I have a Garmin windshield unit - which has always been very accurate. My speedometer is saying Im traveling 70MPH but my GPS says Im traveling at 67MPH...

At every speed I seem to be off by 3 MPH...is this a common issue? Is there an easy fix?

 

I would think the portable units have a less accurate instantaneous speed, since it is calculating it based on distance traveled over a particular time frame, maybe 1 sec, maybe 2...I dunno. But the point is, the less accurate a gps is for horizontal distance, the more error you will observe.

Have you checked it in another 'stock' vehicle, w/o any wheel/tire discrepancies from OEM?

 

I used to always use it on my Audi A6 and the MPH always matched up perfectly...

 

Hey there, I think thats about right Mayor, the discrepancy I mean!

I use my Roadangel GPS /Laser detetector in my Audi A6 and its always been off from my speedo by around 3-5 mph.....In actual fact the faster you go the higher the discrepancy between the two.
I think that the mobile GPS units are actually more accurate as they lock on to 6 maybe 7 or more satellites. Its the speedo that is inaccurate, by up to 10% I believe usually...thats why over here many people caught speeding and over the limit in a 70mph zone and over by 3-4 miles per hour have been let off. Its why when in a 30mph zone the fixed Gatso cameras are actually set to go off at 35mph

 

GPS will be way more accurate. Speedo, not so - US law allows for a +/-5% accuracy on the speedo.

- Tim

 

Mayor, if your speedo is off by 3 mph at all speeds, then your needle for the instrument cluster has gotten moved somehow. I have seen this in the past. Normally the fix for this is to simply pull the needle off, reposition it to where it needs to be and then reattach. But, this is was used on other Ford vehicles. I am not sure exactly how the needle is attached to this instrument cluster. I believe it is just pressed into place. If this is the case, then what I said above can be done.

 

Are you on stock wheels and tires? Wrong sizing can affect speedo readouts...

 

As tires wear you will get up to a 3% difference in your speedo. Car companies don't guess low either as it can be very costly with the men in blue.

 

I have a GPS receiver that plugs into my laptop. My speedo is right at 3mph low also. This reading is backed up by every road side radar unit that I go through. Your GPS unit is going to be much more accurate than a speedo. Some speedo's will be more accurate than others. The only car speedo that I've ever had that was accurate was in an ex-state trooper car I bought that the had been set up and certified to be true.

 

+ -5, is quite the error telerence...especially for a vehicle, but thats just my chem side speaking

 

At least you wont get done for speeding.
By (UK) law speedos must read over rather than under with correct size tyres,correct tyre pressures and fuel

 

Chem? As in chemistry? Aren't we talking physics here? :>)

 

The laws are pretty much the same everywhere. We've all lived in 'ignorance' for years about speedo inaccuracy, now everybodty has a GPS and are 'discovering' something new. My S type over-reads by a few MPH just like Jaguar designed it to.

 

I have five different GPS's (seriously). One time, just for the fun of it, I put all five of them in my Expedition and went for the drive. Every GPS showed exactly the same reading, and two of them even showed tenths of a mile per hour and they were accurate to the tenth! The speedometer was off by a couple of MPH depending on my speed. GPS's are incredibly accurate and speedometers are notoriously inaccurate. Your speedometer can be affected by tire size, tread wear, air pressure, temperature...

Trust the GPS. It's showing you your real speed. It may lag behind by a second or two as you accelerate or decelerate, but if you're driving a steady speed, it's spot on.

 

well there you go, I suppose I was off on this particular theory. Glad some other knowledgeble ones could clear that up.

 

If you ever want to "calibrate" your speedo and don't have a GPS unit, the easiest method I know of is to get out on the highway and set the cruise on a flat road. Now, look at the markers on the side of the road that tell you the length of the highway. As you pass the milemarker, start a stop watch. Time the time it takes to travel from one marker to another (this is best done if you can do it over a few miles). Take the time and convert it to seconds. Now, take 3600 times the number of miles that you drove and then divide the time you go into that number. So, for example, if you timed yourself covering 3 miles in 3 minutes (180 seconds), you would then go 3600 x 3 = 10800, then divide it by 180 and you will get 60 mph. This is true of all speeds. Form there, you can look at what your speedo says and compare that number to what you calculated.

The reason why I recommend doing this over a few miles is that it reduces the effect that errors may have on what you are calculating. The mile markers may not be exactly 1.00 miles apart and/or you starting/stopping the stop watch may not occur exactly in the same spot with the mile marker. Most stop watches on your wrist watch are well within any accuracy to make this a valid way of measuring things.

 

I can just see someone..."Dang, didn't start it in time" Next marker "Dang, did it again"...and a little later "Well officer it's like this, my speedo doesn't match my Garmin..hey got a radar gun with you?"

 

Speedometers are always calibrated high within a tolerance. They're never on or under. Imagine the lawsuits when citations are received because the speedometer read under the actual speed.

 

I think its amazing how Garmin's and tomtoms can predict speed, but i would still listen to the car is saying.

 

My GPS on my BB shows my XJR speedometer is spot on, and my wife's Excursion reads three MPH over. I agree with Sam, the GPS is going to be the most accurate guage of speed that us lay people are going to have access to.