I have a '78 F-150 4X4 that the speedometer doesn't work right. It reads too high and pegs out at around 55mph. I just took off the 15" tires and put 16" tires on, so I know I know I need to recalculate what speedometer gear I need in the transfer case. Is all that I need to do that is the tire size and gear ratio? Also would simply going from a 15 to a 16 inch tire make that much of a difference? Also the speedometer gauge makes a very noticable grinding noise, does it just need to be lubed or replaced? Would WD-40 be sufficient enough to do the job? Thanks in advance.

Deanbob

 

1. If it pegs out, ya got a problem with either the speedometer unit in the instrument cluster, the speedo cable itself, or the gear in the transmission - not the T-case cuz that is not where it is mounted.

A shadetree way to test it is to pull the cable from the tranny and connect it to a drill. Have a pal engage it while you observe the speedometer.

2. Tire size and gear ratio matter. However, it is not a matter of 15-inch or 16-inch wheel diameter. What matters is the tire height and resulting circumfrence. What ya need to do is calculate and the marginal difference of the stock vs your new up- or down-sized tire. There are lots of on-line calculators to figure it out. For example, a larger tire will turn less revolutions per mile so you speedo will read slower than your actual velocity. 15-inch tires (actually the wheel mount size) can be 30 inches tall, all the way to 38 inches tall or even bigger.

3. A dry lube like graphite is best. All WD-40 will do is attract dirt/dust and gum up its internals.