Surprising fuel mileage
 

Took my lemans out today for a long run through upstate New York. It was my first long drive sice doing the engine swap last month.

The car had its born with 350 2bbl (with AC, T400 and 2.56 safe-t-track with factory duals. It still ran nice but I had in mind for many years to transplant a 350 HO into it. Never found the right one so instead I used a 69 YS GTO engine, gave it a quality stock rebuild with a compression cut to 9.5 and a mild 066 equivilant cam. I make it visually correct as a 350 HO with painted valve covers and the unique 350 HO air cleaner which was basically the single large snorkel unit like a 68 GTO but with a black lid.

Really nice set-up with the gearing; smooth, quiet but effortless torque for passing and hills compared to the 350-2V. What was amazing today was my mileage. With the original engine I always ran 14mpg (US gallons) around town and 15.5-16 on country cruises.

Today I averaged 20mpg, 175 miles and I was reading a hair under 3/4 (which is actually halfway of usable fuel) when I fueled. I double checked all my numbers and it's right. I'm curious to see what my around town mileage will be like.

Amazing to have a 25% improvement despite adding 50 cubes. I guess it shows that a fresh engine helps and that a Q-jet driven gently is much more efficient than a 2bbl.

My GTO's on the same run average about 16mpg and 13-14 around town which I guess is related to gearing and cams (my GTO's are ram air cars with 068 cams.

What kind of mileage do you guys get?

milage
 

Ten mpg :gag: 3.90 fun rear gears 27" tires.
I meet a lot of people at the gas stations :D

Attachment 337113

Glad to see such good mileage for you. Back in my high school days I had a 69 GTO. Standard 350 horse (YS), Turbo 400 and 3.23 gears. My good friend had a 69 LeMans with the same 350 cube 2 barrel that you had. We made numerous out of town trips going to ball games. We always competed with each other on gas mileage and I always won. We followed each other so the distance and speed was the same for both cars. I rarely beat him by more than 1 mpg but my car was a whole lot more fun to drive. Jimmy

John, about 3 years ago I changed out the 3.36 rear end in my '67 GTO convertible (TH400, 400 engine, 87cc iron heads (9.3:1) and 068 cam) and installed a freebie 2.56 posi that a friend had. I got the same results. I average 20.5 mpg at a steady 75-80 mph, and the car is smooth and quiet at those speeds. Passing power is excellent, and it'll hold 2nd gear until 105-107 mph at WOT. Off the line, not as responsive as the 3.36 gears, but still just fine. Well worth the trade off. I believe that the Quadrajet has much to do with it: it's a super efficient carb at light throttle cruise. I used to be 3,000 rpm at 70 mph....with the 2.56 rear, it's a tick over 90mph at 3,000. Car runs nice and cool, and gets the same MPG as my modern 4Runner, so I drive it a lot more on long trips now, too.

Good to hear I'm not alone, never thought I would add a bigger engine with an extra 100horses and a much bigger carb and end up with 25% better mpg!

I'm just starting to drive my sprint firebird with a Qjett ohc, wonder how it will compare? 150 less cubes but 3.55 gears.

There was a car life comparison test back in 69 that compared a 69 lemans, Buick special with a small v8 2bbl against a 4bbl chevelle, the chevelle was much faster and got significantly better mileage to boot. I have it somewhere.

That GTO engine, while fresh, is probably right on a sweet spot for torque at the speeds driven with that gear.

I can tell you my Firebird Sprint, back in 75, got about 16-17 highway with the factory 3.55/3-speed. Bumped to a 3.08/M-20 and got 21 highway, then a 2.73/M-20 got it up to 25-26 highway, but was horrible to drive around town; could not get it rolling. It was like starting off in 3rd gear. The first driveline change was when my Dad owned the car. I changed to the 2.73 after the 3.08 broke and I grabbed the first thing for cheap from a junkyard, the 2.73 from a Nova.

North,

Nice MPG numbers.

I always find these fuel milage thread interesting. I wouldn't have thought the 350 would loose out on a MPG challenge to the 400. The 350 numbers of 15-16 seem low to me. I would have guess they should have been much closer to 21-23.

There was another thread where a guy had a 326 2sp 2:56 gear, then went to a th350 trans and then to a 350 motor then a 400 motor. He said the 350/350 combo got the best milage, close to 25 MPG if I recall.

On another thread a guy mentioned how much fun his small valve head 400 with 2:56 gears was and was getting close to 25 mpg.

If you peruse period car mags from the late 60's and look at the mpg of the tested cars it seems like mid size small block cars were in the mid teens, big inch luxury cars like GP's, rivieras etc were in the 11-14 range and highly tuned muscle-cars (like RAIV) were in the 9-11 range. (these are all average city-highway numbers after weeding out reported numbers that were clearly hard performance testing mileage)

I noticed that Gm cars generally fared best and ford was the worst.

A good portion is the gears and the engine setup. My stock '78 Bonneville with a 400 and 2.41s would get 19-20 on the highway and 13-14 in town while my '75 Trans Am with a 400, Melling SPC-7 cam, and 2.56 gears gave me 19MPG when I drove it to San Diego this past summer. My '72 350 2bbl Lemans with a 2.78 rear end never gave me any better than 14.5 in town and 17 on the freeway.

