Anyone know the original highway rating for this combo, please? ( 5.7L of course )

I think it was 16mpg city and 24mpg highway. I've got the 2.77 geared 9bolt in my IROC. My mixed fuel economy genrally ranges between 18mpg - 21mpg.

Thank you! Anyone else confirm or refute?

I have seen a 1988 Formula 350 sticker listing 16/25 and a 1988 TA 1LE 305 listed as 16/26. My Camaro with a 350 is marked at 16/25 and I've seen 1987 Trans Ams with the 350 at 16/25.

http://www.fueleconomy.gov/
http://www.fueleconomy.gov/FEG/bymod...emodelNF.shtml

Dont know what my iroc is rated but I have the 2.77 rear and 350 tpi. Window sticker wasnt in the car when I bought it off a dealer lot in 96. I have calculated the mpg for highway and it was around 21 mpg. +/- a couple miles depending on how my foot was giving it gas on the highway. Good thing is that the engine rpm at 65 mph is about 1750. 40 mph is practically idle rpm.

Mine is rated at 15 city and 23 hwy

Error. Which might explain your below-normal MPG. Should be a red 17 tooth drive gear for the speedometer, and a 37 tooth driven gear. Correct is 1640 RPM at 65 MPH. And 21 is a shame. I had a carbureted '78 Camaro with 350, swapped 700R-4, 3.08:1, 235/60R15s on 8s, and the big rear spoiler, do 25.1 MPG at 65 MPH with no centrifugal advance in the distributor, running from SLC to Wellington. Fixing the dizzy would've been at least 26 MPG. That was 1800 RPM at 65 MPH, with 18:43 speedometer gears.

What I posted is that its about 1750 rpm at 65 mph. Tach doesnt read 1600 rpm but the needle is in that area at 65 mph. I think 21 mpg on the highway isnt that bad for my car or any other thirdgen with the same set-up. Have to remember that its not all highway driving either when I calculated mileage but most of it is highway. MPG on the window sticker is just an EPA estimate. I work for a ford dealership and the window stickers say estimated MPG but actual mpg are around 5 or so miles less than whats on the sticker.

Yeah, but Fords have always been bad for MPG. With GM it's not challenging to do 10% better than the EPA highway ratings. To be fair to Ford, I did get a 2.73:1-geared 5.0 / T5 GT to do 30.06 MPG, once. No tailwind, and the pumps weren't suspect, nor was the odometer. Just keeping the MPH down. This thread started after I looked at the EPA ratings for the '84 Z28 L69 / T5 / 3.73:1, and an LB9 / T5 / 3.45:1 IROC-Z, can't recall what year.

My 89 Formula 350 Convertible actually got 26 Mpg from North Carolina to Flint, MI Highway 70 - 75 Mph...

I would be surprised if your GTA has the 2.77 gears, it should have the 3.27 by default, unless it was a Canadian Car or a Trans Am... There are a few out there but the ones I can recall were export models. Almost all LB9/M5 Formulas and GTA's have the 3.42 or 3.45 gears and the GTA 350 and Formula 350's should have the 3.23 or 3.27 gears...

With an Auto @ 70 Mph in OverDrive
2.73 or 2.77 Gear = ??? I do not remember
3.23 or 3.27 gear = 1750 RPM (approx)
3.73 or 3.70 Gear = 2250 RPM (approx)


John