Testing speed with 60cx

[planewood]

Was out on an interstate last week that had mileage markers. I used the stopwatch feature then calculated the MPH between signs. I had my truck in cruise control and it was a level highway. Also was using a Gillson antenna on top of the cab.

 

The GPS indicated speed was always within 1 mph of that calculated. My truck was always low by about 3 mph.

 

Took about a dozen readings between 60 & 80 mph.

 

FYI - to get MPH just divide 3600 by the time in seconds between the 1 mile markers. My co-pilot used the calculator.

[+Zilvervloot]

I see the same difference. The speed shown on my digital speedometer in my car is always approx 5 km/h (3 mph) lower than shown on my Garmin map60 Cx.

I did not, like you, measure which speed might be the correct one.

 

Zilvervloot.

[kb9nvh]

Vehical speedometers usually indicate a higher speed than the true speed as the manufactuers want to err or tne safe side. I'm surprised your is the opposite.

 

I see the same difference. The speed shown on my digital speedometer in my car is always approx 5 km/h (3 mph) lower than shown on my Garmin map60 Cx.

I did not, like you, measure which speed might be the correct one.

 

Zilvervloot.

[+CJOttawa]

Interesting. My 1992 Toyota Tercel tends to over-read by about 4km/h versus the GPSr. I haven't done the mileage marker calculations but your methodology seems sound. Also surprising your auto speedometer reads low... as kb9nvh says, failure mode is supposed to be the opposite.

[Overland1]

Try that on a Honda GoldWing 1800.... the speedometer and odometer are usually off by as much as 6-8% above actual speed/distance. The only known fixes are slightly larger rear tire or an electronic device which can be installed to correct the calibration.

 

If your vehicle uses a drive in the tranny to feed a speed sensor, there may be a differnt gear available to recalibrate the speedometer/odometer, but the newer vehicles require use of a DRB scan tool to recalibrate the speedometer. Either f these fixes may cost more than they are actually worth to the individual vehicle owner.

 

If you have a built-in Nav system (Chrysler products use one built by Alpine), you will notice there is no speed reading on the display; having such a reading available via GPS would send everybody bto their dealer to complain about the speedometer (in)accuracy, creating major headaches for Chrysler dealerships.

[+wickedsprint]

The more your tires wear the faster they incorrectly show the vehicle speed, my truck with fresh tires is actually dead nuts on with the gps all the way to 85mph, last truck was off by 2-3 mph at interstate speeds..etc.

[planewood]

Try that on a Honda GoldWing 1800.... the speedometer and odometer are usually off by as much as 6-8% above actual speed/distance. The only known fixes are slightly larger rear tire or an electronic device which can be installed to correct the calibration.

 

If your vehicle uses a drive in the tranny to feed a speed sensor, there may be a differnt gear available to recalibrate the speedometer/odometer, but the newer vehicles require use of a DRB scan tool to recalibrate the speedometer. Either f these fixes may cost more than they are actually worth to the individual vehicle owner.

 

If you have a built-in Nav system (Chrysler products use one built by Alpine), you will notice there is no speed reading on the display; having such a reading available via GPS would send everybody bto their dealer to complain about the speedometer (in)accuracy, creating major headaches for Chrysler dealerships.

 

The tires on my truck are larger than what came on it.

 

FYI - After driving for 2 or 3 years with a bad ERG valve and getting, at best, 18 mpg, I finally got it repaired and am now getting 21 mpg on the highway. 1995 Dodge Dakota one owner with 60K showing miles.

[+HaLiJuSaPa]

Both my GPS's (Lowrance IFinder Go and IWay 100M as well as my former one, Whistler Galileo) were always almost exact with our Saturn L-Series wagon on speed, but on my Kia Sephia my speedometer usually reads 5mph higher than the GPS. So I think the GPS is more accurate.

[JingJangJoe]

If you outfitted your vehicle with bigger aftermarket wheels/rims/tyres that does not conform with the vehicle manufacturers' specifications, your speedometer readings will be off by 5% to 15% depending on the choice of wheels/rims/tyres.

 

Even when using wheels conforming to the vehicle manufacturers' specifications, normally the speedometer readings are faster by about 5% for most vehicle makes. I feel that the most inaccurate speedometers tend to be in vehicles that uses mechanical means to drive the speedo when compared to those that use sensors.

[Henny Pa]

Just FYI, I tested my 60csx (and for that matter, my Magellan Platinum and Legend) with a calibrated and certified radar. When on a straight stretch with the cruise set the GPS and radar read the same speed. Believe me folks, your GPS is more accurate than your car's speedometer.

[+Sputnik 57]

The GPSr uses doppler effects to determine speed. Not distance and time, which is good, since the distance (position) is not accurate enough. Garmin reports speed accurate to .1 mph. I believe Magellan is comparable.

[+Thrak]

The speedometer in my 1999 Ford F150 matched pretty darn closely to the speed reading on my 76CS. I'd guess it was generally within 1 mph. I don't think that's enough of a discrepancy to worry about. I haven't checked it against my 76CSx yet.