[footjoy]

What gas mileage should I expext?

I have built a pontiac 400
1965 Tempest
Bored .040
Flat top pistons
stock #15 heads
Crower 60243 cam
factory cast iron exhaust manifolds with dual exhaust.
Quik fuel 735 cfm Hot rod carb
2004r over drive
3:73 rear end x .68 over drive = 2.53:1
Timing base 12 degrees with 34 all in at 3000 rpm.

Cruises 2300 rpm at 70 mph

with about 1200 miles on the engine I took a 500 mile trip to Des Moines Good guys.

I averaged about 11 miles per gallon. Does that sound correct?

[Chief of the 60's]

I get 18 mpg on pump gas with a hell-of-a-lot more power, a 700r4 and 4.30's. Something in your setup or calculations doesn't seem right.

I should say that my number is an average. At 70 mph I am doing between 2600 and 2800 rpm.

[adynes]

Do you run vacuum advance? It will help

[footjoy]

Originally Posted by adynes

Do you run vacuum advance? It will help

I am running vacuum advance 10 degrees on manifold vacuum.

also running a unilite conversion.

["QUICK-SILVER"]

Originally Posted by Chief of the 60's

I get 18 mpg........

I should say that my number is an average. At 70 mph I am doing between 2600 and 2800 rpm.

041 cammed 400 in my car. 3.08 gear w/ 28" tire.
23~2400 RPM (close to 65 MPH) gets 10~12 MPG.
2800 RPM, 75ish MPH gets 16~18 MPG.

The engine just ain't happy (running free) lugging at 2300.

Clay

[Navy Horn 16]

I get about 17 on the highway. Do you have a lock-up convertor or no?

[footjoy]

Originally Posted by Navy Horn 16

I get about 17 on the highway. Do you have a lock-up convertor or no?

No lock up

[Chief of the 60's]

Originally Posted by adynes

Do you run vacuum advance? It will help

No. No lock-up convertor either. 31 rear tire though.

[Chief of the 60's]

OP, is your speedo calibrated correctly? It will make a big difference in your calculation. Mine is right on at 40, 50 and 1/2 mph off at 65 per the local sheriff's radar.

[dataway]

My stock 68 with 2.93, auto, would get 17-18 mpg at around 70-75 mph. (with original Q-jet).

I've heard a lot of people over the years mention 17-18 mpg with anything close to stock and with reasonable gearing.

[android 211]

Do you know the jetting is correct? (AFR gauge etc)

[Bruce Meyer]

What intake manifold?

[geeteeohguy]

11 MPG? YIKES. My tripower 389 powered 3.36 geared 4 speed GTO gets 14-16 mpg. And my '67 GTO ragtop is built almost identical to your car and gets 21 mpg at a steady 75-80 and has gotten 23 mpg at a steady 65. Mine is a '67 400, bored .030, flat top pistons, #15 heads (1970 455 87cc) , 068 cam, factory manifolds, original Quadrajet, original points distributor, Turbo 400 (no OD), and a 2.56 rear end. My base timing is 8 BTDC with all in 36 at 3500 and about 50 at light cruise 2000 rpm. I've been consistently getting 21 MPG highway with this car for the past 7 years and 10,000 miles. I suspect your fueling system is too rich. My car weighs about 300 pounds more than yours and I am getting double the fuel mileage...all with 50 year old parts.

[chiphead]

Quikfuel carb likely doesn't help. Add more vacuum timing. At 2300 RPM, the mechanical advance isn't all-in, so you're running slightly retarded timing. How are engine temps?

[footjoy]

Engine runs between 185 and 190 on a 94 degree day

[footjoy]

Originally Posted by chiphead

Quikfuel carb likely doesn't help. Add more vacuum timing. At 2300 RPM, the mechanical advance isn't all-in, so you're running slightly retarded timing. How are engine temps?

I am not sure what you mean by add more vacuum timing. Does that mean a different vacuum advance?

[footjoy]

The reason I questioned this gas mileage is because my 64 GTO 4 speed tri power got 19mpg with 323 rear

[chiphead]

Originally Posted by footjoy

I am not sure what you mean by add more vacuum timing. Does that mean a different vacuum advance?

Possibly. You've got 12 initial and 22* mechanical adv for a total of 34 degrees at 3000, right? With 10* of additional vacuum advance, that's 44* total at cruise at or above 3000 RPM.

But at 2300 RPM, you might have half the mechanical advance, so instead of 22* mechanical, you've got (I'm guessing) 11* mechanical at cruise.
So 12* (base) plus 11* (mechanical adv) is 23* total. Add the 10* of vacuum and you're at 33* total at actual cruise RPM of 2300.

Now the lower engine RPM may or may not need more timing, but only experimentation will show it. There are 3 ways to increase timing at your cruise RPM:

Add more vacuum advance timing, or
quicken the mechanical curve by changing the springs, or.
Give it a few more degrees of base timing, which will add timing everywhere.
If you could add (lets say) 2-5 degrees of timing at cruise, you can see what effect that has on engine performance, octane requirements and fuel consumption. If it isn't any better, go back to your original settings, as too much timing is just as bad as too little.

The vacuum advance is the "safest" play to add timing, as it is a load sensing device. As such it adds timing at light load and will reduce timing during heavy throttle.

Fine tune the ignition timing and then work on the carb.
And I'm serious about the carb too, a Q-jet would give better mileage than most hot rod holley carbs.

[carbking]

The first thing to check is your numbers.

If you have a highway handy with mileage markers, do a 20 mile run and check your odometer against the mileage markers.

Rear gear change or tire diameter can greatly effect the accuracy of the odometer.

I don't have a 400 A/T to compare, but my highly modified 350 4-speed with 850 Carter does almost double what you posted overall; have not checked highway mileage.

Jon.

[footjoy]

The speedometer is 1 mile off on hiway mileage signs and GPS. At 70 my speedo says 69

It has an Edelbrock performer intake and quikfuel 735 carb

My vacuum advance is not adjustable should I get one that is?

Thanks