I am not surprised at those figures at all. I'm the original owner of a 69 LeMans 355 H-O 2dr hard top. I ordered it as a 330hp, T-400 and 2.56 open diff. The car ran high 14's with a air bag in the rt rear spring. It would go thru the traps in 2nd gear. Funny thing was most stock 69 350hp GTO's just about ran the same numbers. I remember one street race I got into on a rolling 35mph start with one such GTO. the car must have had 3.55's and we stayed side by side longer than a quarter mile. The guy was pretty surprised when my third gear came on at 105mph and walked away ( figuratively speaking ). After a few years a rod bearing went south I picked up a 69 355 2bbl with 9.2 compression a 254 cam and #47 heads ( still larger valves than some performance 389's ). The car was surprisingly quick, but suffered the large 2GC mileage problem just as you said. The 355 H-O got 24mpg on the hwy @55mph ( remember those Jimmy Carter 55mph speed limit days) the 355 2bbl was around 15mpg and it was in excellent shape.
Now the car is 456 ( 462 @ .030 )Powered with 2.56, 3.08 or 3.23 and the car is sometime used for grand touring with all the suspension mods. See the link;
http://www.pismoderelicts.com/photog...s/p1020752.jpg
http://www.pismoderelicts.com/photog...s/p1020753.jpg

Best I ever did was with a '69 Firebird. Original 350 2bbl car with probable 2.78 or 2.56 rear axle. High teens stock. Swapped cam to a mpg grind, the Crower "Baja Torque Master", true duals, dual snorkel air cleaner with hoses to front grill, recurve distributor, 7029268 Rochester and cast intake. 22-23 mpg highway. I did 125 mph in it once late at night on a empty highway racing a Chevelle which I dusted, it had more left.

Here is a pick of my engine bay. The correct air cleaner seems to be a bit of a mystery. Factory photos all show the single large snorkel with chrome lid (ala 68-69 firebird 400 and 68 GTO). But the original part number in the MPC is unique. Maybe that was the single large snorkel air cleaner but with a black lid.

Since I had a mint chrome lid and no nice painted lids I use the chrome one. They never used chrome valve covers on 350cid engines.

Right side up pic as my neck hurts turning around that much :focus:

I don't know how to fix this when posting directly from the phone. All my pictures appear right side up in my photo gallery. Is there a way to rotate the photo on my iPhone??

Yeah the smaller Poncho's make great MPG, with no loss of power!

My 67 Tempest wagon has 27" tires with 2.56 rear with a 4 speed and it gets stellar gas mileage with a 570 CFM Holley 4150 carburetor.

I think the best I got was 24ish but routinely get 20-21. And as far as aerodynamiccs go the wagon is similar to a Chevy Astro van. Brick with wheels!

What that means is that you need to drive it more!!!

I had a 69 4 spd GTO when I was in the service in the early 70's. The gas lines on the base were a lot shorter than those at home during the first gas shortage in 73.
I remember going back and forth between home in NYC and the Naval Base in Newport RI where my ship was homeported on some weekends during the height of the gas shortage and not only trying to stay at 55 MPH, but going out of my way to keep the secondary's closed on the Q-jet.
The result was some pretty amazing gas mileage. I made the roundtrip (about 370 miles or so) a few times on one tank of gas, and still had a decent amount in the tank when I got back to the base on Sunday night. I remember averaging a little bit over 20 MPG back then.
Of course, the trick is to try to keep the secondary's closed. That's always a challenge.

Do u mean when u look at them on your phone or after you download them to your laptop/pc?

You simply must have taken them while phone was not right side up(though iPhone will rotate them when u view them on it)...Cannot rotate them on the iPhone once they are taken but can if you download to laptop using Windows viewer...Vids cannot (unless there is a 1rd party S/W that may now but I haven;t looked for one)...

I don't want you to have an incorrect engine compartment. Just PM me and we can exchange that wrong chrome lid for a painted one.

Thats what I figured

I got lucky, found it on a 69 GP, up close you'd swear it was NOS. Even has the green inspection dab.

Yep. Long ago our 70 400 GTO 3-speed stickwith 2.54:1 Type "O" rear would pull >20 MPG Highway. crazy good MPG from 16 heads and 068 cam.

Efforts to beat that all failed, except my former 455 (48 heads) & big 252 @.050 cam, Th400 into 3.31:1 rear would shoe excelleng highway MPG yet terrible backroad MPG.

I finally figured on OVERLAP EGR as the great equalizer. So my 68 GTO (455, 9.0:1) 236/244cam, Th400 into 3.54:1 rear do both backroad & highway MPH terribly.

And that is why the OVERDRIVE trans is in process.

With the Gear Vendors, TH400, Hughes GM25 convertor, 3.42 rear,27.4 inch tall tires- on my '73 455 with 041 Melling cam, I consistently get 18 mpg on the highway. I did actually get 21 one time, but that was when I had an 066 style cam in it. Haven't checked it around town, but it is mediocre, because I keep my foot planted in the "secondaries" of the Q jet from time to time.:D This winter, I plan on stepping back a little on the cam, as well as installing a stock convertor, I figure my mileage will go up on the highway and around town at that point. A good convertor can affect gas mileage more than a lot of people may realize as well. Nice to hear of a Pontiac 400 with factory gears and tranny doing over 20mpg.:)

68 GTO, original 400 with 100k miles, Holley 650 spread bore, 2.93 rear, TH400, AC removed, windows down, at a steady 65-70 mph it would do 18 mpg on the highway. Around town ... 12mpg, driving like an 18 year old back in 1977.
JohnnyB