what causes my '87 bird to indicate higher speed and higher mileage than actual?
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
Are you running stock wheels/tires?
A mechanically failing speed sensor might cause this but I've never seen it. Usually, it's like DavidH25 says - tires and wheels.
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)
Looking for a Used Thunderbird in your area?
CarGurus has 1,041 nationwide Thunderbird listings starting at $6,499.
Search Ford Thunderbird Questions
Ford Thunderbird Experts