So 3.73:1 with 35" tires is equivalent to 3.09:1 with stock 29" tires, plus a 700R4 with overdrive? Yeah, that's about the mileage you should be getting.
You're baffled because of a fundamental misunderstanding. Horsepower is not a function of how much gas you burn, but how much air you move through the engine. For a given amount of air, the maximum horsepower is at the correct mixture of gas. ADD GAS above the correct mixture and the horsepower and the mileage BOTH go DOWN, for two reasons. One is that the extra gas is not being burned, it's just wasted, killing your gas mileage. Two is that the additional gas displaces air that you could be moving through the engine, so the horsepower goes down as well.
This is why turbocharging and supercharging and nitrous add so much horsepower. Turbocharging and supercharging blow more air through the engine. Nitrous has more Oxygen atoms per molecule than air, so it is as if there were more air going through the engine. People don't blow GAS through the engine, they blow AIR through the engine. You need enough gas flow to keep up with the air at the proper ratio, but more gas alone makes things worse, not better.
Just some dumb numbers. Let's say you are moving fifteen parts of air per second and one of gas per second. That's about the right mixture. So you will get the horsepower associated with fifteen parts of air. Add gas, so you are running too rich, so that now you have two parts of gas per second. Well, the second part of gas won't burn, not enough air, so it's just wasted. But it's worse than that, because now you only have fourteen parts of air. The other part of air you could be moving is the wasted gas. So the horsepower goes down to what you would get with fourteen parts of air, not fifteen. So you are using more gas, and horsepower went down.
You were running way rich, because Edelbrocks are set up for a wide range of different engines, and our truck engines are a poor match to their factory jetting. (Which is also the source of a lot of complaints about Edelbrocks from people who can't be bothered to properly tune them.) When you leaned it out, the gas mileage improved a lot AND the horsepower went up across the rpm band. BTW, going to 8" springs delays the point at which the rods pop up and richen the mixture, so that was also a move that leaned out the engine.
An engine will almost always run OK rich, but go too lean -- above about 17:1 air/fuel ratio -- and the fire goes out. Which is what happens if the accelerator pump isn't set correctly. You open up the throttle and air rushes in, but the gas flow can't accelerate as quickly, so the A/FR goes above 17:1 and the fire goes out until the gas flow catches up, which causes a stumble when jumping on the throttle. The accel pump compensates by squirting raw fuel into the bores until the gas flow in the jets can accelerate up to the rate that matches the new air flow. In this case, by moving the accel pump to the innermost (shortest) hole on the lever, you richened the mixture for that brief time, and eliminated the stumble.
Horsepower is all about moving air through the engine. Fuel economy is all about making sure the right amount of gas for that amount of air is added, and not adding too much. Most people don't add too little, because the point where the fire goes out isn't a lot leaner than the optimum, so the errors are almost always to the too-rich side of things.
Anyway, congrats! Glad you got it sorted. That is very happy-making!
