The correct way to look at it is the effective diameter of the tire. What
does this mean? It's the distance from the center of the hub to the ground of
the loaded (van on its wheels) tire.
Tires DO grow..but not enough at even 1000  wheel rpm that you would
notice..If we were talking Formula 1? yeah I might worry a bit about gaining
that 1/2 inch at 200 mph..
The "wrinkle wall" slicks that are used on the top fuel dragsters get REAL
big when spun up to a few K RPM.. but they are not DOT rated steel belted
radials....

So..pop off your rear wheel cover and measure from there to the ground..take
that measurement and do the diameter of a circle bit ( You know, the Pi*D
thing) that's your effective rolling curcumfrence.
Let's say (because I'm TOTALLY lazy), that this comes out to,oh, 5.280 feet...
One thousand turns of the tire = one mile.. Say you have a 4.00/1 combined
ratio in the tranny (great how my numbers work, eh?) and so the engine turns
4000 rev/mile or 4000 rpm at 60 mph....
It's no big deal to do this..Most tire manufactures supply charts that list
rev/mile for a fully inflated tire at maximum load..this number is REALLY
close to what you're going to see on the van..Really, +/- 2 mph in the real
world ain't gunna make or break ya

steve...

"Hey! nice Jeep, Mister!"
"Look kid, it's NOTAJEEP"