Speedo Accuracy....Testing
11-07-2008, 10:12 PM
#17
11-10-2008, 03:46 PM
#18
Instructor
Join Date: Jan 2008
Location: New York, NY
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My undersatanding is that it is a deliberate calibration to allow some some room for error on the state of tread wear, etc. What it means is you can safely drive about 5 mph faster than you thought you could, LOL.
11-10-2008, 07:31 PM
#19
Poseur
Rennlist Member
Rennlist Member
Back in the early 1980s the Department of Transportation actually REQUIRED speedos to report higher speeds than were being attained (in an attempt to artificially slow us all down).
Here's an extract from one of my service manuals:
As off May 1983 the existing regulations (FMVSS 127) (Federal Motor Vehicle Safety Standards) concerning vehicle speed indication were cancelled by the DOT. Starting May 1983, the only requirement is that the vehicle's speedometer indicate a higher speed that is actually being driven.
Speedometers having European calibration specifications were installed in vehicles for model years 1984 through 1985 (until Dec 1984 production). These speedometer heads have a basic tolerance of (+) 6.2 miles her hour with an additional (+) 5% of the actualy speed driven. For example:
Actual Speed: 50 MPH
+ basic tolerance: 6.2
+ 5% of actual: 2.5
Maximum indicated seed" 58.7 MPH
(with OEM tires and wheels and only normal tire wear)
Speedometers with the former FMVS 127 calibration are installed starting with Dec 1984 production. These heads have a tolerance of (+) 3 miles per hour.
Actual speed: 50 MPH
+ Basic Tolerance: 3 MPH
Maximum indicated speed: 53 MPH
And while we're on it...
ODOMETER
The odometer reading is independent of the speed indication and is regulated by the FTC. The tolerance of the odometer is (-) 1% to (+) 3.75% of the actual number of miles driven.
Here's an extract from one of my service manuals:
As off May 1983 the existing regulations (FMVSS 127) (Federal Motor Vehicle Safety Standards) concerning vehicle speed indication were cancelled by the DOT. Starting May 1983, the only requirement is that the vehicle's speedometer indicate a higher speed that is actually being driven.
Speedometers having European calibration specifications were installed in vehicles for model years 1984 through 1985 (until Dec 1984 production). These speedometer heads have a basic tolerance of (+) 6.2 miles her hour with an additional (+) 5% of the actualy speed driven. For example:
Actual Speed: 50 MPH
+ basic tolerance: 6.2
+ 5% of actual: 2.5
Maximum indicated seed" 58.7 MPH
(with OEM tires and wheels and only normal tire wear)
Speedometers with the former FMVS 127 calibration are installed starting with Dec 1984 production. These heads have a tolerance of (+) 3 miles per hour.
Actual speed: 50 MPH
+ Basic Tolerance: 3 MPH
Maximum indicated speed: 53 MPH
And while we're on it...
ODOMETER
The odometer reading is independent of the speed indication and is regulated by the FTC. The tolerance of the odometer is (-) 1% to (+) 3.75% of the actual number of miles driven.
