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2017 Ecoboost Mustang with Performance Package
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Discussion Starter · #1 ·
I've had my '17 a couple of weeks now and I'm checking my speedometer accuracy. I like know how close it is to correct.

What I'm seeing is, let's say, I hit 70 according to the speedometer and hit the cruise. The cruise comes up and shows 68. If I bump the cruise up to 70, the speedo reads what looks to be like 72.

Does anybody else have an issue like this? Is this a common S550 thing?

It also looks like the speed the cruise shows is a bit fast too. I checked it this morning with a tachymeter and when I had it set at 75, the tachymeter showed I was doing 73.7. I'm going to check that a few more times to make sure.
 

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2020 Ecoboost High Performance Package
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You're close enough I'd guess your wheel/tire package isn't exactly what the speedometer is setup for. Best guess is it's probably configured for their highest selling GT configuration or maybe top end (GT350 in '17) to be accurate?
 

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I imagine that there is some regulation regarding allowable speedometer error and you're probably within it. And since tires vary in dimension for the same size amongst the manufacturers, it is unlikely that you would ever get it matched up to your tachymeter. I have the digital dash and my cruise and speedo always match up, except when you have drift going up/down a hill.
 

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Discussion Starter · #4 ·
I imagine that there is some regulation regarding allowable speedometer error and you're probably within it. And since tires vary in dimension for the same size amongst the manufacturers, it is unlikely that you would ever get it matched up to your tachymeter. I have the digital dash and my cruise and speedo always match up, except when you have drift going up/down a hill.
Tge difference between the speedometer itself and the speed the cruise show is what gets me. I'm not surprised about some variance in the speedometer. But it reading differently than what the cruise shows does.
 

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Tge difference between the speedometer itself and the speed the cruise show is what gets me. I'm not surprised about some variance in the speedometer. But it reading differently than what the cruise shows does.
ah I see, I'm not sure I never have enough open road to use cruse control. If I find some I'll let you know.
 

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All speedometers should read slightly higher than the actual speed to allow a safety margin.
It is set by the factory like that.
Otherwise, drivers could get more speeding tickets, when they think they are doing the speed limit.

Comparing my analog speedometer in my 2017 to my GPS, it reads about 5 Km per hour higher at 100Km/hr than actual speed.
Note that the size of aftermarket wheels and tyres you put on the car, may also change the accuracy or reading of the Speedo, if the rolling diameter changes as well.
 

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Discussion Starter · #7 ·
I"m on the stock tires & wheels. The difference between what the cruise shows and the speedo is show is what kind of has me baffled.
 

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I have owned an S550 continuously since Jan 15.(a 15 and now a 17). The speedos on both behaved the same.

There is a fair amount of hysterisis in the movement. Comparing to the GPS speedo in my phone or the one in my radar detector approaching a speed from below using the GPS as a reference the speedo will read 1 or 2 mph low. Approaching a speed from above again using the GPS as a reference the speedo will read 1 or 2 mph high. I believe this hysterisis is a result of the damping built into the movement to keep it from bouncing around.

The other issue is the lit portion of the needle is off the dial face where the numbers are quite a bit and that causes parallax. So sitting to the left of the speedo makes it appear to be reading high, especially in the 50 to 80 mph range.

As far as accuracy goes I would believe the speed displayed when the cruise is set over the actual speedo. There is a sensor that digitally counts the revolutions of the drive shaft which is mathematically converted to speed which can be directly displayed by the digital display for the cruise setting. Where as the same signal has to go through a digital to analog conversion to get a voltage proportional to speed to drive the speedo. This will introduce some error.



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Discussion Starter · #10 ·
I have owned an S550 continuously since Jan 15.(a 15 and now a 17). The speedos on both behaved the same.

There is a fair amount of hysterisis in the movement. Comparing to the GPS speedo in my phone or the one in my radar detector approaching a speed from below using the GPS as a reference the speedo will read 1 or 2 mph low. Approaching a speed from above again using the GPS as a reference the speedo will read 1 or 2 mph high. I believe this hysterisis is a result of the damping built into the movement to keep it from bouncing around.

The other issue is the lit portion of the needle is off the dial face where the numbers are quite a bit and that causes parallax. So sitting to the left of the speedo makes it appear to be reading high, especially in the 50 to 80 mph range.

As far as accuracy goes I would believe the speed displayed when the cruise is set over the actual speedo. There is a sensor that digitally counts the revolutions of the drive shaft which is mathematically converted to speed which can be directly displayed by the digital display for the cruise setting. Where as the same signal has to go through a digital to analog conversion to get a voltage proportional to speed to drive the speedo. This will introduce some error.



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Thanks dgc333.

I read an post over on the 6GC site this morning talking about he parallax. I had kind of noticed that but hadn't really thought about it. I know when you tilt your head to the right and get perpendicular with the speedo, the speeds look right. Like you and the poster on the other site mentioned, it is worse when the needle is in the upper part of the range.

I've got a performance pack car and one thing I don't like about the car is the way all of the gauges, especially the speedo and the center dash gauges face. They really should have been canted more toward the driver so you could easily see them head-on. Sorry to be nit picky. :)
 

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I agree with @dgc333 on this. My observation has been roughly 2mph difference at highway speed between cruise control and observed speed on the speedometer. I'll also throw out that the speed from the cruise control generally agrees with GPS within 1mph.
 

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Both of my Mustangs are/were PP cars and Ford blew it with the design of the oil pressure and boost gauges for two reasons IMHO.
1. The fact that they aren't angled toward the driver. I keep saying I am going to pull the bezel and see it there is a way I can angle them toward the driver.
2. They could easily have made them a 270 degree sweep and given you more resolution in the gauge.

The straight forward mounting, the distance from the driver and the 5 psi resolution makes the boost gauge pretty much useless. I turned on the boost gauge in the center display with ForScan like the non PP cars have and never look at the gauge in the dash. The one in the display gives you a full 270 degree sweep and 2 psi resolution.

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