Odd MPH Readings

[EddieS'04]

I have had the scan gauge II for several years. Also thru several sets of tires. When I have gone thru radars and also had a local officer check me by radar @ 40 mph on the dash speedo. I would actually be going 37mph. The scan gauge would be reading 43 mph when the radar reading was 40 mph.

The scan gauge is calibrated to the rev/per/mile for the tire size.



Now all of the sudden the factory speedo and the scan gauge read together. The scan gauge is still calibrated correctly. It cant be because of tire wear. that would effect both speedos'.

I havent been able to get it radar checked yet. I just find it odd, that. All these years I knew if I drove with the scan gauge reading 3 mph higher than posted. I was on the money.



Most of us have found that our gen1 tracs have a 2 to 3 mph error with the oem speedo. So I have always used the scan gauge when Im on the highway. Now Im confused, that both speedo's read the same @ all speeds.



Any Idea's? I dont have a GPS, to compare with. It may be a gremlin. Just not battery related...LOL:banghead:

 

[Richard L]

Eddie,

The ScanGauge gets it's readings from the VSS (Speed Sensor) signals passed to the computer. It does not matter what tire size you have, they both get the same Speed signal, assuming the tire size is exactly the same?



I have an UltraGauge that I can calibrate myself using the mile markers on the Interstate. I drive at least 40 miles on a fairly straight section of Interstate, starting exactly at one mile marker and stopping exactly at another mile marker. The longer you drive the more accurate the calibration will be.



When my Hyundai Tuscan was new, the Speedo was reading about 3 MPH faster than the calibrated speed the UltraGauge and GPS indicated. After about 18K miles on the Hyundai, I noticed that the difference in speed is not as much as it originally was....I assume that may be due to tire wear. The Speedo and UltraGauge are getting the same signal from the computer, and the UltraGauge factors in the adjustment, but the GPS never changes and it still matches the UltraGauge speed withing about 0.1 MPH ???



...Rich

 

[DaninVa]

Every vehicle I have ever owned....the speedometer read 1 to 3 mph high. Could it be to use up the warranty faster??? :rofl:

 

[Bob McDonald]

I have always suspected that the speedo reading fast was an intentional design by the manufacturers to reduce the exposure to lawsuits from owners claiming speedo's reading slow as a cause for receiving speeding tickets.

 

[Richard L]

Uncle Bob,

Probably true to some degree, but I also think there was some financial motivation to reduce the length of the Warranty mileage.



A small 3% error (some are even more than 3%) in speedometer/odometer reading can shorten your warranty by over 1000 miles on a 36K mile warranty. Just consider how many of us have had breakdowns within a few hundred miles past the warranty expiration....then multiply that by the number of vehicles the automakers produce and sell every year! I'm sure that save automakers $millions every year.



As previously stated, my Hyundai is 3 MPH too fast at almost all road speeds, however my Mercedes speedometer is only off by 1 MPH fast at all speeds when compared to my GPS and the UltraGauge.



In the end I think the automakers decided it was financially better to make money by having the speed/odometer readings discreetly shorten the Warranty mileage rather than spend money to make the speed/Odometer readings more accurate.



...Rich