Speedo Drive Gear

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MOB993

MOB993

Active Member
Posts
143
Hi folks,
My speedo is currently out showing 70mph when actually traveling at 60mph on the GPS. Thats 17% over reading. Its been like this for a good while and I had simply marked the speedo with tipex on the speed limits. I had looked into this problem and thought I might have the wrong speedo drive in the transfer box and picked up a blue 20tooth drive. I finally got round to taking the old one out which turned out to be a blue 20tooth also. I'm now a bit stumped. There doesn't seem to be a 19tooth drive available and I want to make the speedo a bit more accurate. I want to keep the original clock. Any suggestions? can the clock its self be adjusted?

89 110 with 31X10.5R15 BFG's
 
MOB993 said:
Hi folks,
My speedo is currently out showing 70mph when actually traveling at 60mph on the GPS. Thats 17% over reading. Its been like this for a good while and I had simply marked the speedo with tipex on the speed limits. I had looked into this problem and thought I might have the wrong speedo drive in the transfer box and picked up a blue 20tooth drive. I finally got round to taking the old one out which turned out to be a blue 20tooth also. I'm now a bit stumped. There doesn't seem to be a 19tooth drive available and I want to make the speedo a bit more accurate. I want to keep the original clock. Any suggestions? can the clock its self be adjusted?

89 110 with 31X10.5R15 BFG's
Click to expand...
You can buy speedo correctors that go on the circuit not on the drive gear. Can't remember the name though was expensive. Normally changing the gear works unless you have really weird sized tyres
 
Ah sorry I assumed a electronic speedo.
Is it worth looking at the discovery drive gears? I'm not sure if they read higher or lower, but they're diffirent to the defender teeth. I think it was lower. But not certain
Do you have weird sized wheels?
 
No worries. I can only find the 4 sizes of gear drive. Blue, red, yellow and green. The wheels are 31x10.50R15 very close to the standard size I think
 
MOB993 said:
No worries. I can only find the 4 sizes of gear drive. Blue, red, yellow and green. The wheels are 31x10.50R15 very close to the standard size I think
Click to expand...
There's at least a black one cos I had it in my v8 transfer box but isn't any lower teeth than them i think. There's defo some more for disco ones but the cable struggles to fit in without filing down
 
You can get it recalibrated, I had mine done by speedy cables for £80 iirc about 8 yrs ago. Took ages to come back though about a month.
 
MOB993 said:
Hi folks,
My speedo is currently out showing 70mph when actually traveling at 60mph on the GPS. Thats 17% over reading. Its been like this for a good while and I had simply marked the speedo with tipex on the speed limits. I had looked into this problem and thought I might have the wrong speedo drive in the transfer box and picked up a blue 20tooth drive. I finally got round to taking the old one out which turned out to be a blue 20tooth also. I'm now a bit stumped. There doesn't seem to be a 19tooth drive available and I want to make the speedo a bit more accurate. I want to keep the original clock. Any suggestions? can the clock its self be adjusted?

89 110 with 31X10.5R15 BFG's
Click to expand...
Just thinking out loud here but don't you want to be increasing the number of teeth on the gear rather than decreasing them? More teeth on a 'driven' gear means the output shaft rotates slower thereby reducing the speedo readings?
 
MOB993 said:
Hi folks,
My speedo is currently out showing 70mph when actually traveling at 60mph on the GPS. Thats 17% over reading. Its been like this for a good while and I had simply marked the speedo with tipex on the speed limits. I had looked into this problem and thought I might have the wrong speedo drive in the transfer box and picked up a blue 20tooth drive. I finally got round to taking the old one out which turned out to be a blue 20tooth also. I'm now a bit stumped. There doesn't seem to be a 19tooth drive available and I want to make the speedo a bit more accurate. I want to keep the original clock. Any suggestions? can the clock its self be adjusted?

89 110 with 31X10.5R15 BFG's
Click to expand...
blue is the one for the biggest tyre lr do it was designed for the 7.50/16 which was the tallest tyre lr did at the time
 
Kwakerman said:
Just thinking out loud here but don't you want to be increasing the number of teeth on the gear rather than decreasing them? More teeth on a 'driven' gear means the output shaft rotates
slower thereby reducing the speedo readings?
Click to expand...

Was chatting to an engineer buddy who says the same that more teeth should slow down the Speedo bringing it inline with correct speed. There is currently a blue 20tooth in it so will pick up a 21 or 22 tooth. Just trying to figure out which one now. Knowing my luck I'll pick the wrong one. Not cheap either unlike the blue one
 
Hopefully remembering school arithmetic (a long time ago), and assuming they do one, then 23 teeth should do it 20/23 = 0.87

70(mph) x 0.87 = 60.8 (mph).
 
Yea that's the math I done too. Apparently there is a formula but needs gear diameter not just number of teeth. But if the gear drives are the same diameter then you are probably right. I'll get the 22 tooth and see what happens.
 
Update: I put a 22 tooth drive in and it brought it closer to correct at just 5% over reading. Close enough for me.
 
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