Ever since I purchased my 2015 Mustang Base Model with Automatic, I noticed that the speedometer is reading high about 2-3 MPH depending on the speed I am driving. I believe this for three reasons: If the speedometer is reading 60 MPH (right on the line) and I set the cruise control, the cruise control indicates “58 MPH” and my GPS indicates I’m traveling at “58 MPH”. The third reason is when I drive by those “Your speed is” radar signs, they always read a few miles under what my speedometer reads (I realize those signs may not always be accurate). I asked the dealer service department and they asked if I had larger tires (they are the original tires) and at first they told me the speedometer can’t be recalibrated. Now they tell me it may be possible to fix the problem but it could take a long time to find the problem. I realize that a few MPH is not terrible (it just bugs me a little) but since the speedometer is tied in to the odometer, wouldn’t the odometer show many more miles than the car actually has after several years?
Sorry, but I fail to see the reason for concern. You are going to drive around the world before the miles driven amount even means anything. I doubt you could find 10 cars that the speedometer is 100 % accurate.
I bet (but don’t know) that carmakers do this to keep from getting sued by folks who get a ticket for speeding. If the speedometer underestimated the speed, that would be a bigger problem, ticket-wise. You may have gotten a car at the top end of the ‘acceptable’ calibration range. You could provide them the info to calibrate this by setting your speed with the cruise control, record what’s on the speedometer, and measure how long it takes to drive, say, 10 miles on a fairly straight, flat road, according to the mile markers. Calibration factor = calculated actual speed/indicated speed.
Most of my cars, over the years, where about 5 MPH high. My latest is about 2-3 high (15 Forester).
I’ve always assumed the error was built in for the reasons texases listed.
Just to kill time I did a Google search ( really not that hard to do ) one vehicle testing firm found that out of 60 vehicles 93 % read slower than actual speed and they claim that the guide lines are for the ones reading low should only be off by 4 MPH ( 64 instead of 60 ).
Well, maybe 5% more. Just check it against the mile markers on the interstate.
New tires versus worn tires will make the speedometer read different.
Same size tires different brands will make the speedometer read different.
You’ll never get the speedometer to read exact for the life of a tire.
It’s only 3% off. That isn’t much and it won’t get you in trouble for speeding. You can correct it in your head or not. Either way, drive on.
My speedo was a hew mph high.
I removed the instrument cluster, pulled the needle, adjusted it, and retested it. I did this some years ago and have since checked it against numerous radar “signs”, as well as a calibrated police radar gun, and it’s right on.
I don’t know if this is an option on your car. Mine has an analog needle. But if you have a needle too, it’s something to think about checking into.
It can be fixed with a software calibration update. Maybe the dealer can’t do it but there are aftermarket engine programmers that can be purchased to fix the speedometer error. They cast about $400. Is that 2-3 mph worth $400 to fix? If it is, order one of these
http://www.americanmuscle.com/x4-custom-tuner-2015v6.html
It comes with custom engine tunes to improve mileage or performance but you can ignore that and just use it to adjust the speedometer error.
The problem is not worth $400 to me to fix but the car is still under warranty so I’ll see if the dealer can fix it. Thanks.
I’m pretty sure, they are going to say that 3% is within factory spec and send you on your way. I have a 2016 Mustang GT, mine reads high by a few MPH also, nothing worth worrying about IMHO.
I’ve owned ten cars and this is the first time I’ve noticed this problem although the other cars didn’t have a cruise control speed indicator. I once owned a 1973 Pinto (don’t laugh) - bought it new and the speedometer would fluctuate up and down while driving at a steady speed. Of course it never did that when I took it in under warranty. I guess I’m asking too much when I pay a lot of money for a product and expect everything to work as it should at least when it’s new.
No offense, but maybe you have unrealistic expectations. There is no such thing as perfection in anything, especially a complicated product like an automobile. Variability can be reduced, but it will always exist. My 2005 Accord EX V6’s speedometer is on the mark and has been for the last 188,000 miles. I verified it with mile markers and GPS. When I go through the radar speed checks I notice that they are wrong because my speedo is right on. IMO I’m just lucky.
Hmmm… I have a car with a totally accurate speedometer, and you don’t. Wanna trade? 
That could mean that you had no idea just how accurate the speedometer was. Mr. Sanders is correct, your expectations are unrealistic.
Do the radar speed checks show you going faster or slower than your actual speed?
You can check your speedo’s accuracy with a watch and a road with markers at one mile intervals. One mile marker posts are on almost all rural roads in the western USA, often in urban areas too, but most people don’t notice them. Once you find a road with the markers, drive at what the speedo says in exactly 60 mph. If you pass the one mile markers every 60 seconds, the speedo is spot on.
George, the OP has GPS that shows actual speed so he knows how much his speedometer is off.
Besides what Volvo said. I’d be willing to bet, trying to do this while driving, and the fact that the mile markers themselves are not placed precisely, would have about the same margin of error as the OP’s speedo.
I had a Mazda with a speedometer that read 4% higher than the actual speed. I finally reached Mazda’s USA technical support group who told me “it’s normal” and that they don’t consider it a problem until it’s more than 7% off.
I understand most manufactures look at it the same way.