We are planning to make quite a few long trips in our RAM van this year since it's the most comfortable vehicle we have.

Since these vans are known for being serious gas guzzlers I was wondering if anybody know what would be the "sweet spot" cruising speed in terms of optimal torque and efficiency on the B2500 3.9L Magnum V6 stock transmission (32RH) & gearing?

 

Keep the engine around 1800 RPM. Unfortunately, I suspect that is only going to be about 50 miles an hour or so.

Essentially, you are trying to push a brick thru the air at high speed. With a three speed trans, "efficiency" really doesn't apply. It's just gonna suck gas. At least gas is reasonably cheap at the moment.

 

Does that mean that the engine's optimal torque RPM is the figure to aim at?

This is rolling in Europe at about $4/US Gallon depending on the octane so it can hurt on long trips if not handled right.

Unfortunately we don't have a tach so I'll have to get the RPM from the OBD II port somehow.

 

It's a compromise RPM. It isn't where the engine develops peak torque, but, it is getting into the power band, and the RPM are low enough that it won't eat gas wholesale. I think putting a vacuum gauge on it, and just trying to maintain highest vacuum while driving, might be a better plan. Takes a bit of learning to figure out how to drive 'by the gauge', but, it should help at least a little.

 

I have the V6 and for me 55 MPH seems to be optimal.

 

I have the 3.9 with the 3 speed in my 02 shorty and I usually get around 13.5 and never better than 15. I'm going to use synthetic oil next time but I don't expect too see any improvements in mpg. All the other fluids are fresh and synthetic.

 

Thank you for the replies @alloro and @Eunospeed

@HeyYou I have installed an extra vacuum and RPM gauge in this van - please find in the images attached the vacuum reading at 2000 RPM, 3000 RPM and hot idle.

What would be the ideal vacuum reading to aim for?



Vacuum reading 3.9L Magnum V6 @ 2000 RPMs

Vacuum reading 3.9L Magnum V6 @ 3000 RPMs

Vacuum reading 3.9L Magnum V6 @ hot idle

 

You should have higher vacuum at idle..... Should be closer to 20".

But, as far as "ideal" goes, the higher the better. Same during acceleration. You are going to see your highest reading at idle, with no load on the engine. You will see the worst when accelerating hard. (close to zero....)

 

Thank you! Maybe I should hunt for a vacuum leak after this trip - when on WOT or close to it the vacuum needle tends to jump a lot. Is that normal?

@Eunospeed 13.5 mpg on highway is also the best we ever got with this van. We already run synthetics in the engine (Mobil 1 5/10W30, differential, transmission (ATF +4) and bearings.

Also the ingnition coil, spark plugs, distributor cap, rotor and wires are new but with little to no impact on mileage as far as I've noticed.

Maybe we can try "side-gapping" the spark plugs https://www.instructables.com/id/Save-Gas-And-Incerase-Horsepower-By-Side-Gapping-S/

Had anyone tried this on a B-Van Magnum engine, is there any real benefit to it except shorter-life plugs?

 

Good info. Looks like our vans are optimized for best mpg. I'm not seeing any improved mpg either, even with the tuneup and improved fluids. The mpg wouldn't be so bad if she had another 50 hp.