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I have a 77 f-150 trailer special that had 31 inch tires on it, now I am running P275 60r 15. the truck has a 460 c-6 with 3.25 gears. The motor, tranny and drivetrain are all original. My question is where can you get a new plastic speedometer cable gear to make the speedometer accurate?Any info would be great. Also did the trailer specials have 31's from the factory as the speedometer was correct with those tires?
The gears are available through National Parts Depot(see left hand side of this website for sponsors). There is also a calculator for determining which gear you need.
Actually, the 235/78-15 will be 29.4" Dia. The 275/60 is still less in dia. though, so you might want to do a gear change. You will see an approximate 5% error in your indicated speed, and its in your favor (at least for speeding tickets) - you'll be going slower than indicated.