Speedo Driven Gear Issue

[tadkingnkc]

My Speedo continues to give me fits. And before anyone suggests it, YES, I’m using the TCI auto calculator and trying to do everything by the book.

All new parts New Speedo, cable, driven gear, driven gear housing because TCI has to be correct right? Verified with paint pen while rotating tire that I have 18 drive gear teeth on tail shaft. 3.73 ratio just installed. 34.9 tire diameter verified on Mickey Thompson website. TCI says I need a 39 tooth driven gear. Awesome, Speedo was off and bouncy so I changed my Speedo because I was convinced it was no longer calibrated right. Speedo bounce and noise gone. Now however, When Speedo reads 55, GPS (two different apps) says 48. So defeating.

That said, TCI is supposed to be gospel so how can this be after I e changed litterly everything. I’ve read that each tooth is roughly worth 3mph. Would you veterans swap out the 39 to a 41 or 42 tooth based on this logic and why? It would literally be a fishing expedition at this point.

 

[Turbo4whl]

So if the tire measures 34.9", what is the radius when the tire is sitting on the ground? The tire deflection may be the issue. Measure from the ground to the center line of the axle. Even a one inch deflection can be two inches different from the diameter of the tire.

1/2 of 34.9D is 17.45 radius. Measure from the ground to your axle center line, 16.5" gives a tire running diameter of 32.9", 16".. etc.

Either way the 41 or 42 tooth driven gear will bring your reading closer. Keep in mind most mechanical speedos can be off by 3-4%. 48 MPH to 55 MPH is more than 3%.

 

[tadkingnkc]

My radius is 16.5” from ground to center of hub. So you are saying I should be using 33” instead of 34.9 for diameter?

 

[tadkingnkc]

According to TCI, that would put me right at a 41 tooth. I guess I’ll give it a whirl.

 

[BKING33]

U need to calculate your tire height with it "loaded" on your truck. Most tires have about 3% less height than the spec measurements. A typical 35×12.50R15 tire is about 33.5" tall when installed on a truck. Your tire is probably around 33.8" in height.