I have a B-body (Buick Roadmaster) with LT1 and the following cam + TC installed. 4L60E tranny, the gears are 3.73, the tires are 255/50/17.

I don't race or anything, just want a good MPG car and decent power. I don't like 3.73 gears, the RPMs are too high on the highway for my taste (between 2500-2600 at 70-80) and the corresponding MPG is terrible, I am not even making 18mpg.

Question, possible to swap in either 3.23 or 3.42 for better MPG and to lower the RPMs a bit? On paper, it seems my cam is pretty mild going from 1000-5000 and the TC is also pretty mild and 3.23 should work nicely.

I heard one opinion that it's better to keep the 3.73 gears but swap the TC for a much more efficient 9" TC to transfer the torque more efficiently, something much higher like 3000 stall. My concern here is that it might not be streetable.

Yet another problem is that my PCM could be tuned for power and not efficiency, I also don't know what octane gas it was tuned for.

I heard that it's possible to get mid 20's MPG in a B-body (1990's Chevy Caprice, Impala, Roadmaster) with 3.73 gears and a high-efficiency TC, is that true? I have a hard time believing that.



Transmission Specialties BigShot 12LS

12'' STREET CONVERTERS Our 12" Street converters are the standard of the Industry by which all others are measured. Maybe that's why we sell more than all others combined.
Designed for vehicles with mild intake or camshaft changes, our street converters give you the increased stall speed to complement the HP and torque increase of your combination, plus a smooth ride for the daily driver.
Available for GM and some Ford applications.

GM Big Shot 12LS
Primarily used in cars that are slightly modified, and mostly street driven. An excellent converter for the GM Target Master crate motor. Rear ratios should be in the range of 3.23 to 4.10. Hydraulic and flat tappet cams with good low-end and mid-range torque work well with this converter.
Stall speed range: 2000-2400 rpms.

• New pump hub
• New bearing between stator and pump
• Computer balanced and pressure leak tested
• Rebuilt sprag assembly
• Pump and turbine fins tig spot welded for extra strength
• Competitively priced

The 12" Street Converter is available in Chevy, 700R4, 2004R, 4L60E and AOD in both high stall and low stall.

Here is the cam:

Competition Cams Comp cam LT-1 LT-4 Chevy 93-96

· Application 1993-1996 Chevy 350 LT-1 and LT-4 engines
· Power Range 1000-5000 RPM
· Idle Quality/special requirements-Best for original LT1 with stock computers
· Valve Lift 500/510
· Adv Duration 266/276
· Duration @.050 210/220
· Lobe Centerline 114