This depends between different models because those explorers that came with larger diameter wheels also came with larger diameter tires (Eddie Bauer and some Sports). I know my XLT came with smaller tires compared to my brother's Eddie Bauer. Hence, putting 31" tires on the XLT would offset the speedometer more than putting 31" tires on the Eddie Bauer. I think the Eddie Bauer's stock tires hover around 30". So what I'm trying to say is you can't compare how much the speedometer will be off from an Eddie Bauer when you have an XLT. However, with that in mind, the calculations are not too bad and is of course a function of tire diameters.
% Offset = [ 1.0 - (initialTireDiameter)/(finalTireDiameter) ]*100
Note that the initial tire diameter must be the tire diameter from the factory. If your stock tire diameter was 28", and you changed to 29", and then went on to 30", your initial tire diameter is still 28". Also note that, even though the speedometer is based on the circumfrence of the tire, in our calculations, we don't need to calculate the circumfrence because it is a ratio and the "2 * PI" term would just cancel out once the fgures are stacked on top of each other.
The resulting percentage will be how much off your speedo will read. So if you had a positive 5% Offset, then you will be travelling 5% faster than what your speedometer is displaying. If you got a negative 5% Offset, then you will be travelling 5% slower than whats displayed at the speedometer.
. In other words, distance yourself from the next vehicle, be aware of the vehicles around you, and travel a little slower on country roads where deers are common 
