Tires and speedometer

Jeep speedometers generally read slow stock, you are probably closer to the correct reading now than before. (but still slow)

You need to find a measured mile or use a GPS to see what it is now. I have found that Jeep speedometers are calibrated for the largest stock tire size so yours might be right on now.

Our XJ's became right on with 235's which are almost 30" tall.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail > I upgraded to alum canyon wheels and 30" tires from 195/75R15 steel

Speedometer ratio adjustment calculation. Ever wonder how far off your speedometer is with your new bigger tires? By using a simple ratio calculation, this info will only approximate your corrected speed and depends on the accuracy of tire size diameters (assumimg nothing but tire size has changed). If you know specific information about your vehicle (gear ratios, RPM, etc.) then use the Gear Ratio calculator, it's probably a little more accurate.

Formula used

(New Tire Diameter / Old Tire Diameter) * Speedometer MPH = Actual MPH

Another way of looking at this relationship would be to figure what the indicated speed would be if you were actually going 60 mph. In this case, the tire diameter relationship is flip-flopped to:

Indicated Speed = old tire diameter x actual speed New tire diameter

Using the previous example, your speedometer reading at an actual 60 mph is:

28 x 60 = 48 mph 35

Here's the site.

Easy way to find out, beg borrow or steal one of these GPS navigation devices, they give your speed and are pretty much dead on.