter has more ethanol in it so typically we get worse mpg in the winter than the summer.

[68c10owner]

My speedo is off but I clocked it sing my GPS. At 55mph I'm doing 66-67mph. I usually drive with the speedo between 55-60mph. I got this truck in Sept so I don't know the history UT the carb works well. Sometimes it idles so smooth you can't tell it's running. I don't have any bogs at all. It does smell a little rich but when I first started messing with it I took it to a rinds place and we hooked up a o2 reader. I can't remember what the readings was but wasn't overly rich at idle but did get a little rich with some throttle. I'm sure it could use a rebuild but it does Un really well. The engine is worn out and uses oil so I try not to abuse it. Funny though, this truck replaced my 97 gmc c10 that got otalled and with a 350/4L60E combo and 3.42 gears and the same tires I am running on this truck it got 13.8-15.2mpg.

[Jake Wade]

My '85 with 355, TH700R4, 3.73's, and 28" tires gets 14.7-15.3 MPG

[68c10owner]

Originally Posted by Jake Wade

My '85 with 355, TH700R4, 3.73's, and 28" tires gets 14.7-15.3 MPG

I would be happy with that. Especially since I know your 355 isn't stock. What carb do you run?

[slotard]

Originally Posted by Matt Man

You need to make sure your speedometer is accurate. I think your mpg should be better than that how good of shape is the carb in? Recently rebuilt? Bogging ect... My truck with no overdrive and 33's with 4x4 gets 10 in town and the best I have seen is 14.5 on the freeway, but I usually average 11-12mpg overall. I would check to see if you have a sticking caliper, air in the tires ect. Also you need to remember that the fuel they use in the winter has more ethanol in it so typically we get worse mpg in the winter than the summer.

what gearing? It sounds like his gearing might be too tall. In comparison, your 33s might help your gearing and mileage. My truck, 350/350/4.10/30s is geared way too short to get good mileage, especially if I wanna go more than 55ish. It wouldn't surprise me if a modest tire size increase helped my mileage.

[68c10owner]

Originally Posted by slotard

what gearing? It sounds like his gearing might be too tall. In comparison, your 33s might help your gearing and mileage. My truck, 350/350/4.10/30s is geared way too short to get good mileage, especially if I wanna go more than 55ish. It wouldn't surprise me if a modest tire size increase helped my mileage.

I don't doubt my gearing is far from ideal. I'd be willing to bet goin to a 3.42 or 3.73 would actually increase my MPG but not in the budget just yet.

[Jake Wade]

Originally Posted by 68c10owner

I would be happy with that. Especially since I know your 355 isn't stock. What carb do you run?

I am running a Quadrajet from '78

[Firebirdjones]

I'd say there is a lot more MPG to be had with that combo. Likely need to go over things in the tune, dial in the AFR, check tires, brakes etc...
30" tires and 3.08's isn't bad. I had an 84 blazer with that exact combo with a goodwrench 350 in it straight out of the crate, it got 16-17 MPG regularly.
My 72 blazer with a 6.0 LQ4, 4L60E and 33" tires with 3.07 gears go 22 mpg on highway trips. I added 3.73's and highway dropped to 20 mpg.
My 79 1 ton with a 454/400 turbo and 3.73 gears just tickles 14 mpg on the highway with it's stock 16.5x9.5 tires (I think 31" tall) and that's with no overdrive.
If my 454 can do that I see no reason your small block couldn't do better.

[68c10owner]

Originally Posted by Firebirdjones

I'd say there is a lot more MPG to be had with that combo. Likely need to go over things in the tune, dial in the AFR, check tires, brakes etc...
30" tires and 3.08's isn't bad. I had an 84 blazer with that exact combo with a goodwrench 350 in it straight out of the crate, it got 16-17 MPG regularly.
My 72 blazer with a 6.0 LQ4, 4L60E and 33" tires with 3.07 gears go 22 mpg on highway trips. I added 3.73's and highway dropped to 20 mpg.
My 79 1 ton with a 454/400 turbo and 3.73 gears just tickles 14 mpg on the highway with it's stock 16.5x9.5 tires (I think 31" tall) and that's with no overdrive.
If my 454 can do that I see no reason your small block couldn't do better.

