9.5 MPG!!!! Please Help!!!!

[nosfed2]

Ok so it is a 1984 Gmc 1500 2wd It has a New 350ci with 1200 miles on it with a RV cam. It also has a 700R4 tranny with 2.73 gears in the rear.It has a stock intake and Quadra jet on it that has been rebuilt by national Carbs, It does not smoke or smell like its running rich. I pulled the plugs and they are perfect. I dont care about towing or anything, we drive it to swap meets that are 30 to 70 miles away one way. We need it to get the best gas mileage possible. The thing is that I watch the tech and at 55mph it is at 1500rpm it never gets over 2000rpm even on a hill so I dont understand how it can be getting such bad mileage. We checked the brakes and wheel bearings for drag but they are all ok. It also has 31X10.50 wheels on it.What is the Deal?

[muddnutz]

hard to get good gas mileage driving a brick im getting 10 mpg out of mine with a 4" lift and 35's stock gm350 put in 2 years ago with a 700 r4 how many miles are om your stock cats?

[INSIDIOUS '86]

With those 273s good luck. Change to 308s and it should go up actually.

I'm guessing the stock intake and manifolds and qjet are just having a field day killing your efficiancy
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[Desert1957]

Originally Posted by INSIDIOUS '86

With those 273s good luck. Change to 308s and it should go up actually.
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Very true, I went from 2:73 to 3:42 in this 87 and mileage went up 3 MPG.
It also has 31 X 10.50 tires.



Desert

[85sierraclassic]

Try running it in drive and not overdrive and see if it improves lower rpms don't always make it more efficient.
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[cleansquare]

Yes you have issues. I am getting 12.2 in my 3/4 ton 85 with 350 and 3 speed trans. Course I have 4.10s headers no cats aluminum intake flat top pistons etc. Runnibg 3300 rpm at 70. Still sucks tho which is why im swapping in the 4L80e now
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[dieseldawg142]

welcome to the club!, 6 mpg's, 7 when i can pick up the jet stream

[tucsonjwt]

I would have thought that a lower rear axle ratio would provide better gas mileage. City mpg has always been 9-11 best on any of my 3 squares - 73 454, 76 350, 83 454. I rarely drive highway but the 83 got 13 mpg best on the highway with 4.10 rear axle.

I bought an old beater Cavalier for most of my driving (24-26 mpg), and rent an economy car when I have to drive a long distance out of town (usually costs less than $20/day.) You could also rent a V6 pickup from Uhaul when you need one (or Enterprise), but it would cost $20 to start with, then a mileage fee, but probably still less than owning a truck if you don't rent one very often, considering the savings on fuel, insurance, registration, etc.

My best pickup mileage was 19-20 city/22-24 highway on a 2000 Silverado V6 with 3.42. You might consider looking for one of those, since you don't want to tow with it. Another option might be a late model Tahoe with 4.8 and 3.42 for better gas mileage and still lots of cargo space, plus a very comfortable ride. I have owned several Tahoes/Yukons purchased new in the past 10 years and the worst I got with any of them was 17 mpg city - but I do drive like the senior citizen that I am

[cleansquare]

I have a 2000 volkswagen beetle diesel that is my dd, but with the 4L80e swap in my c20 im hoping to chamge that!
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[SSC's76]

What was posted above is correct to low rpm will kill fuel economy just to same as spinning it to fast granted not as bad. You want your highway rpm to be inline with the power range your camshaft makes the most torque. 500 plus or minus so if your cam specs say peak torque at 2500 you want to be 2-3000 rpm on the highway to see the best results usually the lower limit is better. If you have a vacuum gauge hook it up and jump on the highway drive in Oliver drive for a few miles like normal and then try a few miles in drive. I'm sure you will see higher more consistant readings in drive with your current setup. 273 ratios are great highway gears for a 3 speed but the overdriven will like a 3.23 or 3.42 ratio better. As far as city goes the lower numerically higher ratio will help get it moving a lot better as well from a stop and you should see a gain here as we'll if you were to swap gears. But then again city driving is usually a wash with all the stop and go.
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[bilfman]

Agreed 2.73 with 31's and overdrive is too low rpm to be in an optimum torque/power band. Easiest check for this is as a previous poster mentioned. Try driving in 3rd and see how the mileage goes. If it goes up, gearing is the issue.

[68 TT]

I agree. The gearing is way too tall with the tire height and overdrive. You are making the motor work too hard just trying to push the brick.

In my old 83 C10 I went from 2.56 gears to 3.73's and my mileage went from 8 around town to 14 and from 10 on the highway to 18. It took far less throttle input to get the truck moving from a stop with the 3.73's. Don't know why the factory didn't figure that out.

