Using a vacuum guage and gear selection towing for best MPG?

I dont automatically use tow haul. At times it's better to just run in 5th because my main concern is to minimize the trans shifting in and out. It's RV'ing so I dont pay attention to mpg or worry about differences in fuel prices. I have used vac gauges to diagnose engine problems years ago but I would focus on not having constant shifts going on and get the feel of where it's on the pipe and be able to anticipate upcoming hills and let it roll.

I have driven my diesel with an eye on the boost gauge just to minimize shifts but there is enough other stuff going on that I dont do it any more. Fuel use is what it is so I focus on minimizing wear on my trans parts and not lugging the engine.

I think that it's less of an issue with fuel injection than with a carburetor. The carburetor had a "Power Valve" that opened at set vacuum point, FI controls the mixture with the O2 sensor in the exhaust. So while FI it may still enrich the mixture, it does it on the curve. Not at one set point. So less RPM, less fuel, IMHO.

this is a tough one. for a gass engine

(a) ignition advance goes up with higher vacuum, and that offers a significant increase in fuel efficency, even in a FI engine. the more advance you can get, the better the fuel economy
(b) engine frictional losses are related to RPM SQUARED, so higher rpm= higher frictional losses.

So, in your case (a) is definitely working for you with a noticeable increase in vacuum and likely more ignition advance, but (b) increased rpm is working against you as well.

However, I think that the difference between the two when towing is likely to be 1mpg or less

Need more info. How much more RPM are you gaining with less gear under you? I would pick the higher vacuum reading. Either way it's not going to be much mileage savings at all between the two.

Vacuum gauge is old school. Nothing wrong with old school except a vacuum gauge is not very accurate compared to a TPS reading. A vacuum gauge is to a ruler what a TPS is to a micrometer.

Bottom line is this. The best mileage you will get is the lowest TPS setting with the lowest RPM. Where those two lines cross will be the best mileage you can get is achieved.

There are a lot of things to achieve better mileage.

#1. Tire pressure to max pressure.

#2. Get off of the gas early when you want to stop. This is really important when towing to save fuel.

#3. Both pedals kill mileage. Any time you are on the brakes you are losing fuel mileage. Stay off of the brakes as much as possible. Any time you are on the gas you are losing mileage. Any time you are one the gas quickly you are losing even more mileage.

#4. Go as slow as you can or want. Speed kills mileage by 4X's.

#5. If you can pick a day with little to no wind do it. If you can pick a day with a tail wind so much the better. (I know most of the time you can't pick but if you can, do it.) A head wind will KILL mileage big time.

Above all:

#6. Fuel is cheap right now. Go on vacation!! :)

slow down... lower rpm's WITHOUT Lugging.

as already stated, Vacuum gauge is old school FOR old school fuel delivery systems.

and I am OLD SCHOOL, retired auto mechanic. and yes I understand fuel injection and computer controlled systems.

best way for better fuel mileage. loose the weight.. and then better aerodynamics

I get 9 mpg.. with trailer... 17,000 pounds total.

in 1968 my 1965 Impala SS with 409 got 10 mpg.. just a car at 3,600 pounds.

More vacuum means more efficient. fuel injection or carb, doesn`t matter.

You should be more concerned with transmission temperatures than MPG. Gas is cheap, at least the difference between 8.0 and 8.5mpg that you may might be able to save. For naturally aspirated gas engines - one higher gear will use same to slightly less fuel overall, better throttle response, higher average speed, and lower trans temps. You probably want to be in the 2500-3000rpm range.

Vacuum gauge?

Does your truck have an avg MPG readout? If so you can settle this really easily. Get up to speed on the highway and reset the avg MPG meter. Drive it for a few miles and record the avg MPG. Shift to the other gear while holding the same speed and repeat. Done...

Lower RPM doesn't always mean better mileage if your lugging the motor. Tans may suffer as well.