Being that the torque band is up at 2400rpm, would it be smarter to cruise at 70ish? I'm planning on doing 4.56 gears in it, not for mileage, but for some more go.
The faster you drive, the more power it takes to overcome the air resistance, which is guaranteed to cost you more gas. But as we found out many years ago, if the engine becomes more efficient at the higher rpm, then you can break even or even win. What I'm talking about is early 60s Big-inch carbureted cruisers that got better mileage sometimes at 85mph, than lugging along at 60.
Or like the 340 Mopars, where the cam was all wrong for a cruiser.
Additionally with a higher cruise rpm, comes more internal engine friction., also costing power.
With 31" tires, your 3.55s are comparing to 3.09s with 27s which are the typical tires that most street-rodders run, so, 4.56s would be a 28% improvement all round. and Cruise rpm would be up to 65=2200, which obviously is a more efficient rpm to be at. Whether or not you'll get better steady-state fuel-mileage remains to be seen. But yes your take off will be much improved. and so, it might cost less fuel to get up to speed.
Plus your engine will be working quite a bit less hard at the new Cruise rpm, therefore pulling a higher vacuum, Which your ECM should properly interpret and crank up the timing and down on the fuel. If it does that without annoying the knock sensor, that is guaranteed to decrease fuel usage.
I hope your ECM knows that you are running 3.55s. Cuz if it doesn't then she won't know you vehicle speed. which it uses for some of it's calculations. and if the calculations are wrong, then the economy takes a dive.
I would still do a compression test to get an idea of the health of the engine, I mean it is 25 years old right..... and again, if the pressure is up and even, then I would put a scanner on it, and check the sensor outputs; and I would compare the engine timing at say 2400rpm on the balancer, with what the ECU is commanding, to be sure they are synced up.
And, you gotta be sure your O2 sensors are working ......... you do have O2 sensors right?
Without working O2 sensors, your check engine light comes on, which you might thing not much about, cuz the truck is running just fine; but If the light is on because of the failed O2 sensors, then the ECU has been forced to go into limp-in mode, and the programming has fallen back to the pre-programmed warm-up program, which is often killer-rich and lacking timing, and away goes your fuel economy. So, if you don't have a scanner, and your CEL is on, I'm not feeling sorry for your 11mpg, lol. However, IMO, if your combo is is only getting 11mpg in steady-state cruising, my guess, is that the ECM has been deceived. Check your tailpipes; if they are black and sooty, that's part of your problem.
I highly recommend to buy a good quality scanner and learn how to read it. If everything is working the way it's supposed to, then you will have a hard time improving fuel-economy.
FYI's
The rule of thumb is that a reduction in rpm, by percentage, will improve fuel economy by half of the reduction, in percent. So like, if you drop the rpm by 10% you should get a 5% increase in fuel-economy. That 10% reduction could be just slowing down, too.
But in your case, the 31" tires have skewed this rule up a bit.
If you want to see what 4.56s will do to your fuel economy, Just run the rpm up to ~2400 in Direct/LU, which should be about 58mph, and see what happens.
Warm-ups in winter will decimate fuel-economy. It'll be cheaper to run the block heater for ~4 hrs on a timer, to shut off when you get in the truck. Run a multigrade like 5W20 in winter, cuz your Magnum has roller lifters. Give it two minutes and drive!
Some other things that suck gas are;
soft tires, dragging brakes, tight U-joints, a bad alignment, and an overfilled crankcase. Also, tight wheel-bearings; which if you have those cartridge type deals you can't do much about. Tailgates IDK about.
IMO,
A 5.2 injected Magnum, in a full-size Ram pushing close to 5000pounds (I'm guessing) thru the wind cruising at 65=1600, is gonna have a pretty big throttle opening, with a correspondingly low engine vacuum, and I have a feeling the ECM is not gonna fuel that correctly. I suppose you could put a vacuum gauge on it and see, but the scanner will read that too, and more accurately.
If the vacuum goes below about 13 inches, a carb would be tickling the "main circuit".
By about 10 inches, the mains are flowing.
By 7>/8 inches the power system would be engaged.
The EFI System doesn't have 3 separate systems like that, but on the fuel map, you can still see the injection time rapidly increasing as the vacuum goes down. The problem with a low vacuum, is that the ECM doesn't know exactly why the vacuum is low. It may be thinking your climbing a hill, so it just injects what it's been programmed to do, and cuts timing to keep the engine out of detonation; which for cruising, kills the power, and is the wrong thing to do...
And
since I'm not an EFI guy, I don't know any work-arounds for that kind of chit, lol.
For a carb'd engine with deep hi-way gears, the vacuum goes down too. But I am the ECM, so now it's up to me to figure out how to get the cruise-timing that the engine desperately needs, in that circumstance. And when I get it, I can lean the carb out.
