L31MaxExpress
I'm Awesome
my truck has 3.42s and 30" tire. OD is basically useless unless on flat ground or going down hill. I live in mountainous terrain, I would think that takes a big bite out of mileage. I had a 2bbl 305 w/ 2004R in an early 60s impala about 15 years ago. I would get 23-24 mpg, but this was in the prairies where roads are flat. i also knew nothing about carb or ignition tuning.
@L31MaxExpress I should get your help with my timing map sometime. I haven't had time to mess with it but it is a neutered for sure.
I tested the Express van at one point on a long road trip when it was stock with the 4L60E and 3.42s. OD was useless in terms of fuel mileage compared to 3rd gear. It was constantly in and out of lockup and often downshifted to 3rd on its own. It was not until I started adding power that overdrive became a useable gear. Then I swapped to the 4L85E then put the 3.73 rear gear. At 70 mph the 4L85E and 3.73s added ~400 rpm and overdrive became useable and actually netted a small mpg gain. With the 383 there are roads flat enough here that it could use a taller overdrive ratio and other roads that a split between the current OD and 3rd would be nice. That is where the 6L90E will soon be an awesome addition.
The Tahoe was under geared with the 3.42s and 32" tall tires, but that little 305 did not seem to mind pulling along at 1,700 rpm with basically zero manifold vacuum. By that point I had learned how to make a good shift point and TCC lockup schedule and keep the thing in overdrive under moderate power yet still have ample response to downshift to hold speed without continually hunting the gears. When the 305 downshifted to 3rd as it often did, it still managed decent fuel mileage because it was 45 cubic inches smaller than a 350, thus less air and less fuel. The cammed 305 was like a 5.3L, it wanted to rev to make useable power, so I let it. Running 75 mph if I needed more power it would even downshift to 2nd if the go pedal was pinned to the floor.
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