Originally Posted by jtjohnston
I have just put on 16" snow tires, down from the 17" rim - on my new 2008 JK Wrangler (new for me).
Is my speedometer still working correctly? It seems like I'm driving awfully slow at 100 kph?
To recap what others have said, if the overall size of your tire changed any (32" to 31", etc), then your speedometer will not be working correctly.
If you want to verify your speedometer, use a GPS that shows your current speed, and compare it to your speedometer. If you don't have one, or can't borrow one, a lot of smartphones have free apps that do that too.
If the overall size of your tire did change, to recalibrate your speedometer, you can have your dealer flash the new tire size in (and continue to pay your dealer each time you swap tires), or you can pay for a ProCal once, and make the change yourself when needed.
The second page of the ProCal PDF you uploaded shows how to change tire sizes. Oh, and you need to measure the actual physical size of the tires on your Jeep. Do not go by the size stamped on the side of the tire, as that's often not accurate.