Est. MPG on a 64 L75

[M W.]

Hi all,

I was wondering what the standard or estimated mpg on a all original but restored 1964 327-300 HP with a 3.36 posi-trac should be? I should also add it's a 4-speed. I figure right now it's only getting between 5-7 mpg. Not that I really care but that figure I gave does seem way low for such a light weight car. Any idea's or suggestions is greatly appreciated.

Thanks,
Craig W.

[Donald T.]

Re: Est. MPG on a 64 L75

5-7 MPG would be VERY low for that car especially with a 3.36 rear. Should be around double that for normal driving. Of course, if you really have your foot in it...

[M W.]

Re: Est. MPG on a 64 L75

Don, how would one go about increasing the mpg? I had the card completely rebuilt to GM specs. and really leaned out the card the best I can. Not sure what else to do.

Cheers,
Craig

[Joe L.]

Re: Est. MPG on a 64 L75

Don, how would one go about increasing the mpg? I had the card completely rebuilt to GM specs. and really leaned out the card the best I can. Not sure what else to do.

Cheers,
Craig

Craig-----


5-7 MPG for your application is way too low. Most big blocks do better than that. I would say you should be seeing about 11-12 MPG for mostly city driving and about 15-16 MPG for the highway. Something is "out-of-whack" and it may not be the carburetor.

[Jean C.]

Est. MPG on a 64 L75

Craig,
My '64 has the same engine and drive-train config as your car. I log many miles on my car on long haul trips (5,971 miles to Boston and back last summer) and usually get 15 - 16 mpg when cruising. Last time I checked city traffic mileage it was +/- 10 mpg. The mileage figure you are quoting is really off the mark for this engine/drive-train combo so something needs fixin' ASAP!
Best regards,

[Jean C.]

Re: Est. MPG on a 64 L75

I might add that my '64 is NOT air conditioned...yet.
Cheers

[Robert R.]

Re: Est. MPG on a 64 L75

Craig,
Do you spend a lot of time with the engine running, but not going anywhere?...like letting friends hear the engine, or tweaking it for tuneup purposes? They all add up, or take away from true mileage average.

Just a thought, I am guilty of it as well.

Bob

[Michael A.]

Re: Est. MPG on a 64 L75

Charlie - let me know how the A/C goes. I just completed 5210 miles in 2 weeks without A/C and I can say that I wished I had it! Thanks again for your suggestion on what to pack for the long trip. Your advice was excellent.

Craig - I have a 66 327/300 and just averaged 14.9 for 5200 miles for a trip from Chicago to LA. The best I got was a stretch across the flatlands of Iowa with 17.1mpg. In town I get 11 or 12. The 5 to 6 you are talking about is really bad. Beyond the carb you discussed there could be issues of timing. I know that when I wanted to improve the driveability and performance of my car I followed the info in John Hinckley's series of articles on ignition timing in Corvette Enthusiast from a few years back. They were excellent for a newbie like myself. Driveability improved as well as mpg by about 2 mpg. If you'd like that series of articles I'd be happy to email them to you. Send me a private message and I'll pass them along.

Cheers,

[Don H.]

Re: Est. MPG on a 64 L75

I have a 65 300/327 (automatic) and get approx 14 with the AC on, 16 with it off. Many years ago I purchased an identical car to this one except it was a 64. I drove it home from Florida to Iowa and only got 8-10 mpg. The problem was the vacuum advance. Don H.

[Michael A.]

Re: Est. MPG on a 64 L75

Don - The John Hinckley articles I mentioned in my earlier note really helped me understand how the entire distributor/timing system worked and led me to the fix that helped my car. It didn't double the mpg but it did help improve it by a couple of mpg.

Cheers

[David D.]

Re: Est. MPG on a 64 L75

Craig,
When in the back of the car do you smell a strong (eye burning) rich mixture in the exhaust? Have you ever checked the fuel pressure to the carb? I believe should be 6 or less.....depending on pump.
Have you confirmed you don't have any type of leak(s). I know that seems like an obvious question, but even the smallest of leaks, could be evaporating as quick as leak.....
Good Luck,
David

[M W.]

Re: Est. MPG on a 64 L75

Thanks to all who have offered advise. I have doubled checked everything for leaks, pump pressure and any foul odor's. All is tight and clean. I am just a bit stumped but will ask my local mechanic again today what we should do.

Thanks,
Craig

I will keep all of you posted

[Stuart F.]

Re: Est. MPG on a 64 L75

Are you sure you have the right speedo drive gear in the transmission? Also, what size tire are you using?

Since I put P205-75R x 15 Radials on my 63, I've noticed my speed reads low by about 3 MPH (my son and I did a side by side test on the 4th).

Also, for what it's worth, I still have the mileage data from several trips between Wisconsin and New Mexico back in 1963. I was pleasantly surprised then as I recorded between 16.5 and 20.1 on the road with my 340hp w/Duntov, 3.36 rear and 6.70 x 15 bias plys. The lower readings were in hilly country, or perhaps after I had it broke in and my foot got a little heavy. I would expect better with your engine combo.

Stu Fox

[M W.]

Re: Est. MPG on a 64 L75

Are you sure you have the right speedo drive gear in the transmission? Also, what size tire are you using?

Stu, I have no idea what speedo grear is in the transmission. Unless it's been changed before I bought the car it still has what GM gave it. As for the tires, I am running 205/65R15 radials. It baffles me but I will figure it out. Thanks for everyones help.

Cheers,
Craig

[Louis T.]

Re: Est. MPG on a 64 L75

Hi Craig,

Your tire size wouldn't account for such a low mpg, but in your case, it actually makes matters worse. While there are, of course, variances between manufacturers and models of tires, and a certain amount of expected speedometer error, doing some quick calculations, your tire size results in a nominal wheel height of 25.5 inches vs. 27.1, the nominal height of a current metric radial replacement (205/75R15) of the original 7.75-15 bias ply size (I assume that was the original size for a '64 - it is for a '65). That results in a 6% reduction in circumference (80.04 inches vs. 85.11), which means you're registering 6% more on your odometer than you're traveling. So you're probably getting at least 1 mpg less than you even thought. It seems to me you might be in leaky power valve territory.

Regards,

Louis