Well, my 77 Camaro project is almost done. Just need to put the gas tank back in, change out the9 power steering pulley, and tie up a few loose ends. What I would like to know is the mileage I can expect. The whole idea behind going with a 4.8 was to get decent performance and great mileage in a car that will see about 150 mostly-freeway commute miles a week.

The motor is a brand new (2 miles) 4.8 with a 4L80, all factory computer controlled. Power is put down through the factory 2.56 one-legger. Any ideas on mileage?

The car is just about 3500 lbs.

 

send your computer off and get it tuned for fuel mileage and gear change

 

4L80E is going to make that 4.8 work, are you sure it's an 80? 80's use up a lot of power, you should be in the low 20's though.

 

Yup, it's an 80. It came out of a 1-ton Express van, and all of them have 80's. Actually, I thought I was getting a 60 when I bought it, but it showed up with the 80. I figured I'd keep it, just in case I got tired of the stock 4.8 later.

Anyway, the PCM has been programmed for the gears, so the shift points will be right. I was hoping for 25 MPG on the highway.

 

I would ditch the 2.56 gears. With the 4.8 I would want alittle more gear. Maybe 3.42's. JMO.

 

I wonder what the RPM difference is at 70 MPH between the two? I have a 26" tire on it now.

 

http://www.pszweb.com/car/gears.htm

70 mph would be about 1750 rpm with 2.56s and about 2350 with 3.42s.

 

Thanks!

 

Just to let you know my experience with the 4.8 and 6.0. The 6.0 gets better highway mileage as it never has to downshift like the weaker 4.8. However, you being in a light 3500 vehicle probably makes that moot.

 

With a 2.56 is he ever going to use 4th gear? I would imagine 23-25 is the expected range.

 

I would change the gear's for sure the 4.8 doesn't have anything down low and needs more revs to make power. I was getting 19-21 in 1500 4x4 with a 5spd and 4.8 and 3.42's so I don't see why you won't be able to get 25.

 

I have a 1981 trans am w/ a 6.0L LQ9, 4L80E trans, 3.42 gears. I drove it 450 miles a couple of weeks ago. I averaged 17.6 mpg for the trip. I went over 100 a few times and drove about 80 on the interstate. It varies between 13-15 mpg when I drive it in traffic and around where I live. I have about 1800 miles on the car. I haven't driven the car to maximize fuel economy. The 4L80 has more parasitic losses due to the increased rotating mass. If you're building a car strictly for economy, use a manual trans. My rpms at 75 are 2450. It basically mirrors the rpms of my 06 2500HD truck.

 

My 5.7 LS1 with a 4L60E in my 3400 pound all steel sedan got better than 26 mpg with 3.70 gears and 29" rear tires. Just for drill I thought I'd put a set of 3.25 gears in it to see if I could really knock down some good fuel economy numbers..........BAD MOVE.

With the 3.25 gears it was awful, the engine was never in a "comfortable" range, the drivability was terrible, in short.....it ruined the car, it wasn't even fun to drive anymore. I struggled to get 20 mpg.

A change back to the 3.70 gearset returned the performance to my happy, efficient, fun level. In the back of my mind I kinda knew that might happen, that's why I kept the 9" center section with the 3.70s ready to reinstall.

Just my two cents worth.

 

Looks like we're neighbors! I'm in Simi, too. I'd really like to see your conversion sometime, if possible.

Based on what you're saying, your 3.70 gears along with 29" tire are effectively a 3.31 axle ratio, when substituting a 26" tire, and that seems to be a sweet spot for you. What this tells me is, my ratio is way too low (numerically). I'll have to drive the car to Oregon next week for licensing, but when I get back, it looks like a gear swap is in order. It looks like I'll be cruising around 75 MPG at around 2K RPMs to Oregon. I hope it gets somewhat decent mileage!

 

In my LT1 with 4.10s around 2800rpm doing 75 i get 24mpg, about 2000rpm at 60mph i get around 21mpg. High rpms dont really seem to hurt mileage. I have read because the TB is open a little more therefore creating less turbulence in the intake or something.

 

Given that the intended application is maximum mpg, it would probably be worth your time to go through the car & see where you can drop some weight as well. On my first pass, I dropped around 140 lbs from my car, swapping from an iron/iron engine to all-aluminum (as well as changing to a 'glass hood) should allow me to get somewhere close to that again. (I'm hoping, anyway...)

You might not be able to drop that much, but I'd bet that you could find 60-75 lbs. without working too hard...

Good luck with it, it's a cool, out-of-the-ordinary project!

 

My 79 T/A had a 2:41 rear gear with a 700R4 ..it got horrible mileage. The rpm's were way too low , on any kind of slight grade my vacuum was below 8"- very inefficient. I changed to 3:73 ....what a difference...got around 21 mpg on the hwy.
With the computer control and mild stock cams you can probably get away with 3:23 or 3:42...I wouldn't go lower. Also the coefficient drag on a late 70's Camaro is lower than a 4 th gen...that will hurt you in mpg compared to the 4th gen.
My 74 Camaro LS6/700R4 with 3:42 got about 26 mpg with a good tune , cold air and LT headers( stock motor).

 

My Chevelle is about 3600, 4.11's, T56 and a 99 LS1.. turning 1750 at 65mph.. doing 70-75 I avg about 23mpg... If tuned right, no reason you should get mid 20's to high 30's.. at the speed limits... but it seem that your giving up a bit with that big 80', that 2.56 gear and the smaller troqure of the 4.8L.. might have to step on it a bit more to get it moving. that would be my .02 cents..

but .. your the one that is going to tell us for sure.. post your finding once you get a good avg over a longer distance...

 

I work in Thousand Oaks and usually drive the car to work a couple days a week. Tomorrow (Thursday usually for sure) I will be cruisin' the 'Oaks. I work at your friendly local Chevy dealer. Drop by!!

 

Thanks for the encouragement!

I've thought about weight reduction, things like a glass hood and bumpers, getting rid of the spare tire and such, but I'm not sure just how far I'm willing to go. Since it's a daily driver, I don't want to do anything that will compromise safety. A glass hood is a serious possibility, though.