Help with Speedo Gear

[dirtystanchez]

Hi all!

I'm having trouble figuring out the size of the new speedo that I need.

87 ranger, 5 speed, 17 tooth (white) speedo gear.
Door label says 215/75/15 was original tire size.
I used to have 3.73 gears

Currently installed
31 inch BFG AT = about 30.5 inch tall
17 tooth speedo gear
4.56 gears

Using formulas found while searching
1. (new gear ratio/old gear ratio) * current speedo gear
(4.56/3.73) * 17 = 20.8 or 21 tooth new gear

2. (driven gear*gear ratio*revs per mile)/1000
(7*4.56*650)/1000 = 20.8 or 21, again

So seems like I'm on the right track. The one thing I'm not understanding is how to factor the difference in tire size into this equation. Any help would be appreciated!

 

[bigmark303]

http://www.therangerstation.com/tech_library/Gear_Tire_RatioChange.html

Use the calculator in the link above to find the correct gears you would have needed to first correct your speedometer after the tire change with your stock gears. Then use that corrected ratio and round off to the nearest manufactured ratio then do the speedometer tooth gear calculator with the new ration in place of your stock ratio.

Example

My truck had 3.27 gears and 29" tires. I changed tire size to 33" tires and then gears to 4.56.

When I changed my tire size to 33" I needed a 3.72 gear ratio to correct my speedometer while keeping the stock toothed gear. I never changed to a 3.72 ratio but that is the ratio I needed to correct my tire change first so the speedometer read correctly. With that information and knowing a 3.72 gear ratio was not manufactured I rounded to a 3.73.

Now with a change from 29" to 33" tires and a change from 3.27 to 3.73 gears corrected my speedometer while keeping the stock speedometer toothed gear. then use the corrected ratio in the speedo gear calculator now that everything is corrected from the tire change.

Your truck had 27.5" tires and 3.73 gears stock. You changed to 30.5" tires so the now correct gear ratio you needed would have been 4.14. Round to 4.10 because thats the closest manufactured gear ratio and use 4.10 in place of your stock ratio when figuring the speedo tooth gear.

17x(4.56/4.10)1.11=18.87 round up to 19. Youll need a 19 tooth gear to get as close as possible to the correct calibration.

 

[dirtystanchez]

Thanks bigmark303! I see the error in my ways now that you spelled it out for me!