Reprogram Chip?
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Senior Member
Joined: Aug 1999
Posts: 1,615
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From: Sayre,OK,USA
Reprogram Chip?
As you can see from my signature I already have a superchip. I just went from 255/70 R16 to 285/70 R16 tires. Would a chip reprogram be called for with such a drastic tire change?
Former Sponsor
Joined: Mar 1998
Posts: 13,385
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From: Virginia
Hi Jolly Green,
Nice to see you around here again, it's been awhile!
No, since you have a 1998 model, you can recalibrate your speedometer by changing reduction gears in the tailshaft of your transmission. We do not do speedometer calibrations in these vehicles before the 1999 model year, which is when the reduction gear went away and it can *only* be done via programming.
The ECU will normally use miles per hour (vehicle speed) to determine it's shift points, up and down. It looks at numerous data streams to confirm the basic VSS (vehicle speed sensor) data, and as long as they all agree, then the ECU determines the speedometer is accurate and uses vehicle speed to determine it's shift points.
However, if the speedometer is off, the ECU will revert over to it's rpm tables and use them to determine shift points, so that the vehicle can still operate in a normal fashion.
Once speedometer accuracy is restored, then the ECU once again changes back over to using MPH to determine shift points.
You will need to do the math to determine how many teeth you need on the new reduciton gear, just see what the difference is in tire size in terms of a percentage, and then apply that to the number of teeth on your current reduction gear to determine how many teeth you need on the new one. Then it can be obtained from your local Ford dealership parts counter for about $17 or so. Sometimes you won't find a gear with the *exact* number of teeth you need, but you can usually get pretty close, closer than you are now.
Good luck!
Nice to see you around here again, it's been awhile!
No, since you have a 1998 model, you can recalibrate your speedometer by changing reduction gears in the tailshaft of your transmission. We do not do speedometer calibrations in these vehicles before the 1999 model year, which is when the reduction gear went away and it can *only* be done via programming.
The ECU will normally use miles per hour (vehicle speed) to determine it's shift points, up and down. It looks at numerous data streams to confirm the basic VSS (vehicle speed sensor) data, and as long as they all agree, then the ECU determines the speedometer is accurate and uses vehicle speed to determine it's shift points.
However, if the speedometer is off, the ECU will revert over to it's rpm tables and use them to determine shift points, so that the vehicle can still operate in a normal fashion.
Once speedometer accuracy is restored, then the ECU once again changes back over to using MPH to determine shift points.
You will need to do the math to determine how many teeth you need on the new reduciton gear, just see what the difference is in tire size in terms of a percentage, and then apply that to the number of teeth on your current reduction gear to determine how many teeth you need on the new one. Then it can be obtained from your local Ford dealership parts counter for about $17 or so. Sometimes you won't find a gear with the *exact* number of teeth you need, but you can usually get pretty close, closer than you are now.
Good luck!