Best rpm for the D12

I can't relate to the D12 motor, as my Volvo has the ISX 15. But I can attest to the turbo boosting more so if I fall below 59 mph in 10th gear, and thus losing tons of fuel economy. California is my nemesis, in this case.

 

I thought I read something somewhere(pamphlet at dealership?) that it was 1,350 rpm. Ultimately I think it is whatever rpm at which the engine works the least to get you down the road. Obviously that rpm will vary depending on speed, terrain, load, wind and gearing. I know, really helpful.

I primarily monitor the boost gauge, like it sounds you are doing. I am generally running closer to 1,300 rpm at 55(8th direct) or 65 mph(8th OD) with a 2007 780 with a D12, 13 spd, 3:42 rears and 11 x 22.5 tires. My lifetime average since I have owned the truck(230,000 miles) is 7.1 mpg. My average gross weight has been in the low to mid 50's.

FWIW: I believe Cummins recommends 1,400 - 1,450 as optimum for the ISX.

 

Ive heard once you get above 55 your loosing millage due to pushing the air. 30% more from 55-70. I run at 1350 too 62mph in 10th O/D BUT going down hill I push as much air as I can get away with so I can get up the other side! ( free speed ) 1500 at 70mph and my truck will climb walls

 

I do not know but itmust be something wrong with my truck because iam only getting around 5.3 to 5.8 and that is doing 65 mph and keeping the RPM around 1,500.it has volvo enginewith 13 speed transmition gear 3:42 tires size 22.5 10.5

 

I do local half miles loaded and get around 5.5 mpg.. 630 with 465HP 10 speed 3:58 and 22.5 low .. with empty flatbed highway I get up to 8.7 mpg :/