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3.08 vs 3.23 gears for LT1 Chevrolet Caprice

Have a modded LT1 Caprice with hot Cam, 2400 stall torque converter and 3.73 gears.

I don't like 3.73 gears for all-around driving and am trying to swap them for a highway gear, either 3.08 or 3.23. They don't feel right on the highway, too high RPMs for my taste and MPG is horrible, I got less than 18MPG. I know the B-body can do much better than that.

3.73 work OK up to about 50MPH, mixed city driving but get the engine noisy on the highway. Go 70-80 and the RPMs will be around 2400-2700.

I had another B-body with LT1/2.93 and thought that it was a bit too low (numerically). But it did get 23MPG on the highway. Stock LT1 Impala SS came with 3.08 gears or 3.23 if it has the L99 engine.

I had an L05 Caprice with 3.23 and thought that was OK, a good compromise gear for all situations. Now of course L05 was only rated for 180HP versus 260HP of LT1 so I wonder if I can get away with 3.08 now. I plan to run 245/50/17 tires on stock SS wheels.

Reply to
SQ
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hot cam, 2400 stall converter will not get good efficiency at low rpm.

Have you reprogrammed your fuel injection to match your cam?

Not much better, as you will be driving around at way below the rpm required for good efficiency with a hot cam and 2400 stall converter.

Is the converter rating from the box, or have you had it tested with your combination?

Sounds like you've got a 700R4. Is the lockup working? RPM should be a little lower.

See below:

245/50r17 diameter is 26.7 inches, rear gear of 3.73, no converter slippage

Gear ratios first 2nd 3rd 4th 70mph 80mph

700R4 3.06 1.63 1.00 .70 2300 2600 200R4 2.74 1.57 1.00 .67 2200 2500 TH350 2.52 1.52 1.00 3300 3800 TH400 2.48 1.48 1.00 3300 3800 245/50r17 diameter is 26.7 inches, rear gear of 3.08, no converter slippage

Gear ratios first 2nd 3rd 4th 70mph 80mph

700R4 3.06 1.63 1.00 .70 1900 2200 200R4 2.74 1.57 1.00 .67 1800 2050 TH350 2.52 1.52 1.00 2700 3100 TH400 2.48 1.48 1.00 2700 3100

Factory stock will. It's tuned for best mileage.

Stock LT1

LT1 5.7L(350ci)was only 260 HP? My 2006 Crown Vic Police with a 4.6L 2 valve (281ci) stock is about 275 HP. I get around 20-22 MPG on the highway. Rear gears are 3.27, with 235/55r17 (27.1 dia.) tires. Only mods are shorty headers and freer flowing mufflers. 15.0-15.2 quarter mile times, depending on temperature. Not exactly a hot rod, but reliable, fun to drive, and decent mileage.

I plan to run 245/50/17 tires on stock SS wheels.

Reply to
Steve Walker

Yes, the PCM has been reflashed to reflect the 3.73 gears, 160-degree thermostat and other changes.

The transmission is 4L60E.

The torque converter is lock-up and rated for 2400 stall.

I pretty much made up my mind to go to 3.23 gears and just hope I gain 2-3MPG. At 70MPH, I should be turning 2000 RPMs and not 2300 like with 3.73 gears.

3.73 is nice up to about 50MPH but at highway speeds the RPMs are just too high, noisy engine, 17-18MPG. With 3.23, you should gain some top end speed. Go 80 with 3.73 and you will turn 2750 RPMs.
Reply to
SQ

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