I know my tires are probably low on psi. I usually set them at 40psi and I know the difference in steering when the psi drops. I can try adjusting the mixture on the carb but other than that there really isnt anytihng else i can do to adjust the AFR. I did check it once at a friends shop and it was good at idle but did go slightly fat under load. I cant recall the numbers but we drove up and down the street to test it. I do have a code 54 which indicates a mixture control solenoid but my truck has none of the symptoms other than the code. Ive also seen the code mention the EGR stuff which I have a new EGR valve and EGR solenoid waiting to go one when I get time. I suppose I could just put a new MC solenoid on but dont want to start throwing parts at it.

My thinking is this engine is gutless and I have to put my foot in it to get it to go so Im using more gas. Id love to just put a 350 in there but I`ve heard my carb and ECU wont work very well on a 350 since its calibrated for my 305. I have had a few of these smog trucks but this is the first time I actually had to deal with any of this stuff.

[Tom]

In my 78 my 385 [w/750cfm carb] with 3.73's got 10mpg. Put a vortec 350 from a 99 in it [w/ eddy 600cfm] and still got 10mpg. Same vortec engine in my 79 with a 200-4R and 3.08's got me 10 in town and 15 highway. Lockup in the convertor went bad and 2004R's can't use overdrive without lockup so I was stuck with only 3rd for a week, my freeway mileage went up to 16 lol. So yes I believe your gearing is holding you back.

[TwoFiftyShifter]

Does it really matter?

It's old. Tune up and good tires, just keep rolling.

[68c10owner]

Originally Posted by TwoFiftyShifter

Does it really matter?

It's old. Tune up and good tires, just keep rolling.

There is always one guy.

It matters to me. If that bothers you I don't know what to tell you.

[68c10owner]

Originally Posted by TwoFiftyShifter

Does it really matter?

It's old. Tune up and good tires, just keep rolling.

Originally Posted by 68c10owner

There is always one guy.

It matters to me. If that bothers you I don't know what to tell you.

Sorry, sometimes I get a little heated when someone brushes off my concerns. If it didn't matter, I wouldn't have posted it. I don't make a lot of money so I'm trying to get all I can from what I've got. I do t have the credit or the money to go finance a new truck so thats not a option.

I'm the idiot who thought driving a 30yr old truck was a good idea. Actually I had a 97 gmc with a Vortec 5.7 that didn't get much better mpg wise but it was totalled back in September thanks to all me guy bouncing off the center divide and hitting me on the freeway ay 70mph. Insurance didn't pay me much for my truck and since I couldn't finance anything I was stuck buying someone outright. All of the CK body trucks, like my 97, that were in my price range either had a salvage title or were beat up. This truck was pretty clean for the price and I've always loved square bodys. So there it is.

Also finding out what others are getting will give me a comparison of my truck that might indicate there is a issue . Or I might find out its pretty normal.

[Firebirdjones]

Originally Posted by 68c10owner

Sorry, sometimes I get a little heated when someone brushes off my concerns. If it didn't matter, I wouldn't have posted it. I don't make a lot of money so I'm trying to get all I can from what I've got. I do t have the credit or the money to go finance a new truck so thats not a option.

I'm the idiot who thought driving a 30yr old truck was a good idea.

Completely understandable. I don't consider someone an idiot that daily drives a 30-40 year old vehicle. Matter of fact I view it as someone maybe slightly more intelligent that doesn't want to conform to the cookie cutter world and doesn't let gas prices dictate what you have to drive.

I may be bias with that thought process because I'm similar

We've had plenty of new cars, daily drove LS cars for years, and have done LS swaps with overdrives in classics (still do them for customers) But even my wife got sick of all the computer fuel injection garbage, expensive to work on, updating software constantly, tuning equipment was a pretty good investment all by itself. My wife got so tired of it she sold her brand new SS Camaro and now wants to drive our 69Z daily. I'm fine with that. Until then she is driving my 79 454 pickup. Carbs are fine with us and I don't have to have an overdrive. I've since much more preferred the simplicity and frankly I'm sick and tired of seeing an LS in every damn car I see now. The only newer vehicle I keep in the stable now is a duramax for towing, and it sits in the garage 90% of the time.
So yeah, I understand, I work with our cars to extract all it can offer without the need for overdrive or fuel injection, and I've been very successful at it. I have a few classic cars here that are getting upper teens for MPG with 400+ cubes, and doing it with a carb and no overdrive. It's doable.

