Tire height info is here:
http://www.jekylhyderacing.com/HeightofTires.htm
As far as your transmission, I haven't searched for you model transmiission, yet you have a granny first gear so that should be around 4:00 to 4:07 which is very similar to the NV3500 except yours does not have an overdrive. Knowing all of what I stated have a huge effect on gas mileage.
My 90 Chevy p/u got almost 20 mpg with its NV3500 yet when I installed a T56 six speed, it got 23 mpg and now with it's LT1 and much more HP, it gets around 27 mpg