sparker000 wrote:The rolling circumference will be the same give or take a small margin. The bigger the wheel and the wider the tyre the lower the tyre profile. Lower profile harder ride and easier to curb. Tyre load rating will need to be looked at, they will need to be safe for your axle weight or the insurance will be an issue.
Look at a tyre size converter/calculator, to see what tyre sizes will work against your standard tyre size. I have 18" wheels with tyres size that are fitted to the Sport, they are 101 load rating. If the load rating is safe for your axle weight these tyres will wear down much quicker being softer walled and not being run at as high a pressure.
That said I am very happy with the 18" I have, ride feels better and the slightly wider tyre gives better grip.
As long as the wheels are designed to fit transit custom which they need to be for stud spacing they should be fine. Wheel nuts will probably need changing from original, make sure they are the correct length, too short and they will bottom out on the studs.
Well, it all depends on what profile you use.
I don't know how low you must go on 20" to get a correct speedo reading but it must be sub 40.
On a van I like some more meat on the tires. 18" 235/55 looks good. The speedo should be accurate (50 profile is standard on the sport and the speedo shows 5% less then real speed).
20" 235/50 and the speedo is probably way off.
In order to "fill the arches" you have no choice, the speedo will read wrong.
Going for 20" and less rubber and keeping the speedo accurate only means that you changed the ratio between rim size and profile on the tire. A standard 16" have a small rim but a high profile.