What do you think of my MPG?

[71enthusiast]

Here is the thing I am currently faced with:
My truck is a 71 sporting a 350 with an RV cam.
I have a 700R4 tranny with a 3.08 rear.
On the interstate the best mileage I can get is somewhere in the 10-11 range at 2000 revs. From what I have read from other posts is I should be getting better economy than this.

My question is does this seem right to you?

Chime away!

Other potentially useful info:
500 cfm Edelbrock carb.
28" tall wheels.
2.5" duals running from headers

[Daman]

No!

I have a th350 with a 350 crate engine and a 3:08 rear gear and a Holley 650 I just broke 18 MPG in my '72 K10 daily driver.

I was at 16-17 for ever then I swapped out to 2 1/2" manifolds and changed gas stations.

The biggest thing I tell people is to set idle mixture. That's the next largest part of the equation next to timing.

[71enthusiast]

So Daman you think I need to do some carb tuning to get those lost miles back?

[fstfwrd]

Get rid of the Edelbrock carburetor. Either use a simple 600 or so cfm Holley with vacuum secondaries, or switch it to a Quadrajet (which requires either changing the intake or using an adapter).

[leftybass209]

Originally Posted by fstfwrd

Get rid of the Edelbrock carburetor. Either use a simple 600 or so cfm Holley with vacuum secondaries, or switch it to a Quadrajet (which requires either changing the intake or using an adapter).

I'm trying to find the logic in this. A 500 cfm eddy doesn't waste gas if it's tuned appropriately. In fact, a 600 cfm Holley would struggle to produce the same economical results the eddy can.

Op- your numbers are certainly off base but don't spend a single dollar on that engine until you've confirmed it's a solid, with a compression test, verify the burn on the plugs, and timing.

[jessemthompson]

Your luggin the engine with a 3.08 gear and a 700r4. This definitely has a negative impact on your mileage. You need a lower rear end gear or no O.D

[Daman]

Yes and a little timing and make sure all your plugs are gapped right
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[fstfwrd]

If you have a GOOD HEI distributor, with verified 12 volts to it, you should be able to jump a spark a half inch or better on a tester. R43 comes to mind for the spark plug, but don't quote me on that. Get new plugs and gap them at .45 - Mark the balancer with a white paint pen, then loosen the distributor hold down bolt (9/16), verify the location to "0" with a timing light.

[71enthusiast]

Thanks for all the replies!
This gives me a few things to try

[68350]

That doesn't sound right at all. My 496 gets 9.8mpg with a th400. I'd definitely check what others have said, timing n spark plugs n fuel mixture. Does it smell like it is running rich?

[Daman]

One other thing is if everything is moving freely. A lot of people forget about bearings and dragging brakes.
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[SS Tim]

That is like 75 MPH with engine turning 1960 RPM, trans O/D of 0.7:1 through a 3.08 rear gear turning 28" tires. Somebody check my math here but that is like running a 2.15 rear gear direct.

[71enthusiast]

I could be wrong on the tire height but at 75 I am doing 2000 revs.
I have Dakota digital gauges so I am positive that number is correct.

NOTE:
I was wrong on the carb. it is marked 1405 which is a 600 CFM Edelbrock

[jessemthompson]

That is way too low. Your killing your engine and MPG

[71enthusiast]

Originally Posted by jessemthompson

That is way too low. Your killing your engine and MPG

Too low in which way?

[SS Tim]

The rear gears are too high (low numerically) to pushing that truck. since your engine is probably below its power band it lugging. Try driving it around with the O/D locked out and it will probably get better mileage at thoses speeds. Something in the 3.73 or lower (higher nummerically) is a lot more suitable for pushing a high drag vehicle at these sort of speeds.

[71enthusiast]

SS Tim
I have been looking into 3.42 gears. Are they still too (numerically) low?

