Gear ratio and fuel mileage

I got a MACK with a 460 and 13speed. 4:30 gear ratio. Im happy if i get over 4.5MPG. I think 3.91 is way to high in a dump truck. Unless you are stocking black top plants all day and never go off road.

 

wow i have a tri axle freightliner not alot of hp but plan to turn it up a little next yr. to 350/1450 i hope it will go there..anyway now its a 280/1050 i think with a rto14709mll trans and i get 6.5/7 mpg with 4.11 geras and the truck will run 80mph loaded its a snall off the line but once i get it going its ok....

 

What MM141 said.
If i go off road or haul heavy, give me low gears every time

 

I think 3.91s will be ok if you dont do alot of heavy off road. If you think you need to change the trans also get an 8LL or 18 speed. They both have a lower low hole than a 9,10, or 13 speed. Dont bother changing your 10 speed to a 9 or 13. High gear is all the same so their will be no value as far as fuel milage. If you shift a 13 properly you can get a very slight increse in fuel milage but nowhere's near enough to ever pay for the swap. The 9C is convertable to a 13 but its not the old RTOO double over. That trans has not been made in 20+ years. All Road Ranger trans that are over drive have .73 or .74 OD so swaping trannys to gain fuel milage is a complete waist. (15speed .79 but not worth talking about)
Alot of new dump truck are being spected with 3.91s but the newer engines make power at lower RPMs. 3.91s with an 8LL or 18 speed would be more fuel efficient than what you have now but I dont think it will change a whole lot and your talking a lot of money for 2 new rears and a new trans. I dont think you will ever recoupe all the money spent swaping things out. 4.8 to 5.2 aint that bad for a dump truck. Im guessing after all the time, labor, and cost you will only gain 2 or 3 tenths. Not worth it in my oppinion.