I'm curious what kind of mpg you guys are getting?
The best I've gotten so far was about 18 with a 305 and I think 3.42 gears.

 

stock 305 LB9 with auto, 2.73 rears got sumthing like 25mpg. i tested my 350tpi with 3.23, and an auto and everything in my sig below with AFPR set at 48psi and im getting 18-19mpg highway... done at 65- 70mph.

dont quote me but ithought the stock 305s got around 25mpg and 350s got 20mpg

 

So far I haven't drove my 91 trans am much, needs new front tires before I really drive it (dry rotted) and nobody stocks 245/50/16's around here so I had to special order them

Anywho, the trip home when I bought the car was about 120 miles and I drove the car with a rather light foot as I didn't want to break something before I gave it a good once over in my garage. about 90% of the trip was on the highway at 70-80mph followed by 55-65mph country roads (very little stopping).

Once I got within 10 miles of my house I opened it up a bit with a few hard launches and WOT runs to 90 mph.

Enough of my life story lol.

Stock (as far as i know) L98 5.7L, 700R4 trans and 3.23 rear gears (according to RPO, seems right) I averaged 18.3 MPG from the gas station where I bought the car to my house plus a 4 mile drive back to the gas station in town.

I'm positive I can hit at least 23mpg if i drive with a light foot and keep it under 65, those 3.23 gears aren't the most efficient at 70+ but at 60mph I'm at about 1600 rpm in OD.

 

I average 19.5 mostly city driving, 22-23 highway in my 91'.

 

My 85 T/A, LB9, 700R4, 3.27s, shorty headers, 3" back to a Spintec muffler. Mixed driving, I get about 24. When we were cruising on the Power Tour last year, I was getting ~30, when we weren't spending hours sitting in traffic.

 

3.73s w/ 2500 stall converter, no lockup - I can do 18.5mpg mixed if I'm good to it. Turns about 2700rpm at 70mph on the highway. On friday nights w/ lots of speed competitions, I get 15mpg if I'm lucky. Mild carbed 305. Back when I first got the car, on the trip home with it from NYC I managed almost 20mpg w/ the lockup working on the converter and the CC-Qjet. Wiring harness was a cob job and was screwing all sorts of things up, so out w/ the CC stuff and in w/ the computerless setup - havent looked back since, I'll trade that 1-2mpg difference for the easy of operation, tuning, and soggy response of the computer controlled system.

 

i would love to get 15 mpg, my carbed 305 5 spd with 3.23 gears gets maybe 10 mpg. i dont really do any highway driving and i really cant complain cause i do drive it a little on the hard side cause the exhaust sounds great lol

 

hmmm.... maybe I don't have 3.42s, my car does 2000rpm at 60 mph, and I assumed they were 3.42s but with John81 turning 1600 rpm at 60 with 3.23s, makes me think maybe I have 3.73s?? anyone know what rpms I should be turning at 60 with what gears??

 

http://www.f-body.org/gears/

3.42's should be tuning aprx - 1860rpm w/ a locked torque converter, 245/50/16 tires and a 700R4 at 60mph. 3.23's should be doing 1750rpm in the same scenario, and 3.73's turn about 2050rpm. Stock tachs are known to have quite a bit of variance.

Raise the speed to 80mph and you see a lot more variance between the gears, 3.73's do 2750rpm, 3.42's do 2450rpm, and 3.23's are turning 2350rpm. A 2.73, just for fun, turns 1500rpm at 60mph, and 2000rpm at 80mph.

So, whats your rpm at 80mph or even 90mph? That will give you more of a difference to better figure that ratio with.

 

I just drove from California to North Carolina, got 23.5 mpg at 65-70 mph, 25 mpg at 75 mph and I get about 17 in the city.

 

john81 rpms are really low... i dont see how tho. i have the same and im pulling 2000 at 65mph... still i have a 18-1900 stall but his is like having a 2.73.

i would jack the car up and do a simple test. mark the driveshaft and the tire. rotate the tire one complete full turn and watch how many times the driveshaft turns... thats your rear gear ratio. 2.73 times of the driveshaft per 1 complete wheel turn means you have 2.73, same for 3.23, 3.42, 3.73 etc

 

I have got anywhere from 18mpg to a 26mpg with it falling in near 24 mpg usually
89 350tpi 2.77 gears

 

I need help posting a thread and I can't seem to figure out how to do this.......and another question....I am going to try to post a thread looking for a transmission for a friend of a friend....and I am not sure if my information I am posting is all messed up think you can give me a hand with it?

 

I have an L98 in a 72 El Camino. Engine is all stock with the 700R4 behind it and 3.73's in the rear. I run a 275/60 R15 rear tire and with the A/C off 22 mpg and 20 with it on. This is on my daily commute of 21 miles each way on rural roads at 60 mph.

Here are some photos of the engine with the cowl induction air box.

 

I finally got new front tires this morning and actually paid attention to my RPM's on my drive to work. I'm at 1800 at 60 and 2400 at 80. With what atc3434 posted for RPM@MPH it appears that my RPO is correct.

Next chance I get I will be jacking the back up and counting to be sure but so far this trans am seems to be completely unmolested except for the aftermarket american racing rims, even has the original AC Delco stereo lol.