Speedo Accuracy

[joeyearl]

My speedometer reads about 20% off. When it says 70 mph, my GPS says 84. Interestingly, the odometer is accurate. I've checked it with two iPhone apps, and my other vehicle. It is spot on in my F150 Lightning. Anyone having these issues? The other day I accelerated a bit on the Interstate, and the Speed Box app read 136.4, when my speedo said around 114.

 

[FiestaSTdude]

Are you running a stock tire/rim set up? I know from personal experience that can mess with the speedo.

 

[iammacey]

This is going to sound like a dumb question, but you verified this without the phone plugged into the car via USB?

There is, or was, an odd behavior with Sync + CarPlay showing higher MPH through CarPlay than what's displayed. GPS apps would show well above the indicated speed on the car. Unplug the phone and it worked fine.

I use Android Auto and have not experienced this issue but when my girlfriend would plug in her iPhone speeds were way off. Unplug her iPhone and the speeds line up.

Otherwise, it's a speedo calibration issue. As noted above, verify overall tire diameter matches the original size or you'll need to calibrate the speedometer.

 

[Sam4]

Can this be used in Traffic Court? "But yer honor, I was only doin' one fourteen...."

 

[jmrtsus]

Easy way to check your speedometer

I have found over many years that the car makers usually have a speedometer read about 2% high. If a speedometer reads low they would get grief with each customers speeding tickets. Tire pressure and size will also affect the readings. I checked every car I have ever owned back to the '60's the old fashion way and again if I change tires. I time 2 measured miles at an indicated 60 mph using cruise control on a relatively flat interstate with mile markers. A perfect speedometer will would take 120 seconds for 2 miles @ 60 mph. 60 mph = 1 minute per mile. If your car took 123 seconds divide 120 by the time. For example; 123 seconds to cover 2 miles divide 120 by 123 = .975 approximately. Move the decimal 2 places to the right to convert to percent. So your reading is 97.5% of 60 mph or 97.5 x 60 = 58.5 mph at an indicated 60 mph. If your time is for example 118 seconds it would be about 1.02 X 60 = 61 mph. So go the techie route and recalibrate if you have the time and equipment. I still do it like I did pre-tech, timer and calculator will tell my how low my speedo is reading with both on my phone, LOL. And do it once and nothing will change until you get new tires then do it again.
My Fifi runs about 2 mph over with the OEM Bridgestone's replaced long ago with General G-Max A/S.
I don't need to recalibrate, I know I am approximately 2 under the speedometer at 60 to 80 mph. That is all I need to know. So I cruise an indicated 75 in a 70 knowing at 73 mph no cop will give anyone any grief. On the other hand when driving hard I never look at the speedometer, I look at the tach.

 

[joeyearl]

Easy way to check your speedometer

I have found over many years that the car makers usually have a speedometer read about 2% high. If a speedometer reads low they would get grief with each customers speeding tickets. Tire pressure and size will also affect the readings. I checked every car I have ever owned back to the '60's the old fashion way and again if I change tires. I time 2 measured miles at an indicated 60 mph using cruise control on a relatively flat interstate with mile markers. A perfect speedometer will would take 120 seconds for 2 miles @ 60 mph. 60 mph = 1 minute per mile. If your car took 123 seconds divide 120 by the time. For example; 123 seconds to cover 2 miles divide 120 by 123 = .975 approximately. Move the decimal 2 places to the right to convert to percent. So your reading is 97.5% of 60 mph or 97.5 x 60 = 58.5 mph at an indicated 60 mph. If your time is for example 118 seconds it would be about 1.02 X 60 = 61 mph. So go the techie route and recalibrate if you have the time and equipment. I still do it like I did pre-tech, timer and calculator will tell my how low my speedo is reading with both on my phone, LOL. And do it once and nothing will change until you get new tires then do it again.
My Fifi runs about 2 mph over with the OEM Bridgestone's replaced long ago with General G-Max A/S.
I don't need to recalibrate, I know I am approximately 2 under the speedometer at 60 to 80 mph. That is all I need to know. So I cruise an indicated 75 in a 70 knowing at 73 mph no cop will give anyone any grief. On the other hand when driving hard I never look at the speedometer, I look at the tach.

Excellent idea. I also think that the issue with being plugged in via USB may be the reason the GPS is off. Twice recently I’ve have the Maps app go all crazy on me while using it to get to a destination- I’ll be going down the turnpike, and all of a sudden it thinks I’m about a quarter mile to the right, and keeps giving rerouting corrections for streets I’m not on.

I’m pretty sure that my speedometer is accurate, judging by other traffic, and compared to my other vehicle in the same traffic. I drive the same route daily with either vehicle.

 

[joeyearl]

This is going to sound like a dumb question, but you verified this without the phone plugged into the car via USB?

There is, or was, an odd behavior with Sync + CarPlay showing higher MPH through CarPlay than what's displayed. GPS apps would show well above the indicated speed on the car. Unplug the phone and it worked fine.

I use Android Auto and have not experienced this issue but when my girlfriend would plug in her iPhone speeds were way off. Unplug her iPhone and the speeds line up.

Otherwise, it's a speedo calibration issue. As noted above, verify overall tire diameter matches the original size or you'll need to calibrate the speedometer.

I think you may be on to something. Twice recently I’ve have the Maps app go all crazy on me while using it to get to a destination- I’ll be going down the turnpike, and all of a sudden it thinks I’m about a quarter mile to the right, and keeps giving rerouting corrections for streets I’m not on. I’ve called Apple, and they had no idea what was going on.

 

[MikelSS]

I think you may be on to something. Twice recently I’ve have the Maps app go all crazy on me while using it to get to a destination- I’ll be going down the turnpike, and all of a sudden it thinks I’m about a quarter mile to the right, and keeps giving rerouting corrections for streets I’m not on. I’ve called Apple, and they had no idea what was going on.

I don't know if it's Sync or Apple (I use Carplay), but around every month I get a new issue, they don't show up at the same time and they always get fixed by unplugging and plugging again except the one for the speed discrepancy which got fixed some time ago with a Sync update. The latest issue that I have is the volume automatically going down to like of what I had it at and not being able to adjust it.

Previous issues have been, like you, the fiesta suddenly turning into a helicopter having no idea where it is; Carplay not connecting at all with phone not even charging; random apps just deciding to start playing (although I think this would be an Apple thing and they keeping track of everything you do at all times and suggesting stuff); and so on... and so on.

 

[PhoenixM3]

I use Apple CarPlay W/ phone directly connected. When I have the Waze application displayed, my speed reads 10MPH higher than actual.