[68c10owner]

Originally Posted by Firebirdjones

Completely understandable. I don't consider someone an idiot that daily drives a 30-40 year old vehicle. Matter of fact I view it as someone maybe slightly more intelligent that doesn't want to conform to the cookie cutter world and doesn't let gas prices dictate what you have to drive.

I may be bias with that thought process because I'm similar

We've had plenty of new cars, daily drove LS cars for years, and have done LS swaps with overdrives in classics (still do them for customers) But even my wife got sick of all the computer fuel injection garbage, expensive to work on, updating software constantly, tuning equipment was a pretty good investment all by itself. My wife got so tired of it she sold her brand new SS Camaro and now wants to drive our 69Z daily. I'm fine with that. Until then she is driving my 79 454 pickup. Carbs are fine with us and I don't have to have an overdrive. I've since much more preferred the simplicity and frankly I'm sick and tired of seeing an LS in every damn car I see now. The only newer vehicle I keep in the stable now is a duramax for towing, and it sits in the garage 90% of the time.
So yeah, I understand, I work with our cars to extract all it can offer without the need for overdrive or fuel injection, and I've been very successful at it. I have a few classic cars here that are getting upper teens for MPG with 400+ cubes, and doing it with a carb and no overdrive. It's doable.

I'm not sure I would call it more intelligent bit the older trucks certainly have more character.

I would love a LS engine in here not only for the mpg they bring but the increased power. My smogged out 305 won't hardly spin the tire in the rain. It's also down about 2 mpg from my last 305/700R 4 powered square but tire size is different and freeway speed is too. If I could get 15 mpg average from tank to tank I'd be happy. Next time I fill up I might try going a little slower like I did with my old 83 c10 and see if that changes anything. Even my old 90k5 blazer with 31x10.5's and 3.42's would average 16-17mpg tank to tank. I'm sure the tbi had a lot to do with that though.

[Firebirdjones]

Originally Posted by 68c10owner

I'm not sure I would call it more intelligent bit the older trucks certainly have more character.

I would love a LS engine in here not only for the mpg they bring but the increased power. My smogged out 305 won't hardly spin the tire in the rain. It's also down about 2 mpg from my last 305/700R 4 powered square but tire size is different and freeway speed is too. If I could get 15 mpg average from tank to tank I'd be happy. Next time I fill up I might try going a little slower like I did with my old 83 c10 and see if that changes anything. Even my old 90k5 blazer with 31x10.5's and 3.42's would average 16-17mpg tank to tank. I'm sure the tbi had a lot to do with that though.

I was the same way, first it was the LT-1 bandwagon, had many of those and even put one in my wifes scrambler back in the day, Then thought I'd love LS engines and jumped on that bandwagon. We had 4 LS cars at one time, and then I did a 6.0 LS swap in my 72 blazer and daily drove that for a few years. Been there done that. Just got sick of it all. Architecture keeps changing, and now we are starting another LT1 generation. Just don't care for it anymore, too expensive and it's ever changing. If you like to tinker like I do, it will cost a small fortune.
I can make power with older engines and carbs and do it cheaper and still have fun with it. I like the fact that there isn't much that can go wrong with old stuff, and when it does I can likely fix it on the side of the road with a small tool box. And like you said, it has character. But I'm getting older too and don't want to deal with that new stuff anymore, and I don't like big fat car payments and expensive insurance that comes with it either.

[68c10owner]

Originally Posted by Firebirdjones

I was the same way, first it was the LT-1 bandwagon, had many of those and even put one in my wifes scrambler back in the day, Then thought I'd love LS engines and jumped on that bandwagon. We had 4 LS cars at one time, and then I did a 6.0 LS swap in my 72 blazer and daily drove that for a few years. Been there done that. Just got sick of it all. Architecture keeps changing, and now we are starting another LT1 generation. Just don't care for it anymore, too expensive and it's ever changing. If you like to tinker like I do, it will cost a small fortune.
I can make power with older engines and carbs and do it cheaper and still have fun with it. I like the fact that there isn't much that can go wrong with old stuff, and when it does I can likely fix it on the side of the road with a small tool box. And like you said, it has character. But I'm getting older too and don't want to deal with that new stuff anymore, and I don't like big fat car payments and expensive insurance that comes with it either.

Making power isn't a issue. I've got that covered. My deal is trying to build for economy. I've never worried about it in the past but these days money is tight and even though gas prices are coming down its still not cheap to fill up twice a week. But I agree this older stuff is usally easier to fix. This smogged out stuff can be tough to work on but I'm getting the hang of it.

