Speedometer inaccurate??

[Petrograde]

This is odd. Today, driving home from work I happened to look at my speed as indicated by my GPS. It was different than my speedo read.

The GPS speed was about 3 mph faster. hmmmm.... Is Ford trying to slow us down a bit? or,... perhaps my GPS is wrong,.. I doubt that since it perfectly showed my car's position on the map.

weird,...

Tom

[martyo]

Tom: How long were you holding your speed steady? The GPS averages your speed over time (many units can adjust the averaging speed as an option) and can lag behind real time speed.

[woaface]

Check the manual that came whith the GPS unit, see if it has an error rate at all.

I've seen some stock speedo's off by a couple of mph. I guess you might want to take it to a shop and see if it's calibrated ok.
*Edit*
This was on the Crutchfield website for the Garmin StreetPilot 2610, which is or is close to what you have right? Anyways

GPS Accuracy: This unit's GPS positioning capability is accurate to less than 15 meters (49 feet).

Speed might be off a lil too.

[RCSignals]

Our speedometers aren't calibrated to read that accurately, otherwise they'd have one or two mph increments on the dial.
It's not unusual though for a speedometer to be calibrated to read slightly under actual speed.

[Bigdogjim]

Tom: Easy to check the speed. Hold on cruise control at 60 and re-set the trip. watch the mile markers and your watch. Lots of level road in Texas:):):)

[Petrograde]

Thanks for the info guys! I'm not sure if my GPS 'averages' my speed or not,...

I'll try that Bigdog!

Tom

ps- hey Sarge,. Have you noticed this with your SP3?

[Marauderjack]

Speedo's are supposedly calibrated at 60 MPH?? :confused:

Mine reads "dead on" at 60 with my GPS..... :)

I did notice a 2-3 MPH difference when I had 26 psi in the rears....Speedo read a little high, which makes sense!! :up:

Inflated rear tires to 38...(BTW, rear center wear seems to have stopped) and speedo & GPS are happy again!! :)

Marauderjack :pimp:

[RoyLPita]

During a test drive in a 300a spec MM, I went past one of those police speed monitors. I noticed that the speedo was 2mph faster than what was posted. Go figure.

[Paul T. Casey]

Speedometers, as with any gague, is most accurate in the center third of its range. I, too have noticed the 2 mph difference with the speedometer and traffic gun (mine is also 2 higher.0 in the 45 to 70 mph range. Had my brother the cop also check me with his Radar, same result. I couldn't pin him down on the police radar's accurancy (trade secret I guess, like the handshake to avoid tickets) but he says that's why they don't ticket anyone for 2 mph over the limit.

[DOHC 4.6]

Speedometers are typically accurate within +/- 3%. At 60 mph that is about 2 mph.