[nosfed2]

Ok so I have a set of 3.23 and 3.73 but i was wanting to put a posi in it also but the posi currently has 3.08 will either of these gears fit on a 8.5 10 bolt 28 spline?

[Skunksmash]

Guess I won't be complaining about my 14 MPG anymore.

[piecesparts]

You can get a set of 3.73, that should fit up to your posi unit. The change should get you back to at least a 14 MPG range.

[SSC's76]

Originally Posted by nosfed2

Ok so I have a set of 3.23 and 3.73 but i was wanting to put a posi in it also but the posi currently has 3.08 will either of these gears fit on a 8.5 10 bolt 28 spline?

It's been a while but the 373 will use a different carrier. The 323 gears should be fine. What kinda cam is in the truck do you have the specs? That will give us a better idea of what ratio you should run for best economy
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[piecesparts]

Originally Posted by SSC's76

It's been a while but the 373 will use a different carrier. The 323 gears should be fine. What kinda cam is in the truck do you have the specs? That will give us a better idea of what ratio you should run for best economy
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On this information, I disagree. We had an 86 truck that had a 3.08 geared rear end and a 350C tranny. We changed it to a 700R4 setup and lost gas mileage, due to overloading the engine to accomplish simple things like driving down the road. We then put a rear end, with 3.73 gears, under the truck and gained back the mileage we lost and then some. The 3.23s will help, but if you are running anything close to the factory height tires, then you are going to not gain very much. You will still be too low in RPMs to be effective. There are formulas out there that you can use to calculate your RPMs on the road and where you need to be to get the optimum out of your engine. You should target around 2000 to 2200 as your RPMs for cruising at whatever highway speed you like.

[nosfed2]

Well me and my uncle dug in the snow at a junk yard to find a eaton posi with 3.08 and i am going to buy 3.73 gears off ebay and put them in...A custom posi shop here says that the 1984 10 bolt 8.5 only needs a 3 series carrier to use 3.73 and I am going to put them in and test the mileage again. I will keep you posted. Also someone earlier asked if my cats where old? I am running stock 2.5 inch y pipe exhaust with a walker quiet flow muffler in place of the stock cat and the stock muffler also. I want it quiet as possible because my neighborhood is picky..lol... It does not seem to restrictive. But that is 2 mufflers on a single exhaust system? Thanks for the help

[nosfed2]

Ok another question is that my truck may have 2.56 or a 2 series carrier in it which is fine because the posi i have has 3.08 on it which will accept the 3.73 but i read somwhere a 10 bolt axle housing is different from 2 series to 3 series? if this is true I will need a different axle housing all together for a 3 series carrier? This sounds wierd???

[nosfed2]

Well today I installed the 3.73 gears into the housing and it went smooth..I drove it around town and it felt much easier on the accelerator... less pedal to get up to cruising speed...tomorrow is the big day to see if the mpg has gone up, taking it to work...will keep you posted

[INSIDIOUS '86]

Exhaust will help much too. My neighbors hate my truck since its practicaly a dragster going Down the road. The *****ed till I had freinds with real dragsters have a cruise in on my street. The now think I am perfectly quiet.

Two mufflers will be as quiet as it is now abouts but will help mileage some more.
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[Rufton]

In many street applications safe WOT air/fuel is approximately 12 but cruise A/F should be around 14. Wideband O2 will likely pay for itself in short order if you know how to jet carb.
What does plug look after cruising several miles then immediately turning off ignition?

[Rufton]

RV cam should have adequate torque for good mileage above 1500rpm.
If you don't have a torque curve for your engine then you can download a trial version of engine analyzer pro.

[nosfed2]

Ok so i drove it to work again and the numbers got me concerned so i am gonna do another test tomorrow. But here they are The First Time i went to the gas station and filled it till it popped and than filled again and it popped real quick so i know it was full. I wrote down the beginning milage which was 40093 and drove it straight to work and back to the same gas station and same pump and wrote down the milage again 40139 and filled it up again till it popped and than filled again till it popped real quick and it used 4.84 gallons which comes out to 46 miles drove and divided by 4.84 which comes to 9.5 mpg so I took your guys advice and switched to 3.73 rear gears and drove it today. I went to the same gas station and same pump and topped it off again doing the same steps as before and than wrote down the beginning milage 40458 and went to work i than went back to the gas station and wrote down the milage 40511 and filled it up again till it popped and than filled again till it popped real quick so i knew it was full and it used 2.78 gallons which comes out to 53 miles this time but i did go to my house and than left for work which added 7 more miles but that comes out to be 19 mpg? that seems like a huge gain but i did notice that i was giving it a lot less pedal and last time on my way home the gas gauge was dropping down to a line below F but this time it never moved from over F? I will test it again because this seems to good to be true...lol

[Rufton]

Where can I get one of those 3.73 gears