Fuel economy related to fuel temperature?

A little bit of History.

My Dump truck is a 1999 T800H, I bought it with a 3406 E 475 hp 1650 lbs torque, 13 speed Eaton, final drive .73 at 13, rears 4.56:1, 46,000lbs seating on New Way suspension rated at 52,000 lbs on 11 R 22.5 tires, big single 150 gal fuel tank and 75 gal Hyd tank, empty weigh was 28,000 lbs (too heavy for my operation), max speed at 2,100 rpm was 75 mph, sucking fuel average at 7 mpg, my runs are around 200 miles round trip, sometimes 300 one occasional load 500 miles round trip, always one way empty with a loaded weight of 70,000 lbs, so my payload was 21 tons, I don't get pay by the ton, but I like to provide my customers with the biggest load possible.

Well last November I took it to the operating table and replace rears with suspension for a Pete low air 40,000 lbs and 3.73 gears, to drop the rpms and in search of more mpg and lighter MT weight, also replaced was, fuel and hyd tanks and replace all cross members and removed a sub frame from under the dump body.

Long story short the truck now weights just shy of 24.000 lbs (GOOD) and my rpm's are now low, but at the shop they disconnected the fuel cooler, still there but unhooked and fuel economy is 6.81 at best but with the increased temperature lately is as low as 6.4, I purchase a laser temp gun and have been monitoring the temp, at 8:00 am before start up 90* to 99*, running has gone to 146* 156* and today 160*, tank is really hot to the touch could this be the reason for my poor fuel economy? engine runs perfect, lots of power, but you know after so many years of trucking, once you get to know a truck driving a few days or so, you could tell when something change, a little vibration, different sound, when the truck feels heavy etc, and I can tell when it gets really hot is not as powerful as when cool, that's why I purchase the temp gun.

Now I come here and ask you guys to chime in and give me your opinion which is highly appreciated.

From a fellow trucker down South almost at the end of the world, Thank You.

 

Thanks for all the posts, I haven't calibrate the speedometer yet, but I'm using the GPS to get my speed and miles traveled.

Also I do know as a fact that the cooler the fuel the better performance but just wasn't sure on the mpg difference being so big.

I'm hooking the cooler this weekend my self and was thinking of installing a second cooler on the return, just to keep it as cool as possible, this morning when I cranked it up was 110* inside the tank, am I being paranoid or will the second cooler actually help?

Thank you guys and keep posting.

 

I'll try to take a picture and post tomorrow.