With stock-size tires, you should be seeing around 1,700 RPM at 60 MPH today. If you go from 3.73 to 4.10 rear axle ratio, that's a ten percent change, so your RPM should increase 10 percent at the same speed. 1,700 plus 10% = 1,870, or around 1,900 RPM on your analog tach.
Because of the horsepower curve, 1,900 RPM is a lot better than 1,700 as far as power is concerned, at least on a stock engine. And even with your hot-rodded engine, you should prefer towing at 1,900 instead of 1,700 RPM because the tranny won't downshift for every little bump in the road.
So for towing, I think you'll really like the 4.10 ratio if you keep the speed down to around 60 MPH.
Unloaded, your RPM is around 2,150 at the 73/74 MPH ticket limit, and it would be almost 2,400 RPM with 4.10 rear end. That's why the fuel mileage goes down. But if you can make yourself cruise at that same 60 MPH unloaded as when towing, you'd get a lot better MPG.
My trailer weighs around 8,000 pounds when wet and loaded for the road. I think the 3.73 ratio with stock-size tires is about perfect for everything except mountain climbing. I wouldn't want a 4.10 ratio in mine. I normally cruise at 1,800 RPM or 62 MPH when towing. Even with my hot-rodded engine, 1,700 RPM or about 60 MPH is not quite high enough on the power curve to prevent downshifting too often, so unless I'm in really-flat country, I settle for 1,800 RPM. At 1,800 RPM with the DP-Tuner 60-tow tune, mine rarely downshifts for smaller hills.
I would prefer to tow at the traffic speed of 2,150 RPM/73 MPH, and my rig will gladly cruise at that speed all day without any complaint. But fuel mileage takes a beating if I do that. Mine costs about one MPG for every 100 RPM more speed, so cruising at 73 MPH instead of 62 MPH would cost me around 2.5 MPG.
It's not the engine RPM that hurts fuel milage - it's the increased aerodynamic drag at the higher speeds you get with the higher RPM. So you probably won't notice any change in MPG when towing at 60 MPH with the 4.10 gears
