MPG when is OK Good Enough?

[LMN8UR]

I just finished a round trip (900 miles) from Richmond, VA to Ohio and back. My truck has an aluminum cap and only hauling two pieces of luggage. I maintained 65 to 70 mph and I had a trip average of 18.6 mpg.
I have a air fuel ratio (Glow Shift wide band) and at idle she's running around 16, on a steady level cruise at highway speed it's about 14.5. (Through the mountains it was all over the place!)
My Edlebrock 600 has 125K miles on it and has never been touched (Other than external adjustment screws). I hear that 14.7 is optimum, should I even try to adjust the carb?

So what's everyone's opinion?

Thanks,

[67ChevyRedneck]

18.6? Leave it the hell alone

I've got a 350/700R/3.73 and the best I've ever gotten going 60mph ish for a trip was about 16.7-16.8 or so.

[Longhorn Man]

the average on these trucks is 10 - 12 MPG.
Bolt the hood shut and walk away with a maniacal laughter.

[ChevLoRay]

So, you're really bragging, right? Okay, like the others said, just leave it alone or start fixing it until it breaks and 18.6 is just a fond memory.

[LMN8UR]

Originally Posted by ChevLoRay

So, you're really bragging, right? Okay, like the others said, just leave it alone or start fixing it until it breaks and 18.6 is just a fond memory.

Actually I've heard of the newer trucks getting in the low 20's (I know ours are as aerodynamic as bricks) but I was hoping with my overdrive and 2.76 rear, and if I could tweak the timing and carb to get closer to 20 mpg.

[ksjh1316]

Be carefull if you touch anything under the hood you will loose 10 mpg instantly just change the fluids till she blows up

[69GMCLonghorn]

I think you have a major carburetor and timing problem. My advise is to buy a new carb that has twice the CFM's as the current one. Make sure it's a double pumper! Then, just to make sure you don't run lean and burn a valve, increase the jet size to the largest size. Then back your timing off by 5° to make sure it doesn't ping...

[Longhorn Man]

Originally Posted by LMN8UR

Actually I've heard of the newer trucks getting in the low 20's (I know ours are as aerodynamic as bricks) but I was hoping with my overdrive and 2.76 rear, and if I could tweak the timing and carb to get closer to 20 mpg.

Your combo is one that has made a lot of the old truck group cry in the past. Many people have done overdrive and a 3.07 gear and LOST MPG...
walk away... i doubt you'll get it any better.

[Skirkpat]

My wife is a super model, and she is spending all my millions, should I divorce her and look for triplet supermodel gymnists?

[67ChevyRedneck]

What are you getting "around town"?

With the set up I posted above I get anywhere from 11-12 ish depending on A/C, if I romp on it a bit... etc... I can consistently get around 12 if I'm easy on it the whole tank. I've gotten as low as 10 on some tanks, but those were my "fun" tanks

[Jim_PA]

Originally Posted by LMN8UR

I just finished a round trip (900 miles) from Richmond, VA to Ohio and back. My truck has an aluminum cap and only hauling two pieces of luggage. I maintained 65 to 70 mph and I had a trip average of 18.6 mpg.
I have a air fuel ratio (Glow Shift wide band) and at idle she's running around 16, on a steady level cruise at highway speed it's about 14.5. (Through the mountains it was all over the place!)
My Edlebrock 600 has 125K miles on it and has never been touched (Other than external adjustment screws). I hear that 14.7 is optimum, should I even try to adjust the carb?

So what's everyone's opinion?

Thanks,

I can run up to about 16.5:1 AFR at a cruise, and I don't get any surging or drivability problems. 14.7 is optimal for the least amount of emissions, not necessarily best for performance or economy.

If you don't have any very low speed surging or hesitation, the idle AFR is probably OK too, but I find with mine that if I go to the 16.x range at idle, I get a little bucking at very low speeds with light throttle applied.

Would you see any improvement by leaning out the cruise even more? Who knows Write down exactly where everything is, and try to lean out the cruise a little. You can always go back if it has negative effects.

Timing plays a huge role when getting intimate with AFR's too. Advance for lean mixtures tends to help get the burn going a little sooner, so you reach peak pressure just before TDC.

[Jim_PA]

One last thing to say is that when tuning, try to do it at the heat of the summer (or eye-ball it accordingly) because when the temps drop, the amount of o2 in the atmosphere goes up.

A "perfect" cruise AFR (wherever that falls for you) can become slightly lean in the winter months, and cause a driveabliity problem. Tune to be slightly richer than you want to be, so in the winter months you don't run into problems.

[prairewolf]

I read somewhere, that wind resistance starts to really affect mpg at or around 60 mph. I believe there is a lot of truth in that. Our Class C motorhome will get about 10 mpg at 55mph, above 60 it really starts to drop.. head wind effects the whole story but all things equal speed does matter.... my Mom use to get about an honest 21/22 mpg with a 72 Gaxaxie with a 351ci motor, If I had been driving it it would probably have gotten about 14

mpg is good enough when the one buying the gas says it is.


life is good,kent

[americanmusc1e]

Originally Posted by Longhorn Man

Your combo is one that has made a lot of the old truck group cry in the past. Many people have done overdrive and a 3.07 gear and LOST MPG...
walk away... i doubt you'll get it any better.

I'm there!! 700r4 and 3.07's are not a good idea!!! luckily, my next upgrade planned is 3.73's

the best combined mileage that I have got so far with the 383/700r4/3.07 combo is 12.1mpg (I did take straight highway with the short-lived 350/700r4/3.08 and got 18mpg that was 15miles, half 55mph and half 70mph) ..........................the worst was 7.8mpg (choke on the holley was stuck halfway open

the best combined with the old 350/th350/3.08 was 12.5mpg

with a properly tuned 383/700r4/3.73 I'm hoping for mid 13's in the 1/4.............................. sorry....I mean 14mpg

[Blue71LWB]

At this point I wouldn't touch the engine. You may want to bolt one of these in under the front bumper, It sounds like you you've got it dialed in, and a front air dam may get you the best bang for your buck.

[scottrod71]

Are you sure your neighbors aren't putting gas in that truck overnight??? We actually did that to a pastor once but had to quit when he started bragging from the pulpit about his big Buick getting 35 mpg. I think several folks thought he was out of his mind.

[hounddogs!]

Leave the tune alone!!



Everything else the same, drag increases with velocity squared. In practice, going from 60 to 70 increases drag by 36%, even though your speed only went up by 17%. Going from 60 to 80 increases drag by 78%.

Cd matters as well. I'd bet that if the rest of us ran toppers instead of the open bucket behind the cab, we'd be closer to 19mpg as well.

Here endeth the fluids class.

[68C10withA396]

I'd personally leave it alone too. I get about 8 gallons per mile it seems with premium gas, and that runs me currently $2.80 per gal . (That gallons per mile is not a mistake by the way) of course I'm running a built 396 with 4.11 gears...