The default Fuel Burn Rate of 1 is VERY slow. Your track is about the same length as the ones I've run on with RCS64, and it took forever to burn a tank down with the default settings. It's been recommended that to make the new version more like the old one, the burn rate should be 10. Just be sure to only change one thing at a time, so that you can see how it effects the burn rate.
I don't think track length has a bearing on fuel burn, but it certainly could. If it does, I'd like to know. I believe it's just a calculation of total throttle output level, so the more full throttle you use, the faster it will burn. Keeping the throttle low will burn less fuel. Depending on the car and driver, this can make a pretty massive difference.
Hopefully, and I believe this IS the case, fuel burn is based on the speed being sent to the CAR, and not the throttle level reading. This was a problem with SSDC, where your throttle curve could cause you to burn more fuel, and you could game the system by setting up a curve that let you keep your throttle at the low end while the car is being told to go fast, and burn LESS fuel than you would otherwise.