[71meangreenc10]

Originally Posted by 71enthusiast

SS Tim
I have been looking into 3.42 gears. Are they still too (numerically) low?

Most of the trucks running in the late 80's and early 90's came stock with a 350/700R4 and 3.42 gears. The other option was 3.23 from GM. I am running that same ratio in my 71 and got about 16.5 on the interstate without locking in the OD. Now I have that fixed and I'll see how much that changes. 275-60-15 tires, 2wd.

Smitty

[MARKDTN]

Originally Posted by 71enthusiast

SS Tim
I have been looking into 3.42 gears. Are they still too (numerically) low?

3.42 is a great all-around gear. That is what is stock in my '83 K20. With stock 350/Q-jet/700R4 I used to get 14 around town. Now I swapped to Tuned-Port fuel injection and I am over 17 in town.

[1972RedNeck]

Depends on the weight of your truck along with a whole bunch of other variables.

As previously said, make sure you have a solid engine and tune it right.

[kell490]

What type of distributor are you running? You should have a vacuum advance one what you want is when your cruising for MPG total advance can be as high as 32+ degrees. When you put your foot into it then it should retard it so you don't ping. Here is a good write up on it. Nothing wrong with the carter carburetor as long as it is working correctly. If you think it needs to be rebuilt take it to someone who rebuilds them. Unless you want to covert to an 1850 Holley but you might find it to be a new can or worms doing that. I have rebuilt and tuned Holley's, Carter, and Quad-jet's over the years all have their own quarks but they work. I always suggest if you suspect the carb is old and leaking take it to someone who knows what they are doing. Also check for vacuum leaks with some carb cleaner spray it around the gaskets at the base of the carburetor and other areas where vacuume is see if engine idle changes. Make sure you only spray little at a time.


http://www.hotrod.com/how-to/additio...nition-timing/

[BigMike]

I agree 100% with Tim on this, before you go spending money on carb swaps try driving around with the O/D locked out (3rd gear, don't let it go into 4th) and you will probably start seeing better numbers. If nothing else it's an option that costs nothing but time, if that doesn't work then look into plugs/timing/carb adjustments, etc.... I would look for a 3:73 like he said, that would be your best bet and I think one of the best ratio's for a 700R4 trans. That rear end would do great with a 3 speed but the O/D is what is killing it like others have mentioned.

BTW, how does the truck run? Is it smooth? Any hesitation or sputtering? The 1405 is a decent carb when it's working well.

[kell490]

Does your torque converter lockup I think the stock 700R's are suppose to lockup? The 700R you can add a switch to manually lock it that helps to be locked up on the highway. 2000 RPM at 65 MPH sounds about right I would not throw a new set of gears in it unless you want more low end torque or want to run bigger tires.

[jessemthompson]

Originally Posted by kell490

Does your torque converter lockup I think the stock 700R's are suppose to lockup? The 700R you can add a switch to manually lock it that helps to be locked up on the highway. 2000 RPM at 65 MPH sounds about right I would not throw a new set of gears in it unless you want more low end torque or want to run bigger tires.

He's doing 2 grand at 75. That is too low

[bMr]

Aren't most newer trucks turning that RPM (or lower) at interstate speeds?

In any case, to the OP, try the free things first. I'd start with parasitic losses in the drivetrain by checking resistance on all four wheels, all engine accessories, and the engine itself by pulling all the plus and turning the engine over by hand. While they're out, re-gap the plugs and check the timing. Verify the torque converter is locking.

Once all the maintenance things are complete and before spending a dime doing mods, genuinely assess how your driving style could negatively impact MPG. This is the "imagine an egg under the accelerator pedal" part of the post... Also, while I disagree with the too-low RPM theory, it is free so absolutely try keeping it out of O/D and monitor MPG.

Only after exhausting the free stuff would I start spending money and even then it'd be a modest amount on things like retuning the carb.

Lastly, let us know how you find improvements. This is one on which all of us benefit.