I've been towing in 4th, a 0.83 OD gear in my Lightning with a 4 speed automatic. Turns over about 2300 at 65mph.

However, I have a Predator tuner with the truck that allows me to monitor OBDII stuff as well as a manifold boost/vacuum gauge.

In 4th I'm pulling a vacuum, no boost. But it isn't as much vacuum as I pull if I switch to third, which is a 1:1 gear, at 3000 RPM at 65mph. And, in third it will not creep into boost like it will in 4th pulling a hill. In short, 3rd requires less throttle angle to get the job done and isn't likely to generate boost. Either gear has plenty of power and doesn't result in downshifts, even with medium grades.

I also looked at the OBDII stuff and found that at steady cruise something called LOAD is around 50% in 3rd and under the exact same conditions LOAD is 60% in 4th (and LOAD goes to like over 200%, if I floor it and get 14 lbs of boost it hits mid 200s).

Seems to me I'd be better towing in 3rd maybe, but I didn't figure all this crap out until like 30 miles from home on my last Roebling trip, so I didn't get to test it.

Anyone else have some empirical data on towing in 3rd or 4th and which is better? And, I just realized I didn't specifiy better, I mean better mpg! I'm at like 8-9mpg with a 20ft enclosed weighing in around 5000 to 6000lbs.