good ratio for mpg

Could always drop a gear and see how it goes.
If you are loaded light it may be okay, heavy you may need a few more rpm.
I changed the gears on a previous truck, cost just over $5,000. Hard to make that money back at today's fuel prices. But I went from 3.58 to 4.10 as I was moving up to 140,000 lbs. After when I went back to 80,000 lbs my mpg improved 0.6 mpg over the 3.58, but I had to keep it under 62 mph.

 

Freightliner Century with Series 60 pre EGR Trans: 10 speed Axle 2:79 ratio with 22.5 low pro tires 7.3 mpg measured over 50,000 miles included loaded heavy and light and mt mileage.

 

Its all about torque and hp. If your working engine its using fuel. If it acts like its happy and not working hard then go with lower gears. All engines have a sweet spot-find it and work from there.

 

Halfway between peak HP and torque at 65. If peak TQ is at 1200 and HP is at 1600 somewhere between 1350 and 1400 is where you'll want to be. Also depends on the rubber. I went from tall 24.5s to 22.5lp rubber and I turn 1450 at 65 now. Which used to be about 72. Same RPM same weight mpg difference is about oh 1.25mpg.

Mpg is a difficult game and you have to understand the dynamics involved. I have pretty tall gears for running 90k gross (3.42) but I run a lot of stop and go and don't have too many steep hills. Truck never sees the better side of 65 loaded unless it's headed down a hill. Average is between 6 and 6.5 every month.

For what you do your 3.36s are probably fine unless you're having to drop gears when you pull hills it's fuel out the stack might be better suited for 3.55s if you're having to drop and run hilly terrain which is where you'll burn a majority of your fuel.

 

I ran 3.70 on 04 ex-Kinard t600, low pro 24.5, 4 row radiator, bar plated air cooler, 6nz file 500 hp, 10 sp standard. Avg load weight 30-40k lbs. Getting between 5.7-6.5 mpg when driving 65-68 mph around northeast-southern region.

 

I got the best mpg from an FLD 120, engine M-11 Cummins, RTX 10 speed, 3.91 axle 22.5 LP tires

 

M11 was one of my favorite engine to play with when I was at training at fxf on doubles. Their pre 2000 daycabs had that m11 on them.