The original Goodyears are just about gone and most of my research on new tires is almost done but there are a couple of questions about the two sizes that I need some help on. What is the real world difference between them? Will the 30x9.5 affect my speedometer? Waste of money? My Explorer mostly sees freeway driving, no offroad except to the mountains for skiing/backpacking.

Jeff-

There's an excellent site called http://www.4lo.com that has a tire radius calculator, with adjustments for speedo, gear ratio. Check it out.

Jeff,

I used to run 30x9.50-15's on my 91 XLT. The real world difference between the two depends on which manufacturer you go with and which style and model of tire. I happen to have the specs for Bridgestone Dueler A/T's handy so here goes(different specs for different tires from different manufacturers, but they are all fairly similar):

235/75-15
tire diameter- 28.9"
tread depth - 14/32"
section width- 9.1"
tread width - 6.9"

30x9.50-15

tire diameter- 29.6"
tread depth - 16/32"
section width- 9.8"
tread width - 7.3"

So you can see that the 30x9.50's are .7" taller than the 235/75's. This is not much but you will notice a drop-off in power slightly. Any time you increase the height of your tires without changing your gearing, the vehicle will act more sluggish. My experience with the 30x9.50's was that I liked the additional ground clearance but did't like that decrease in performance. It all depends what is important to you.

The 30x9.50's will affect your speedometer, it will read slower than actual speed. This is easily remedied though through the changing of the speedo gear. It is located where the speedo gear cable goes into the transfer case. It is easy for anyone to change, but if you don't want to bother, the dealer can do it for you. You will want to increase the number of teeth the gear has probably by one. To recheck accuracy after the change I time my vehicle between mile markers on the interstate. This works well.

If you're looking for opinions, mine is that if you are not looking for more ground clearance or just bigger tire looks, stay with the 235's. Your performance will stay the same as it was. If you put on 30x9.50's, your performance will suffer slightly.

Good luck with your decision.

DOGMAN