To adjust for changing tire size:
 

Old tire x Old Gear / New Tire = New Gear
OT x OG / NT = NG

(some one told me that 31" tires usually has a 19 tooth gear, and 235/75/15 tires has a  21 tooth gear)


Example:

Old Tire - 29"
Old Gear - 19 tooth
New Tire - 33"
New Gear - ?

235/75/15 = 28.9"

Old Tire
 Old Gear
New Tire
 New Gear
28.9
 x 19
 = 549.1
 / 33
 = 16.639 (17 tooth)
29
 x 19
 = 551
 / 33
 = 16.697 (17 tooth)
31
 x 19
 = 589
 / 33
 = 17.848 (18 tooth)



If you change tire size and gears (or just gears), you have to go one step further:

New Gear / Old Ratio x New Ratio = New Gear

NG / OR x NR = NG
17 / 3.55 x 4.56 = 21.8
 

21 / 33.5" * 28.8" = 18.1
18 / 3.55 * 4.56 = 23.1
 

Each tooth on the speedo gear adjusts your speedometer reading by approx. 5%. So when you add a tooth, you reduce the speedometer reading by 5%. 





Ford (truck) Speedometer Gear's
Tooth Count Colour
 Part Number
 Notes
16
 Wine
 C0DZ-17271-A
17
 White
 C3DZ - 17271-C
18
 Yellow
 C0DD-17271-B
19
 Pink
 C0DZ-17271-B
20
 Black
 C1DZ-17271-A
21
 Red
 C4OZ-17271-A

22


23
 White?
 aftermarket


Here is what speedo gears I have had to use:

302, AOD / BW1345 / 4.56' r&p / 35" BFG KM's = 19 tooth gear (pink)

302 / AOD / BW1345 / 5.13 r&p / 36" Irok's = 21 tooth gear (red)



 

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If you have any comments, suggestions, or see any errors, please let me know..... cbradley@telus.net

Last updated on July 29/2007