W900 that's way too slow.

4.11 is your issue.

I have 4.11 and 24.5 on my T800 and can’t break anything higher than 5mpg ( and I only drive 55 )

 

Those 4.11 gears are the problem. With that super 10. A set of 3.42 would probably be what you want.

 

355s w/ LP 24.5s. Or 3.31 w/ LP 22.5s might be the ticket. Final drive ratios on the trans are the same.only difference would be when down shifting on a hill w/ 13 or 18 spd. Best to look up Eaton RPM charts. Compare the numbers with what you already have on both Trucks. Ideally matching cruising speed to most efficient RPMs in torque range for the engine. Might be best to go with 331s or 335s whatever, keeping yourself 22.5s. You could always go up to 11R22.5s or Lp 24.5s if lower cruising RPMs is needed. LP24.5s will be a very little bit lower RPMs. Hardly worth the extra cost to change out Wheels and tires. But supposedly Low pros get better mpg’s than tall tires. Due to less flex and rolling resistance. Aside from mpg’s , I’ll bet your Trucks fun to drive now, with the 4.11s. Walking up hills. Going much lower than 331s, causing more downshifting, especially with the 10 spd vs. a 13 spd. Might defeat any fuel economy gains. Study the charts, they’re interesting.

 

Cost vs reward. How many miles are you gonna have to turn to see a return on investment from a (generous) 1 mpg gain?

Personally I wouldn't stick a penny into it until needed. When tires wear out, change them. When a diff is on the way out, regear it. When the transmission is worn out, upgrade to the 13 speed.

 

Any minute someone is make a mathematical post to tell you exactly how long and how it's such a good idea. They cant realize in theory and in real world applications rarely match

 

I'll do it. At $2.50 a gallon, going from 5 might to 6 would save in a year, based on 100k miles would save $8,334 in fuel costs. For that kind of money, it would seem that waiting until rearends are needed would make the most sense.

 

Do you have a nose cone on your Conestoga?

 

You really think goin from 4.11 to 3.55 will bring a W900 from 5 or less to 6?


I do agree run it it breaks then make the wanted changes.