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?

[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

[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.

[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.

[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.

[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.

[Firebirdjones]

Another way to look at it. We spend over $100 a month to have internet service at the house. It's more than $1200 a year just to sit here on this computer and type, lol.

I know people that just have to have their smart phones and pay well over $100 a month for that service (something I don't care to do)

That's more than I'm spending in gas driving a classic.

[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.

[68c10owner]

Thanks for the input guys. If I could afford a newer truck i would but I cant so Im stuck with what Ive got. So since Im stuck with it I want to make it better. Unfortunately I live check to check so Im not thinking so much about how much it will cost me per month or year. Or even by the day for that matter. I put in $20-25 at a time and since gas prices are fairly low here right now it will last 3 days. Maybe more but since my gas gauge doesnt work I dont want to push it although I have before and that is why I now carry a gas can.

I know my gearing is far from ideal. I just got this truck in September so its going to take me a bit to get it sorted out. Had it a little over a month and lost the transmission so thats new. I know its needs a new engine but dont really have the funds for that yet so that means the gearing wont get changed for a while now.

I didnt mean to turn this into such a big discussion but I do appreciate the input. I was mostly curious what others are getting with similar combos to see if it was normal or way off. Sounds like Im about right for what I have.


firebird, the year cut off is 1975 and older for smog. you still can get a ticket if you are inspected and the smog is removed but there are no biannual smogs required. Some counties up here dont require them either depending on where it is. Once I get it smogged I plan to register it in another county that doesnt require biannual inspections and I do happen to have a address to use in that county. Once that is taken care of I can install a free flowing exhaust, remove the computer controlled carb and run something more efficient. Im not really going for max power with this since its my daily driver and only vehicle but I will be going for strong low-mid range torque.

[68c10owner]

Small update--

Filled the truck up today and did the math and it was 14.77mpg. What I've been doing is driving to work at my regular speed, 66-67mph and then driving home slightly slower. I'll have to GPS it and see hat it actually is but 55mph on sped is 66-67mph and I dropped it to 50 mph on the speedo. So guess it isn't too bad over all. I still think it can be better but probably not by much. I might start collecting parts for a gear swap and maybe that will increase it some and give me a little better pep too.

[knockretard]

Where are you getting your odometer readings from? I know you said your speedo is off, if your getting your mileage from the speedo your results would be off by the same % as the speed readings.

[68c10owner]

Originally Posted by knockretard

Where are you getting your odometer readings from? I know you said your speedo is off, if your getting your mileage from the speedo your results would be off by the same % as the speed readings.

I am getting them from the odometer. I know that will be off as well but I write down the mileage when I fill up and then again the next time I fill up. Then I subtract the mileage and then divide that by how many gallons it took to fill up. The odometer reading will not be correct but its accurate enough to figure the mileage the way that I am. Plus I know how many miles a day I drive and the math seems pretty close. Based on how much it takes to fill up my truck each time its fairly accurate. Its certainly not getting better or worse mpg because the speedo isnt correct. It gets what it gets. The odometer is still reading miles.

[knockretard]

it's reading miles but it reads them slower in direct proportion to the speed your traveling.

For example:

if the odometer thinks you are traveling at 55mph for an hour it will read 55 miles.
Now if you are really traveling at approx. 65-67mph that's a difference of 10-12 miles in just an hour of drive time.

If it's only off 10-15% you could actually be getting 2-3mpg better mileage than you think.