Dan,
Are you wanting the overall wheelbase of the vehicle or the axle spacing between axles.
Here is both measurements.
Overall wheelbase is from the center of the steer axle to between the tandem axles gives you overall wheelbase. For example 265" wb.
Axle spacing is measured from the center of the front drive axle or trailer to the center of the second drive axle or trailer.
Most configurations are what we call closed tandems will be 50" from center of the front drive to center of the second drive.
On trailers a closed tandem will be the same 50" and when you get into a spread axle like you see on flatbeds or some reefer trailers the min is 10'-2" and up to 12' or 144".
Ok the picture below is a closed tandem and the measurement is from the center of the axle cover to the center of the second axle cover.
This shot shows overall wheelbase which is measured from the center of the steer axle to between the drive axles. In this case the overall wheelbase on this truck is 303" wheelbase.
This truck the overall wheelbase of the tractor is 331" wheelbase of the truck with a 53' trailer with a 12' or 144" spread on the trailer.
I hope this answered all your questions.
Good Luck
Wade