Speedometer Correction with New Tires
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Junior User
Joined: Feb 2005
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Speedometer Correction with New Tires
I've got a '96 F150 5.0L. I went from 235 75R 15s to 31 10.5 R15LTs tires.
I know going to the larger tire was going to throw off my speedometer but I wondered what else. When I asked a mechanic? at the local Ford dealer about it, he seemed in a fog about it and said he thought maybe they could change a gear in the speedometer to correct it but he didn't seem sure. Lets say this guy didn't inspire alot of confidence in me.
When I looked in my owner's manual it said the following: "Your vehicle contains a speedometer which receives its input from the anti-lock brake sensor. The speedometer is also used to provide a speed signal for correct operation of the vehicle's Powertrain Control Module, electronic transmission, and speed control. It is very important to change the speedometer's Internal Conversion Constant if the size of the tires of the vehicle is changed. The Conversion Constant can only be changed 6 times once the vehicle leaves the factory."
1. How does Ford change the Conversion Constant, and is it simply a different gear put in the speedometer?
2. I have noticed since I put on the larger tires that the transmission is shifting too soon, so will changing the Conversion Constant correct this?
I would appreciate any feedback I can get on this subject, Thanks.
I know going to the larger tire was going to throw off my speedometer but I wondered what else. When I asked a mechanic? at the local Ford dealer about it, he seemed in a fog about it and said he thought maybe they could change a gear in the speedometer to correct it but he didn't seem sure. Lets say this guy didn't inspire alot of confidence in me.
When I looked in my owner's manual it said the following: "Your vehicle contains a speedometer which receives its input from the anti-lock brake sensor. The speedometer is also used to provide a speed signal for correct operation of the vehicle's Powertrain Control Module, electronic transmission, and speed control. It is very important to change the speedometer's Internal Conversion Constant if the size of the tires of the vehicle is changed. The Conversion Constant can only be changed 6 times once the vehicle leaves the factory."
1. How does Ford change the Conversion Constant, and is it simply a different gear put in the speedometer?
2. I have noticed since I put on the larger tires that the transmission is shifting too soon, so will changing the Conversion Constant correct this?
I would appreciate any feedback I can get on this subject, Thanks.
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