durasmack

What is the optimum RPM to run when empty on the highway for best fuel economy?

TxDiesel007

Id say


70 mph at 2K RPM, has always been most efficient for me.... It all depends on ur gear ratios in the pumpkins... I like anything under 2K rpms...

Tx

Bobcat698

Cool! I achieved 70MPH @ 2k going with my 4.10s & 315s

infidel

RPM makes little difference in fuel economy until you exceed 2000 then it takes a big dump.
It's speed, not rpm, that kills mpgs.

durasmack

Ok, so which is the best speed to travel?

ds1rider

Best speed is around 55

infidel

Hard to say but my best mpgs with my auto were 25 driving about 300 miles on a hilly, winding road with OD locked out at 40-50 mph.
I thought it was going to be my worst mpgs, was very surprised at the pump.

durasmack

is 50mph better in 3rd tc locked or in 4th tc locked?

HOHN

12Vs have best BSFC at 1600rpm. Technically, the engine is most efficient at making power here.

But mpg is MUCH more influenced by HOW MUCH POWER the engine is required to make (load on it), not how WELL (efficiently) it makes power.

Most efficient hwy method of driving a 12V would then be 1600rpm in tallest gear. Not wise if you have a 5sp and want to keep 5th gear. Autos might be going really slow at 1600rpm.

BigBlue

I get my best mileage at 72mph which is 1850-1900 rpm with 3.54's and 315's with the comp on 2x2.

TxDiesel007

Cool!

answered my next question... Now for this one, does the ram have to be taken back to factory to "reflash" or calibrate for tire changes in speedo? I know for sure my 285's are throwing mine off, just dont know by how much....

Does anyone know how to calculate that percentage of change??

Tx

Bobcat698

For my 98, they took it in and adjusted the Revs per mile with the OBD2 tool. I'm not sure if thats possible with yours.
Using this site assuming you have a .73 OD ratio:
http://www.car-videos.com/tools/speedrpm.asp

You are going 72.9MPH at 2000RPM with 285 tires
You are going 66.4MPH at 2000RPM with 235 tires

So you are running about 6.5MPH faster than your speedo says.

wcbcruzer

What if you just look at the boost gauge since the amount of boost is directly proportional to the amount of fuel that's being burned. Going 55 mph uphill with 10 psi boost is hurting your fuel economy way more than going 80 downhill with 2 psi boost... But generally, I try to keep it right under 2000 rpm's while cruising on flat ground.

infidel

Reflash won't help a '96, you have to change the speedo gear at the transfer case.

signature600

If you can figure out what size gear you need, let me know. I have a couple extras that I bought to try and fine tune my speedo!!

Chris