I've got 305/70R16 ProComps and 4.10 gears. I wanted to fix my speedo so I replaced the 19 tooth gear for the 21 tooth gear. The speedo is now more accurate, but still reads 5-10% fast, I estimate. Can someone re-figure how many teeth I should have to correct the speedo. I'm assuming I need the 23 tooth instead of 21. Or, do they make a 22? I might just have to do a trial and error with the gears and have my buddies drive next to me to tell me which on is right!

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1997 Ford F-150
4.2 L V-6
Thrush Turbo Tube catback exhaust
K&N Air Filter
Window Tint
Pioneer 7000R CD player
2 Rockford Fosgate DVC HX2 10's
Rockford Fosgate 500a Amp
Lund bug shield
Vent Shade Vent Visors
GTS headlight blackouts
Bedliner
Chrome Toolbox
3" Body Lift
Lift Lips
305/70R16 ProComp Muds
16x10 Eagle 589
Chrome Westin Nerf Bars
4.10 Precision Gears
Custom Key racing stripes down both sides thanx to some a**hole in the HS Parking lot.
My WebPage

 

Mav07,

Even though my tires are not as tall as yours (mine are 29.6 inches), I had to go to a 23 tooth gear to get my speedometer corrected. I too tried a 19 tooth gear and then a 21 tooth gear but was still about 10% off. Each tooth corrects 5% so the 23 put me within 1 mph by dyno. I got mine from steeda. Hope this helps!

WarEagle

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98 F150 Lariat extended cab, Pacific Green/Silver Frost,
Grey Leather Capt chairs, AM/FM/Cass/6-cd changer,
full tint, color matched bug deflector, vent shades,
A.R.E. hard bed cover, Ultra type 50 polished wheels,
BF Goodrich 315 55R x 16 Sport Truck Radial T/As, 4.6L,
AT, Towing Package, Airaid, Superchip, JBA headers,Flowmaster dual
exhaust, Flowmaster 50 series muffler, 4.10 gears

 

It's much better to use a measured distance to figure out the accuracy of your speedometer. Most vehicles aren't all that accurate.