What's the absolute optimum speed for fuel mileage?

My truck was set at 75mph but I changed it to 100mph in Cummins Insite but it will only do 85mph lol.Don’t know if it’s because it has 3.90 rear ends or what.

 

On an Cummins ISX you’ll save fuel but lower the longevity of it.The lowest one should run one is 1500rpm.

 

My old Trucks with 12.7 and final gear at .73 always get the best mpg at 1375 rpm. With Lo pro 24.5’s that’s 62 mph. Light load, flat land, fuel economy is excellent. Add some weight and some hills, while different set of driving habits applies. Lack of torque, and a small window between 1350-1550 RPMs makes it a challenge. Newer engines with their long flare torque curves, kicking in at lower RPMs makes simply slowing down effective in all Drive good terrains.

 

Back then I did not wear a 'front air bag' on my body.
After 21 years in reefer I'm proudly wear one.
Should I have done flat bed?

 

Not Ukrainian, but I trained 2 great guys from there.
Добрий день, і Бог благословить вас.

 

Tom Cruise with a cane. Behold.
Soon to be upgraded to a walker.
I'll give it a rest now.

 

62 mph for the best fuel squeezer. I post mine daily when I'm out. There's one troll on my thread it will cause to stroke out someday.

I don't run 62. Usually 65.

62 will get me in 8+ mpg.

Wind has destroyed me in The Plains the last few months. You are indeed fortunate if you're not getting your mpg obliterated by wind in that neighborhood.

Blackie II is 7.8 lifetime. Last trip from driveway to driveway was 7.7

That was one load at 15K, three at 44K, one at 25K, one at 3K.

 

The crown vics are indestructible and the 4.6 will go forever. Later on they fixed the spark plug bs. Ive drove lots of tritons in the oilfield with 300k +

I'd actually prefer a 3v 5.4 over my 04 5.3

The later years 5.4 and the 6sp was a good combo. They only made them for a few years before the 5.0 came out