[MRGTO2U]

Hi everybody!
My 68 GTO has a 428, Carter AFB, Headers, 6X Heads, Ultradyne 1/4 Cam, etc. It made 385 rear wheel HP at the dyno. Even driving easy only punching it once for a whole 21 gallon tank of gas I only get 7.5 MPG. Does this sound right? I fill it up with 93 Octane and I get 150 miles, thats it.
Please tell me if that number sounds ok. I really do not want to change anything because she runs like a beast.

Thanks,
JG

[Ollie]

Who cares about gas milage when you have the FUN FACTOR.

[68lemans462]

Originally Posted by Ollie

Who cares about gas milage when you have the FUN FACTOR.

yup. I purposely do not calculate gas mileage in my pontiac. If you want a fuel efficient car, get a honda.

[BVR421]

Similiar 428 in a heavier Bonneville. I get 12+ around town. I suggest a tuning problem.
Vacuum advance on the distributer? on mine it makes a huge difference on the street, not only with making the idle circuits work better with a 230 degree cam, but the increased timing gets WAY better gas mileage and healls greatly with heat in the summer. That car has a Qjet, Ive used a late type AFB and it ran well but used more gas though not as much of a gas hog as when I tried a Holley.

[BILTIT]

Tranny? Rear gears? My 400 gets 11.5 on the highway, is that good or bad? It runs like hell thats all that matters! Maybe a fuel leak?

[JKGTO]

My 68 GTO with 400@350HP, M20 and 3:55 gear only got about 10 mpg around town and 12 on the highway. That was when I drove it easy too! Didnt want to check mpg when I was hammerin' on it. Jim

[THEGTOCHISM]

Gas milage what's that? Seriously have you checked your distributor vacume advance, and total timing?

[Karch]

Carb settings, float levels, fuel pressure, vac. advance, initial timing, cam, so many variables.

Biggest is driving style.

What else...slipping trans, rearend bearings, ft. wheel bearings, brakes dragging, going uphill too much

Altitude...

[Geoff]

Sounds like there is a Holley under the AFB skin!! LOL.

Is the choke adjusted correctly? Check the power pistons are not stuck in the UP position, check jetting if the carb history is unknown. Check the float level; Carter publish a chart that correlates fuel pressure to float level & needle & seat size. These factors effect final float level & are all interrelated. Also make sure the vacuum advance is connected to manifold vacuum for best idle quality & fuel economy.

[MRGTO2U]

Tranny: Turbo 400 with B&M street mild shift kit.
Rear gear: Brand new 3.55 Auburn Posi
Car had when I bought it last year a 2.78 open rear that I replaced 2 months ago(major improvement). The only draw back is that at 60mph she does around 3100 rpm. But acceleration is so much fun now. As I can see I am not so far off on mileage. I have the original Quadrajet but I understand that if I use it, performance will decrease.

[66bonne]

I don't get mileage with my 455, I get yardage.

[BVR421]

Originally Posted by MRGTO2U

I have the original Quadrajet but I understand that if I use it, performance will decrease.

what sadly misinformed person told you that?