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speedometer cable calibration help
- Thread starter gd71eb
- Start date
bknbronco
Bronco Guru
No clue! There is no way we can answer that question for ya. You have to answer it foryourself!!%)
Tell me what speed your speedo says and what your actual speed is. Then tell me what speedo gear you have in the t-case now. Then do the math and then youll know what speedo gear you need.
Here is the different gears available
http://broncograveyard.com/bronco/c-902-speedo-cable-gears.html
Changing tires will affect it about 3-4 MPH each tire size, but you could always change speedo gears when you change tires. Takes about 30seconds to do so.
Tell me what speed your speedo says and what your actual speed is. Then tell me what speedo gear you have in the t-case now. Then do the math and then youll know what speedo gear you need.
Here is the different gears available
http://broncograveyard.com/bronco/c-902-speedo-cable-gears.html
Changing tires will affect it about 3-4 MPH each tire size, but you could always change speedo gears when you change tires. Takes about 30seconds to do so.
Last edited:
- Joined
- Aug 23, 2007
- Messages
- 24,322
Less, but if you accurately measure the variables (actual speed, indicated speed, current gear tooth count) you can determine exactly what you need to make it right. Better than buying a couple gears with less teeth. It's possible that no combination will work if you go to an extreme tire size like 37's and keep the current axle ratio.
bknbronco
Bronco Guru
less teeth
take speedo speed divide by actual speed and multiply that by the number of teeth you have now. That will give you how many teeth you need.
with 33's mine says im going 50/ actual 58, X that by 21 (teeth i have now) and i need a 18 tooth gear.
with 37's mine says 30/ actual 40, X by the 21 and i need a 16 gear.
Make sense?
SPEEDO
---------- X TEETH = NEW GEAR TOOTH
ACTUAL
take speedo speed divide by actual speed and multiply that by the number of teeth you have now. That will give you how many teeth you need.
with 33's mine says im going 50/ actual 58, X that by 21 (teeth i have now) and i need a 18 tooth gear.
with 37's mine says 30/ actual 40, X by the 21 and i need a 16 gear.
Make sense?
SPEEDO
---------- X TEETH = NEW GEAR TOOTH
ACTUAL
Count the teeth in your existing drive gear.
If you need the speedometer to read slower, add teeth.
If you need it to read faster, decrease the amount of teeth.
Drive gears are available in 16-21 teeth. Each tooth is an adjustment of about 4 mph.
http://www.wildhorses4x4.com/product/Speedometer_Drive_Gear/Bronco_Dash_Parts
If you need the speedometer to read slower, add teeth.
If you need it to read faster, decrease the amount of teeth.
Drive gears are available in 16-21 teeth. Each tooth is an adjustment of about 4 mph.
http://www.wildhorses4x4.com/product/Speedometer_Drive_Gear/Bronco_Dash_Parts
Quick & Dirty
Sr. Member
- Joined
- Feb 15, 2004
- Messages
- 828
Speedometers are set so 1000 rpm is 60mph.
If your tires are actually 35" tall, with 4.56 gears, the driveshaft rpm is 2626 at 60mph.
Your speedometer then needs a ratio of 2.62
If the output shaft has a 7 tooth gear, an 18 tooth cable gear will work best. 2.57 ratio
If the output shaft has a 6 tooth gear, a 16 tooth cable gear will work best. 2.67 ratio.
If your tires are actually 35" tall, with 4.56 gears, the driveshaft rpm is 2626 at 60mph.
Your speedometer then needs a ratio of 2.62
If the output shaft has a 7 tooth gear, an 18 tooth cable gear will work best. 2.57 ratio
If the output shaft has a 6 tooth gear, a 16 tooth cable gear will work best. 2.67 ratio.
Bronchole
Bronco Guru
That is an interestig statement.
It doesn't matter what transmission you are using or what gear it is in. With the Dana 20 it doesn't even matter if you are in low range. The relationship between the tires, diff gears and the speed of the driveshaft stay the same.
To the OP (gd71eb):
I think what you are looking for is a "best guess" for what gear you should start with. My guess is a 20 tooth. I just installed a 21 tooth on mine and it is within 2% now. I run 35" tires and 4.88:a gears. With your 4.56's I'm guessing that 20 or 19 is what you are going to end up with. Of course this assumes that you have the same gear on the shaft inside the T-case. Don't ask, I don't know which one I have. Like most everyone else I dirve it, calculate the difference and install the best compromise. The formula that bknbronco gave you is spot on for that.
You could even put the transfer case in front-wheel HIGH or LOW and the rear in NEUTRAL and the relationship between the tires, diff gears and speedo sitting at the end of the driveshaft would still be the same.
