My speed will not get right with my gps. It is 7 mph low and my speedometer reads about 5 off. I have 295/65/r20 trail grapplers and even when o enter the right tire size and reprogram it's off? What am I doing wrong? I have an fx4 so my gears are3.73 right?

 

What's the axle code on the door jamb sticker?

 

Yes, your Gear Ratio MAY not be correct, but if you entered 295/65/20 into the Tire Size position in the "Options" menu, it will not account for "squish", and will make your speedometer read fast. The PCM thinks your tire diameter is about 35.1" without squish.

Next time you program a non-stock level, say "yes" to "Create A Custom Program?" and then enter a value of 34.0" or 34.1" in the Tire Size data field. This will override the other value and should be much closer to the actual size you need.

- Jack

 

Yeah I understand that, but its not changing it at all. When I enter the bigger size it doesn't change at all?