Tires: Tire sizing on my F150, ford f 150, tire difference

Question
I have a 1999 Ford F150 XL regular cab, short bed truck.  When I bought my truck, it had 245/75-16 tires on it, rather than the 235/70-16 usually found on this model.  It was my understanding that making the tire slightly wider, by going to 245s instead meant that the aspect ratio should go down, not up like it did.  I have a few questions:

~Is this the case?
~Were these tires supposed to go on this truck? As in, do they properly fit the rims?
~Will the speedometer have been affected significantly by this tire difference?
~Does it matter if I replace the current LT tires with P tires?

Thank you in advance!
Alun

Answer
Alun,

The first thing you should do is find the vehicle placard.  It lists the original tire size and the proper pressure for that size.  If I remember correctly, on a Ford truck, the placard is located on the drivers doorpost near the bottom.

I looked up a 199 Ford F-150 and I see LT245/75R16's listed as one of the possible sizes, but I also see P235/70R16's and P255/70R16's, so looking at the placard will be very important.

LT vs P - yes, there are some major differences in the way the tire carries the load.  LT tires are the little brothers of the big over the road truck tires and take more pressure for the same load, but you get a much more rugged tire.