My 2nd gear is great for that now with 5.13 gears, LT265/75R16s and the 6,500 rpm capable 383. I get 45 mph out of first with 200 rpm converter slippage shifting at 6,200 rpm. 2nd gear I get 77 mph @ 6,200 with the converter locking about 4,500 rpm. Turns 2,850 @ 70. It will accelerate and pull grades in lean cruise with nearly no load on the engine. I have seen 17 mpg on a recent road trip and it is not tuned as well as I would like it to be yet. My timing map is ultra conservative because of the 11:1 compression ratio and small cam with Rhoads lifters making the cam effectively even smaller and I am only running at 16:1 air fuel ratio. I ran the old 350 closer to 17.5:1 without misfire thanks to the coil per cylinder ignition.
The Tahoe is geared pretty well for me too. Marine cammed 8.1 that makes about 450 hp @ 5,100 rpm. 4L80E and 3.42 geared 9.5 14 bolt. Has 32" tall P305/50R20s. Shifting at 5,200 rpm I get 55 mph out of first with about 150 rpm slip in the stock B82 converter and 98 mph out of 2nd. I lock the converter about 65 mph or 3,500 rpm in 2nd. The 8.1 does not seem to mind pulling against the tall gear since it makes something like 400 ft/lbs @ 1,200 rpm and 550 tq @ 3,500. The 8.1 turns about 1,850 rpm @ 70 and stays in lean cruise with the empty Tahoe. I have gotten nearly 18 mpg on a long trip. As crappy as the combustion chamber is, thanks to the low compression ratio and the larger marine cam it has no issue running at 16.5:1 air/fuel mixture and 42* timing at 1,850 rpm on 87 octane. I am thinking of pulling the intake and swapping out the lifters for a set of Rhoads lifters to recover some of the low-end torque the 8.1 loses from the marine cam as well as some of the intake vacuum. Making the cam effectively smaller should make the mileage 1-2 mpg better.
