Does a taller tire w/o calibration increase or decrease the speed/miles on the speedo/odo?

 

A taller tire will show less milage on current speed and accumulated miles

 

It decreases it, just like it does your speedo. For instance, if your running 35's and your speedo shows 40mph, you may be doing more like 45 as an example.

So if you have larger tires and your odo shows, say 20,000 and you've had the tires on there for quite sometime, you may actually have more like 22,000 miles.