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How to calibrate speedometer ?
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KCinKC
Reg. Apr 2010
Posted 2010-06-30 10:02 AM (#5030)
Subject: How to calibrate speedometer ?





Posts: 15

I have a 2000 Freightliner FL70 with a Cat 3126 and 6-speed manual transmission. When I bought the truck I knew the speedometer had been replaced. It seems to show abou 5-7 MPH faster than the GPS says we are going. I have not pulled the speedometer out yet. I was hoping someone could tell me the proper way to calibrate the speedo first. Thanks.

KCinKC
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hogtownboss
Reg. Jan 2009
Posted 2010-06-30 10:53 AM (#5031 - in reply to #5030)
Subject: RE: How to calibrate speedometer ?





Posts: 176

Not sure on the manuel tranny, but my automatic it electronic so it would have to be reprogrammed.

You might check to see it you have an actual speed o cable going to the transmission, if so there will be small plastic gears inside that could be changed out to make the adjustments. But you will need to call someone who deals in that tranny and find out what gear you need for that adjustment.

Something else, this could mean that someone might have installed different tires along the line.

If I remember right if you was to install a shorter tire your speedometer is going to read faster than you are going, if you install a taller tire the speedometer is going to read a slower speed.

Lke my wrecker has 245x70R19.5's factory and I was to drop down to a 225x70R19.5 my speedometer is going to be reading faster than the actual speed I am driving. Look on the door jam of your truck and it should show you the factory tire size and see if the tires match.
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KCinKC
Reg. Apr 2010
Posted 2010-06-30 3:10 PM (#5032 - in reply to #5030)
Subject: RE: How to calibrate speedometer ?





Posts: 15

I had to go to my Freightliner dealer over lunch to get a new water temp guage so I stopped by the service department and talked to them. They told me my speedometer should have 10 switches on the back that need to be set correctly. They gave me the following formula to figure out:

RPM of Tires X Rear End Ratio X 16
Divided by 120


Once I had that info, I called them back and they told me what the correct settings should be. So tonight I'll go home and check to see if the dip switches are set correctly or not. Hopefully that fixes it.

KCinKC
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