KyleC,
You will need to change the speedo gear if you want the readings to be accurate. You should just reduce the gear by one tooth, I'm pretty sure.
You should be able to do this change yourself, since it is very simple. The speedo gear is located on the back side of the transfer case. You'll notice the speedometer cable entering the case, which is secured by just one bolt. Remove this and the gear is attatched to the cable with a C-clip. Just take a small screwdriver and remove this and off comes the gear.
The gears are color coded for different number of teeth. I've changed mine so many times I can't remember the color for use with 30" tires, but anyway, determine the number of teeth and purchase a new gear with one less tooth than the present one. Any Ford dealer should be able to help you. The price of the new gear shouldn't be over about $10.
Be aware that any time you increase the diameter of the tires, your performance will suffer. In this case, it's a slight drop-off, but a decrease nonetheless. It all matters what's important to you. I did what you're thinking about, go from 235's to 30x9.50's, and I had stock 3.55 gears. The performance drop-off was slight, but still noticeable. I don't know what your gears are, but if you have 3.73 or 4.10, the 30's won't have as much effect on decreasing performance.
Also, a good way to check to see that your speedo is correct is to time yourself between milemarkers on the interstate. The obvious one is 60.00 seconds for 60mph, but I've calculated more normal cruising speed times. I actually have these computed for every one mph but for the sake of space:
60mph = 60.00sec
65mph = 55.38sec
70mph = 51.43sec
75mph = 48.00sec
80mph = 45.00sec
85mph = 42.35sec
90mph = 40.00sec
I use this method all the time and it is suprisingly accurate.
Sorry for a long winded answer to a simple question, but once I get rolling..........
DOGMAN
Good luck!
DOGMAN
