I know that cat isn't the greatest for fuel mileage. The truck I have is a 1995 T-600. It has a 3406E model Cat that is 475 hp with a 13 speed and 3.55 ratio. My father has a 1996 T-600 with a N-14 525 hp, with a 13 speed also and it has 3.42 or 3.63 ratio and its getting 6.3 to 6.8 mpg. I haven't had a chance to drive mine yet cause of a injury from a previous job but I was driving a 1998 W900L with the same step up as I have in my T-600 and it only got 5.12 to 6.0 mpg. What is the best way to get fuel mileage out of a Cat motor?

I had a Freightliner CLassic XL (2000) with a 3406, 500 hp with a straight 10 speed, and I was getting close to 7 (except when bucking a strong headwind). But, I also had 3:43 rears. I never had any complaints about the fuel economy. Also had a 2000 Classic with an N-14+ straight 10 and 3:42 rears. Same mileage no matter how I ran of what I did. The Classic isn't as big a "fly-swatter" as a W900. There's a lot of wind resistence with a W9. If you don't get the mileage, you might try going to "tall rubber". (Assuming you have 22.5 now.) Might mean lowering your air bags an inch or two so the front of your trailer isn't too high. Then, too, is your sleeper a condo? Flat-top sleeper means your trailer catches more air.

Moving this to Truck Maintenance.

Well, considering everything technically OK, sweet spot for 3406E is 1400 RPM, so try to keep it between 13-1400. Use a progressive shifting, shift fast drive slow. Can't promisse much, but i did get 6.8 lifetime average (at the pump, ECM showed 7.3) from my 1995 3406E.

keep your rpms betwwen 1400 -1500, get a fuel temp resistor it helped my mine about .5mpg and dont mash the peddle take it easy when getting upto speed,shut the truck of every time you stop have the over head run if you havent in 100,000 miles. check all your ire pressure also

Slowing down and drive like you have an egg under the accelerator should help. I seem to do well on fuel mileage with CAT engines. The worst that I can recall averaging is 6.29 mpg for the quarter. The most is 7.88 for the quarter. These were different trucks and pulling open deck trailers.