Speedo Correction

[85_Ranger4x4]

I just put 31's on my truck, according to the checker thing in the library it is 5mph faster than it originally read at 55mph.

My truck did have 235/75-15 tires, and I do think it was kinda close but I can't see the PO being smart enough to change the gear when he upsized to them from the donuts it came with. I am afraid of my previous margin of error stacking up with my new one and causing problems.

In the library article:

Ford Motorsports Also Uses This Formula:

Driven Gear Teeth = (Drive Gear Teeth x Axle Ratio x Tire Rev. Per Mile) / 1000

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

How many teeth are on the drive gear? I have a BW1350M if that matters at all. If anyone has a simpler formula I am open to them too.

 

[Spott]

I just find a long empty country road, and cruise down it with a GPS and some notepaper. Maybe take a friend along to help you out. Get your actual speed when the speedo reads 20, 30, 40, 50, 60, etc MPH.

Then figure out the ratio between indicated and actual speed, and use that ratio to change the speedo driven gear.

Spott

 

[85_Ranger4x4]

I just find a long empty country road, and cruise down it with a GPS and some notepaper. Maybe take a friend along to help you out. Get your actual speed when the speedo reads 20, 30, 40, 50, 60, etc MPH.

Then figure out the ratio between indicated and actual speed, and use that ratio to change the speedo driven gear.

Spott

That would be easy if I had a GPS.

Ford had used this speedometer drive since the 50's, I kinda figured somebody out there would know how many teeth are on the gear that drives the speedo gear.

 

[Spott]

The BW1350 (and the 1354) uses either a 6-tooth or the 7-tooth driving gear. The 6-tooth gear is dark red, and the 7-tooth gear is "natural" plastic, sort of an ivory color. Aftermarket gears, if they were ever made, could be any color.

Get a mirror and a flashlight, and you should be able to pull the speedo cable and peek in the hole, to see the color.

Knowing the circumference of your tires (do a roll-out measurement), and the ratio of your differential, and the number of teeth on your driving gear, you should be able to calculate out the driven gear you need, and determine the error as well.

Spott