what causes my '87 bird to indicate higher speed and higher mileage than actual?

Asked by Jul 18, 2013 at 07:49 PM about the 1987 Ford Thunderbird Base

Question type: Maintenance & Repair

both the speedometer and odometer are indicating much higher values when the car is driven than are actually occurring.  if this system is all electric, there must be a pulse generating sensor somewhere and it may be the trouble source, but I don't have a schematic or manual with details about this problem.  tnx

3 Answers

Are you running stock wheels/tires?

27,565

A mechanically failing speed sensor might cause this but I've never seen it. Usually, it's like DavidH25 says - tires and wheels.

1 people found this helpful.
1,855

My thought is the same as DavidH25's; a dramatically larger or smaller diameter tire will make your speedometer inaccurate by about 1.5 mph PER 20 difference ( i.e. 205 v 225 tires)

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