33" Tires and 4.56 Gear Ratio
33" Tires and 4.56 Gear Ratio. Help. By my calculations, I need a gear that doesn't exist.
Right now I have a black, 20 tooth gear in my t-case. When I had 31s and 3.73s, my speedo was perfect. Now, with 33s and 4.56s, my speedo is 10mph fast at 55mph (it reads 65mph). Because of this, my transmission doesn't shift when it should.
From my research, the 4406 tcase (like all other ford t-cases) uses an 8 tooth drive gear.
Now here is where it gets weird:
According to BFG's site, my old 31x10.5s (actual diameter 30.7") rotated 677 Revs/Mile
http://www.bfgoodrichtires.com/specs/all-terrain-t-a-ko/44.html
According to Interco's site, my new 33x12.5s (actual diameter 33.0")rotate 678 Revs/Mile
http://www.intercotire.com/tires.php?id=11&g=1
Question 1: How can a tire which is 2.3" larger in diameter rotate 1 MORE time during a mile. That makes no sense mathetmatically. It should rotate significantly less. One of these F'ers is lying to me, my guess is Interco
With my old tires:
Driven Gear Teeth = (Drive Gear Teeth x Axle Ratio x Tire Rev. Per Mile) / 1000.
20.2 = 8 [drive gear] x 3.73 [axle ratio] x 677 [BFG 31" revs/mile] /1000
Which is why the 20 tooth black gear worked. So that confirms:
1) I have an 8 tooth drive gear
2) BFG's 677 revs/mile figure seems right.
Now, with my new setup:
New Gear Teeth = 8 [drive gear] x 4.56 [axle ratio] x
636 [BFG 33" revs/mile] /1000
New Gear Teeth = 23.3
Which is double checked by the equation:
New Teeth = (Current Teeth x Speedo Reading)/Actual Speed.
23.6 [New Teeth] = 20 [current teeth] x 65 [speedo reading] / 55 [actual speed]
So it seems I need a 23 tooth speedo gear.
Question 2: It seems Ford does not make a 23 tooth speedo gear. What Should I do? What have people done?
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