Wheel base

[Poconobb]

Maybe silly question ,but looking at different single axle Jayco TT's and seeing different wheel base specs.
What is wheel base on a TT? Vehicles are from front to rear axle,however how is it determined on a TT.
Considering a 18BH and a 154Bh with different wheel bases.Is distance measured from rear axle to hitch ball location?

[Threebutchers]

In the realm of "tractor-trailers", the wheelbase is measured from the kingpin to the center of the trailer tandem....except in California where they measure from kingpin to center of the rearmost wheelset.

[Poconobb]

Thank U for the info.The 18 has 14 in tires and the 15bh has 13 's ,yet the wheel wells and leaf spring looks identical except for different axle rating,and wheel base. I ordered a 15bh and would like to put 14 in tires on without flipping the axle.looks like it will have sufficient clearance.Kuhmo has a 185/14 tire rated at 1840 lb load rating and a 25.2 diameter which should work ,that's the reason for the question.Dealer wasn't helpful on advise,mostly deals with big rigs and motor homes however he is close by,why I choose to buy from him. Thanx