JVinOlathe
Active Member
While there is no offcial speedometer tolerance, manufactures here adhere to the 4% rule which inidcates that the speedometer can read as much a 4% higher than actual speed but not lower than actual speed. So, if you are going 60 MPH, the speedometer could read 60MPH + 4% or 2.4 MPH or 62.4 MPH but in no case less that 60 MPH so in most case you would be driving slight slower than the indicated speed o you don't risk a ticket. European rules are more flexible and allow a much higher reading so a car that is going 100 MPH, the speedometer could real 113MP and still be in compliance. European sports car manufacturers are known for high reading speedometers with BMW and Porsche being the worst offender, perhaps owners feel good abou thinking they are going faster then they actually are.
As far as figuring your actual speed. GPS unit are now very accurate and you can use one to compare your speedometer reading; there are also cell phone application that do this but be sure you are on a flat, straight section of road. A better way (police uses it for their own cars) is to drive in the Interstate, preferably when you can drive with cruise control and no changes in speed, there are mile markers with mile number clearly marked for even miles and smaller markers every 0.10 miles. Use a timer to time how long it takes you to go 1 mile at 60 MPH and compare to your reading and you can easily calculate the speedometer bias. At 60 MPH it should take yu exactly 60 seconds, if it lakes you less then you are driving faster than 60 MPH and need to be careful about speeding and if it takes you more than 60 seconds then you are driving slower than the speedometer indicates and you should be safe.
As far as figuring your actual speed. GPS unit are now very accurate and you can use one to compare your speedometer reading; there are also cell phone application that do this but be sure you are on a flat, straight section of road. A better way (police uses it for their own cars) is to drive in the Interstate, preferably when you can drive with cruise control and no changes in speed, there are mile markers with mile number clearly marked for even miles and smaller markers every 0.10 miles. Use a timer to time how long it takes you to go 1 mile at 60 MPH and compare to your reading and you can easily calculate the speedometer bias. At 60 MPH it should take yu exactly 60 seconds, if it lakes you less then you are driving faster than 60 MPH and need to be careful about speeding and if it takes you more than 60 seconds then you are driving slower than the speedometer indicates and you should be safe.
