1970

Planned final vehicle is:
1970 Chevelle, 5.3 LS engine with 4bbl, MSD, Turbo350 transmission, 2.73 axle ratio.

Wondering if I must change transmission or rear axle.

The REAL QUESTION is:
What is the best RPM for best gas mileage?

Jimbo1367

as low as poss(best RPM). If you really want best mileage, ditch the carb and run OEM EFI. Or even FAST's EZ-EFI Not really expensive at all. Also, switch to a OD tranny. A 700R4 or 4L60E would help immensely with its steep 1st gear getting you going and OD with your cruising gears should get you mid 20s.

chuckd71

iTrader: (23)

Speaking in generalities, the lowest rpm possible once you shift into your highest gear.

garys 68

iTrader: (1)

I'm with chuck and I'm looking at the same situation, 72 442, 2.73 rear, Muncie.
If you look at GM trucks with the 5.3 or 6.0, their cruise rpm (70mph) is about 2000 rpm with 3.72 gears and 2250 with 4.11s. Come to think of it most LS based motors seem to aim for about 2000 rpm cruise.
Both yours and my car will be running about 2500 rpm, 1:1 trans ratio.
A 0.7 OD trans with your 2.73 rear will put you at about 1750 rpm, maybe a little low considering the weight and aerodynamics of the car. But for a couple hundred bucks, you can change to a 3.08 or 3.2-3.4 ratio later.
Since most 5 speed manual trans are 0.6..something, that's going to be too much gear for me with the existing rear. I'm going to live with the Muncie and 2.73.

usc2001gc

iTrader: (1)

I currently have 2.73's in my Chevelle ls2/TKO-600. With the .64 OD and a 26" tire, I'm only turning 1250 rpms at 55 MPH. When I drove it to Daytona in November, I only averaged about 21 for the round trip (including cruising). Not bad, but not as good as I was hoping for. I've purchased a 3.31 gear, new 3 series carrier and plan to install them in the near future. This will put my rpm's at 1500 @55 mph. (I'm probably going to install a performance camshaft in the future and I wanted to get my rpm's up a little.)

Pop N Wood

iTrader: (1)

Interesting. I have the same TKO in a Datsun 240z, 25.5" tall tire and 3.7 rear. I turned I think 2400 RPM at 70 mph and get 25 mpg. Carb'd LS2 crate motor with a 750 dp. Over the winter I swapped to a 3.54 rear to get the revs down a bit. Will be interesting to see what it does to the MPG. I know guys with similar cars with T56 and a 0.5 6th gear and they get 27-28 MPG from the heavier gears.

garys 68

iTrader: (1)

Maybe a little too much gear?
My 6.0 LQ9, Richmond 6 speed (0.76 OD) 3.08 rear puts me at 2000 rpm at 70.
I'm getting 26mpg (mostly hiway).

usc2001gc

iTrader: (1)

I need to check the tune. I sent the PCM with the harness to have it modified for my build and it was reflashed as a SSR (something to do with getting the cruise control working...which I still haven't been able to do). The guy that did the work said he usually puts the original tune back in (for me this is a 2005 Corvette), but I recently purchased HP Tuners Pro and and from what I see, it appears to be the SSR tune. I also bought a wide band O2 sensor. Once I get it installed, I'll look at playing with the tune a little. I've never worked with EFI before so I was a little hesitant to try to mod the PCM and harness myself, but I figure I'll never learn it if I don't try it.