Best possible fuel mileage specs?

[jegzus]

I don't bitch and whine. I keep my costs under control, and I don't take bull****.

No you just deal the bull**** right.

[uncleal13]

I have an 06 Century with 450hp MBE 4000 Mercedes engine, 18spd tranny.
When I bought the truck last August it had 11R24.5 rubber with 3.58 rears. At 105kph it turned 1325 rpm. I had the dealer change the tires to 11R22.5 which brought the engine up to about 1380rpm. I'm based out of Edmonton and after much experimenting I found I got the best economy at 102 kph netting 5.8-6.1 us mpg pulling 53' vans into the lower 48 states.
At the end of January I got the bright idea to try super B flats, but I knew I wouldn't get it started with that rear end ratio. I paid $4,975 to change it to a 4.11 ratio. I went from Revelstoke/Rossland,BC to Toronto/Montreal. Got 3.8 to 4.1 us mpg.
At the end of march I came back to my senses, and still at the same company I switched back to vans into the US. I left the 4.11 rears in place and now after some testing I found 97 kph to be the right speed. I now get 6.3 to 6.7 us mpg turning 1450rpm.
When I had the 3.58 rears the engine just lugged too low and didn't do as well at speeds below 102 kph.
Hope this helps a bit.
I like listening to Kevin on XM also. I was reading on the net that American specs don't do as well for Canadian trucks as we have heavier gross weights eg. 37,400 lbs on a tandem as opposed to 34,000 lbs. If you ran at US weights or less then Kevins ideas would work for you. If your doing Canadian weights and as you point out, Canadian roads (Northern Ontario) then I would suggest specing 100 - 200 rpm higher than Kevins ideas.

[tracer]

I'm based out of Canada too just switched to tandem flatbed division. My rears are 3.73 and my 435 hp/1650 lb-ft CAT C15 turns at 1,300 RPM at 58 mph. I got a 13 speed Eaton with .73 top ratio and 22.5 tires that rotate at 495 revs per mile. Depending on the load and terrain, I get anywhere from 6 to 7.3 mpg. I just installed a little spacer ring in the air manifold intake ("Air Cell" from aircellfleet.com) and this has somewhat improved my torque and - hopefully - fuel mileage. My next "big" project is to switch to low-profile tires by winter. This should increase torque and RPM, and then I can slow down to 56-57 mph even when fully loaded and driving in hilly terrains. And if I had 5 grand lying around, I'd definitely replace my 1,650 tranny with the one rated for 1850 torque so I could uprate the CAT to 475 hp and 1850 torque... The current tranny is still on warranty and I don't want to do the engine's rerate without upgrading the tranny. I"m pretty sure higher torque engine would improve fuel mileage...

[Orangetxguy]

I have a 2001 9400, with 3.55 rears, a C-15, up rateable to 550hp,but set at 450hp and a 13 speed. 2 weeks ago I was getting 6.8 mpg on average. The last couple weeks I have been down at 5.2 mpg. Seems the culperate in my mileage loss is either the Charge-air...or the fact that Pilot has switched to selling an 80-20 mix of Bio-fuel. I always use the cruise as much as possible, and keep my speed down to 64 mph, because for the first 3 month's of owning the truck, after much speed adjusting, and math doing, 64-66mph came out to be the best over-all speed for good fuel mileage. Over 67 mph and under 60 mph,the fuel mileage tanked.

Am gonna have the charge air checked in the morning.

I'm thinking it's the Bio-fuel mix that is costing me.