View Full Version : Would the Speedo Being off Affect Mileage?

06-30-08, 05:59 PM
Just like the title says... the speedo is off in my 05 STS and it is exponentially off... it gets worse as you go faster. The car only has 9,700 miles on it so it makes me worry... Would that problem affect the odometer reading? how about the MPG readout?

06-30-08, 06:27 PM
It depends on the nature of the error. It wouldn't affect the actual milage but it might affect how the milage is figured and reported.

06-30-08, 10:01 PM
I suggest you test out the odometer/speedometer by taking a ride on a rather flat section of an interstate. All interstates are marked in two tenths of a mile increments. To check the speedometer drive at 60 mph and when you get to one of these markers immediately check the second hand of your watch, drive one mile until you come upon the same mileage marker again and check your watch again. It should take 1 minute to travel that distance. Then do the same thing to check the odometer, when you get to a mileage marker note the two tenths and mile number also the odometer reading on your DIC, drive ten or 20 miles(speed doesn't matter) based on the mile markers, when you reach 10 or 20 miles check your odometer, it should have increased by the 10 or 20 miles you just traveled. A longer trip of 100 or more miles woukld be a better test.

08-19-08, 10:49 AM
yes, you will get worse MPG. along with other things, the ECM will not lock up the converter as soon and go into richer tuning to prevent damage when it doesn"t know what is going on with the car.

08-19-08, 11:12 AM
How far off is it? Which way is it reading; slow or fast? Did you mistakening hit the MPH to Kilometers button? Did you change your tire size when this started?