I'm currently running a 3.42 Camaro diff with a 1985 .72 OD T5, behind my tweaked 235 in a 54 truck with a 28.5" tall rear tire. Having tried a 3.08 behind the .72 OD T5 I can tell you that the 3.08 was too tall a gear for my setup. The 3.42 R&P and the .72 OD T5 is the optimum combo.
With the 3.42, my truck cruises all day in 5th gear at 2000 rpms @ 70 mph and will drop down to 45 mph and accelerate up with the normal traffic flow without a downshift. My area on Florida's Ridge, is quite hilly and at 60-70 requires no downshifts out of 5th gear on the long hills. The optimum range I've found with a 261 grind cam, dual Carter/Webers, and Fentons is between 1600 and 2400 rpms.
Numerically, it appears that you could run a .86 OD T5 with a 3.08 but this presents another problem. My 3.76 1st gear in my .72 OD box has me needing to shift out of 1st gear very quickly. The .86 T5 with the 4.03 1st gear would require shifting even quicker, especially with the 3.08 and with a shorter tire height. In essence, the 3.08 worked well as a 4 speed but was flat unless your ground speed was in excess of 70 mph. Ideally, a little taller 1st gear would be just the ticket in the .72 OD box.
The beauty of the 3.42 setup with the .72 OD T5 is that you can cruise around town in 4th gear at 35 mph or if you need to pull a trailer you can run in 4th gear very comfortably at 60-65 mph and still stay very close to the 1600-2400 power band. My mileage AVERAGES between 18-20 mpg with 50% of my driving over the open road. I run an average of a tank of gas through my truck each week so you can see it is not a barn Queen.
Another thing I observed with the 3.08, was the lack of oil pressure due to the tall R&P keeping my cruising rpms down unless I was running 70-75 mph. With the 3.42 I'm running 45-50 psi of oil pressure at 60-70 mph and it idles at 30 psi when hot with 10-30w oil and my operating temperature stays at the 180 thermostat temp even with the air conditioning operating.
When I set up my truck I took into consideration that the 235 was a 4 main engine and that it was initially designed to make torque at the bottom end. With the 261 grind cam, hydraulic lifters, intake and exhaust setup, I feel I've optimized the engines potential while stretching my fuel dollars without sacrificing performance. I hope this helps.
Dave
