Currently we have a x2 multiplier based on the short range between bases, so it is less time consuming to travel from base to base. But we also have real climb rates and the performance of planes is adjusted to real performance at real heights.
So, when traveling from base A to base B at fast speed, it seems correct to use a x2 multiplier to compensate the short distance. But when climbing from 0 to 20k it would be incorrect to use x2 multiplier as you spend the same time to alt as in real world.
There is a difference between traveling from base to base and climbing: the speed.
I would propose the following system:
Any plane traveling at or below its optimum climb speed, and with auto-climb engangend, uses arena setting / 2 multiplier (usually X1). Any plane traveling faster than its optimum climb speed or with(edit: out) auto-climb (in fact, auto-speed) enganged, uses arena setting fuel multiplier (usually X2), so the multiplier is adjusted dinamically depending on the speed and the auto-climb(speed) option.
As an example, a F4U1D would use X1 multiplier as long as it keeps below 160 mph while climbing in auto and X2 above this speed or when auto-speed is dissenganged.