[78w72]

Originally Posted by 3tas4me

I should clear up the fact that my 12.5 mpg is both at 55 and 65-70 mph. Speed doesn't seem to matter. My 1979 W72 T/A has the factory 3.23 gears and a 4-speed with the only mod being a true dual exhaust system. I am able to get 17 mpg with that one. 15-16 mpg shouldn't be out of the question with the '74.

Yes, these cars should have been built with overdrive. That would have been an easy solution to increase gas mileage. Maybe someone can chime in why GM (and others) didn't pursue this solution?

yeah sounds like something is off at 12.5, hopefully fixing the vac adv helps that. i know my MPG changes pretty drastically at higher speeds on my carb'd cars & on my modern FI daily driver SUV, the onboard mpg calculator although not 100% accurate notices a 15mph increase.

as for the OD on these cars, wasnt that before OD was around or used on a wider scale? OD didnt come around until the early to mid 80's for GM with the 700 & 200 trans.

[71GP76TA]

Originally Posted by 3tas4me

I
RPM at 70mph is about 2750 with the P255/60 tires I put on the car. .

Wide tires can have a negative effect on fuel economy. You could be losing 1-2 mpg due to the wider tires.

I never checked the mpg on my '75. I'm guessing its around 14-15mpg. the car has been de-smogged and has true dual exhaust.

My '73 averages 13-14.

my old '70 RAIII 4 speed got 13 on the highway.

[RWS]

That's terrible mileage for a 400!

Even before installing a Tremec TKO 600 5 speed, i was averaging 20 mpg in my '70 Formula 400 RA III with a 4 speed and 3.73 rear end at 60 mph.

With the Tremec, the car averages 25 mpg at 75 mph.

While better fuel economy is certainly a plus, the real benefit to me was dropping the RPM at highway speeds by a third and gaining a steeper first gear (the car came with an M21).

As I've said in previous posts, that's the best money I've spent on the car (and I've spent lots!)...

[3tas4me]

I have bypassed the vacuum solenoid but have not had a chance to drive the car and test it yet. I will report after I drive it.

[TAKerry]

WoW, I need a couple of you guys to tune up my car. I have a 77 with a 400. Slightly warmed over but nothing crazy. I figure about average 12 mpg. I remember when these cars were new they didnt get much better than that. Heck, my wifes new mustang with a v6 barely hits 24 mpg!

[77 Canamman]

My '77 consistently hits 17MPG on longer trips. The long block is totally stock. Recurved HEI, rejetted Q-Jet, headers, PYPES tranverse exhaust, 3.23 gears.

[3tas4me]

I have driven the car now with functioning vacuum advance. The fuel mileage is better, but I have not given it much of a test yet. I can say that it is not running quite right yet, I will do some more trouble shooting.

[Formulabruce]

Couple points.
1. My Buddy has basically same car, but a Formula factory 3.42 posi, Muncie, late April build. Has emissions unhooked, so no EGR Or vacuum modification.
He runs 235 60's which are 26" tall tires.
He is going 3k at 70 easily. Fuel economy is around 12 with varied town, highway driving.
74 was last year of factory Duals and a 3.42 gear in second gen ( for the most part) .
2. Your 455 is working less than the 400 as it has more torque down low.
3. All cars today on alcohol fuel get WORSE fuel economy than when new. Look at a window sticker on any new 1/2 ton truck. Cruising range is cut by 40% compared to "real" gas.
4. Even 2.46 rear wouldn't help as much as a .68 overdrive .
Silversport has a bolt in 5 speed, No mods needed.