The facts:
69 GTO
Matching numbers 400 with Comp Camps 260 Magnum (212, 212 at 50, 440/440 lift), Q-jet, ram air exhaust manifolds, 2.5" exhaust system, stock distributor with Pertronix
Turbo 400, Holeshot 2000 converter
3.55 gears

How can I get better than the 10 mpg or so I get? I may switch the gears to 3.08. Anything else I could do?

The facts:
69 GTO
Matching numbers 400 with Comp Camps 260 Magnum (212, 212 at 50, 440/440 lift), Q-jet, ram air exhaust manifolds, 2.5" exhaust system, stock distributor with Pertronix
Turbo 400, Holeshot 2000 converter
3.55 gears

How can I get better than the 10 mpg or so I get? I may switch the gears to 3.08. Anything else I could do?

If you go back to a mild camshaft grind
and install the higher rear end ratio you should
be able to get 16 miles per gallon out of it.
The only other thing that would help is transplanting an OHC 6 cylinder engine or trading it in on a Toyota.

HPP had an article (Jan 2004) on the installation of Crane's adjustable vacuum advance, in which they picked up 1.5 mpg.

The article says engine modifications, and today's fuels, can cause the stock advance canister to not work optimally, or not at all, so you are not obtaining the optimal spark lead.

Summit sells Crane kit 99601-1. Its inexpensive and may make your car run better anyway, so its worth a try. I bought the kit, but have not put it on yet.

dont drive it, THESE are muscle cars designed to go fast, dont turn too much & hopefully stop.

GM wasnt looking for gas milage, only BEATING the competition.

just my opinion

I have a supercharged 428 in my GTO and my mileage went from 6 yes six mpg on the highway to 14 by installing a 200 4R overdrive trans.

All you have to do is recalibrate your odometer!

a gto is not about gas milage,sell or trade it for a bug

i think there is a bug forem on the net

My Goat runs high 10's and with OD can cruise at 2000 rpm on the highway. Mr. Nobs how about yours?

Not to sound TOO smart-ass, but inquiring minds want to know--what mileage does your 10-second Goat get, Bob W48? Better than the 10 mpg 71 T/A gets?

I get 15 mpg with my 4100 pound, 2.73 geared, 400 w/ turbo 400 62 Grand Prix and it runs 13.80@100 mph. I think you should get better then 10 mpg.

I got a 66 caddy, 429 that will get 13-14 on the highway!

Pontiac6269,
You have 2.73 gears while my GTO runs 3.55s.
There's the difference. I think I'll be swapping to 3.08s as that's the lowest I can go with my 12 bolt's 3 series carrier.

While you change to the 3.08 make sure you take up the advice to swap the th400 for the tr2004r overdrive trans , due to the shorter first and sec gear, your car with the 3.08 will acselerate as fast as a th400 with a 3.73 rear ratio . 3e is 1on1, 4 is 0.67 , so you have your stoplight funn AND you highway crusing milage and top end speed with it , nouw ask yourself wat will be the smart thing to do!!!!

Let's see, a TH400 has a low of 2.42, so the overall low in front of a 3.73 is 9.03, while a TH2004R will have an overall low of 10.25 with a final of 2.5, but one with a 3.08 rear will have a low ratio of 8.47 and a final ratio of 2.06, right? Figuring on spending most of the time in third or OD, it sounds like it would be more fun to swap the trans and leave the rear alone! A lot more responsive.

Kind of a oxymoron. MPG and GTO. Try this, push it around, you'll get great mileage with it that way.

Seriously, get a overdrive and a really tall gear.

Good luck.

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Shadowjack:
Let's see, a TH400 has a low of 2.42, so the overall low in front of a 3.73 is 9.03, while a TH2004R will have an overall low of 10.25 with a final of 2.5, but one with a 3.08 rear will have a low ratio of 8.47 and a final ratio of 2.06, right? Figuring on spending most of the time in third or OD, it sounds like it would be more fun to swap the trans and leave the rear alone! A lot more responsive. </div></BLOCKQUOTE>............Eather way 3.08 / 3.55 its a win win situation . i got 3.42 with the od trans . due to the high perf engine , i will have serius traktion problems as first and sec gear will act like 4.11 / 4.56 . thing is he didnt askt for FUNN ,but more mpg!! , but yes your right trany swap alone will dubbel the funn lololol

Shadowjack if you look at my original post I said it gets 14mpg on the highway. I have no idea what it gets around town but its not good.

Man, I have the TH400 and Iam changing my 2.93 Rear to a 3.55 posi as we speak...costing me $900...did I make a mistake? I can still change my mind.

Yeah, Bob, I eventually saw that a day or two after I posted. Fast fingers, slow eyes.

You think your eyes are slow you should see my typing. When I took my car on the highway for the first time after installing the overdrive I couldn't believe the mileage difference. I wish I changed to overdrive 10 years ago.

I have a 3.23 gear, continental 2200 stall converter, Turbo 400 transed '68 GTO. I get 10 mpg in town, 15 on the highway.

The best way to get better mileage than you're getting now is pretty basic. Fresh tune; light right foot.

Get the distributor checked to make sure the mechanical and vacuum advance are in good working order. The electronic conversion is a good way to go; less tune ups and more reliability. I've used the Mallory Unilite conversion with good success. You can gain a bit by bumping the initial timing up two or three degrees.

I have a Pertronix unit in the stock distributor. I'm happy how the car performs with it and how well the engine starts quickly in the morning. I have the distributor advance hooked up to ported vacuum in the front of the carb.

A new 200r4 would cost a lot and would be tough to justify with a T400 that was rebuilt 1000 miles ago. Is the Holeshot 2000 12" converter costing me gas mileage. Would a change to a lower than stock stall converter be a worthwhile investment?

Come on:

G - Gas
T - Tires
O - Oil

They use it up.

HAVE YOU CHECKED YOU GAS TANK FOR A SMALL LEAK. MY 2+2 GOT 10MPG. I WAS AT A STOP LIGHT AND A MOTORIST BEHIND ME SAID EVERYTIME I WOULD TAKE OFF FUEL WOULD SPILL OUT. SMALL CRACK IN FILLER TUBE. MADE REPAIR AND GOT 15MPG IN CITY. WITH 421 HO 3SP MANUAL.

Gas tank is new. There are no fuel leaks. 3.55 gears need to go.

Before you lose the 3.55 gears, make sure your carb is properly tuned to operate as efficiently as possible. Also, look into recurving your distributor. I have mine set up for 22 degrees mechanical in the distributor all in by 2,800. I also went to the adjustable vacuum. "Suntuned" on the classical pontiac board set up my distributor. Cliff R. is real good with quadrajets. Making sure these are dialed in can enhance performance and mileage.

If you still have the stock convertor it will help around town but at highway speeds the other convertor will be above the stall speed anyway.

if you have to worry about gas milage in a gto ,then maybe it should be parked,no my gtos are not all built up,my convert is stock exept for the ram IV cam and a hollybut i get between 6-8 which aint all that bad considering i drive it the way it was ment to be driven,now if you want to look cool in your gto and get good mileage i would suggest parking it on the frt lawn and just sitting in it lol

and i really dont measure the gas milage i measure how much fun i get out of a tank