1998FordTK

Ok, I have a listed 1998 model F-150 regular cab short bed 4x4 with 4.6 v8 and 4 speed automatic tans. Truck's door sticker has it with H9 rear and tire size as 235/70/R16. I verified that both the front and rear diff's are 3.55 LSD units while changing the fluid for them. While under there I took the speedo gear out and its the black 20 tooth one.The current wore out tires on it now are B.F. Goodrich AT's 265/71/R17. I had a friend use his GPS and when the truck says its going 60Mph by the speedometer the GPS says its going 63. What I'd like to know is what tooth count gear do I need to make it read more to the true speed that the GPS says its running due to having the "bigger" stock tire size on it?

techrep

iTrader: (1)

Not sure about the speedo gear but, I don't think you have a LSD in the front.

marshallr

For 3 mph I wouldn't worry about it. The diameter of a new set of tires will change enough after 40,000 miles to cause it to be off 3 mph. Some of the guys who make radical tire changes causing a difference of 7-10 mph probably need to worry about it.

1998FordTK

Techrep, front axle code is C3. Had the slot with the clutches in carrier when I change the fluid, it may have been a 3.54 I just looked to make sure it was the "match". That and to see if I needed to put in the bottle of friction additive in.