We recently put rims with 235/45/17 tires in place of the 15s that came with the van. How much is this throwing the speedo off? Thanks.

 

You didn't say what size your originals were.
If your new ones are lower, it will cause your speedo/odo to over register.
On the interstate check you odomoter against the mile markers, the longer distance the better.
Or get someone driving a vehicle with stock rubber to run with you and compare differences.
I put a 2" smaller diameter tire on my Dakota and it threw mine off 5% according to the above mentioned tests and police radar. Ha!
But I'm adding 5% more miles than I'm actually driving to the odometer, therefore expiring my warranty sooner.
Do you have any rubbing issues?
I'm about to put 225/55x17s on my T&C.
They are just a fraction of an inch lower so my speedo won't be affected much.