[Firebirdjones]

Oh that's right, you're in Sac. My wife's side of the family is up there. We visit occasionally and enjoy the drive. Yeah they still do smog up there, but I thought they set a year limit on that now.
We don't have that here in Prescott which is one of the reasons I picked this town to move.

[barry1982]

I too love driving / restoring the old square bodies..... have had one or two on my insurance policy since 1983. Although I love the small block Chevy's....gas was killing me on my 70 mile daily commute to work. In 1990 I got a great deal on 1985 Sierra Classic 2WD...... equipped with the 6.2 Diesel. I have to admit that it didn't have the passing ability of my 350's, but what I lost in power I more than made up for in mileage. I basically cut my daily commute cost in half, and still got to drive a square body instead of an economical "puddle jumper" . It is not uncommon for a 6.2 cruising on the highway at 65 MPH to make 25-28 MPG.

[slotard]

The reason I want to go LS - probably GenIV - is for mileage when I'm not having fun, and firing right up with no hassle. Stock LS vs what I have now I doubt power will be all that different.

[TwoFiftyShifter]

I can empathize with your situation. But you have to crunch the numbers. You have to play the long game.

If you drove say 1,000 miles per month, and got gas at 1.99 a gallon and got your 12.8 mpg, that's $155 a month in gas.

If you were getting 14.5 mpg, that's $138 a month in gas.

That's $4.25 per week.

In my opinion, your gears are too tall. You are running 2300rpm at 65 BEFORE you hit overdrive. In OD you are 1650 at 65. So in drive, you are spinning a bit too fast for good economy, and in OD you are spinning too low for optimal economy. A V8 is most comfortable with a 1800-2000 rom cruise.

If you were to change to 3.42 or 3.73, you might pick up that 2mpg.

But how long at $5 per week would it take to reap the gas savings? 2 years?

[1redbowtie3]

Originally Posted by TwoFiftyShifter

I can empathize with your situation. But you have to crunch the numbers. You have to play the long game.

If you drove say 1,000 miles per month, and got gas at 1.99 a gallon and got your 12.8 mpg, that's $155 a month in gas.

If you were getting 14.5 mpg, that's $138 a month in gas.

That's $4.25 per week.

That's $ 4.60 per DAY.

[TwoFiftyShifter]

Right... He'd pay $4.60 per day for gas with 14.5mpg. At his current mpg, he'd pay $5.17.

He'd be better off leaving earlier and driving 60mph.

[Firebirdjones]

I look at it this way.....and this was also one of the deciding factors for us to just drive classics.

A new $40-$50k car was $500+ a month payment, tags are based on retail value here in Arizona, which can be several hundred dollars a year just for plates, full coverage insurance is mandatory when making car payments and that's $5-$600 every 6 months, Just this alone totaled about $8000 a year just to own the damn thing. Let that soak in for a minute
Plus the maintenance of the car, tires cost more, over $200 a piece for a performance 18" tire they all seem to have nowadays, Brakes cost more....When I do the math, by the time I paid for that $40-$50k car with taxes and interest, plus tags and insurance over a several year period (that most people keep cars for these days) I'm getting dangerously close to spending nearly 6 figures $$$ to own that car for an average of 7-8 years.

All this just to get what......25 mpg?? Where's the savings?? I can buy a hell of a lot of gas with all that money.

Easy math here....
I can drive one of my classics, paid for, tires and tags are dirt cheap. Even if I only got a very poor 10 mpg with it (I don't have one that horrible) and driving our average of about 10,000 miles a year, that's 1,000 gallons of gas over 12 months. At our current price of $2.19 a gallon for premium (that's all I run) that's $2,190 for an entire year of fuel.... Hell of a lot cheaper than those $500+ a month car payments alone, not to mention all the other expenses.
The worst classic I have on mileage gets 14mpg. Even using the same 10,000 mile a year formula I only use $1564.00 in fuel. Shucks I can buy $4 a gallon gas and still be saving money over paying for that brand new car. This is on a vehicle that's paid for, insurance is only $100 every 6 months and tags are $22 a year.

Yeah the gas mileage in our classics with carbs and no overdrive isn't what a new car gets, but I'm still saving a boat load of money. One of the best parts of living in AZ is we have weather year round that allows us to drive the classics without worry of road salt and other adverse weather conditions. Besides that, the smiles per mile are well worth it. It's all about how you put things in perspective.