Which MPH is accurate?

[Varekai]

Travelling down the interstate the other week, I noticed that I had three different MPH readings. My spedometer showed nearly 90, cruise control was set to 85, and Nguage showed 84. Which reading do you guys think is the accurate one?

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[plc268]

Your ngauge is the most "accurate".

That pic shows the parallax of the speedo. Shift your head to the right (so that you're looking head on to speedo, and suddenly you'll notice it'll read ~85. Ford should have angled the speedometer towards the driver, but instead they kept it flat and moved it out.

The discrepancy between the ngauge and cruise control is nothing. Car is just trying to maintain the cruise speed, but it'll fluctuate +/- 1 mph.

If you want a true accurate speed, open up waze on your phone and get your gps speed (or any app that'll display gps speed). If you changed wheels and tires, you may have a discrepancy there of a few mph.

 

[GT Pony]

Use a real GPS unit. Here's the comparison data from my GT when it only had ~ 200 miles on it. Have the Pirelli "P Zero Nero" (255/40ZR-19) "Ultra High Performance" All-Season tires.

 

[jbailer]

Some more info:

Analog gauges typically have about a +/- 3% accuracy add to that the parallax error that Patrick mentioned which in our case is pretty bad with the gauge selection and position and the analog gauge is very inaccurate.

GPS will be the most accurate but has the slowest sampling rate so will display fluctuations much slower than the others. GPS eliminates variable from the calculation since it is truly telling you how fast you are moving over distance.

The ngauge, although I'm not familiar with it, would be the most accurate for fluctuations or changing speeds. I'm assuming that you can enter in parameters like final drive ratio and tire diameter. Many good calculators (maybe the ngauge provides this?) will allow you to enter the tire's "Revolutions per Mile" which can be found from the tire manufacturer and is much more accurate than simply entering the tire diameter. 5 different tires that are all 27" could provide different revolutions per mile and move the vehicle different distances on the same number of revolutions, even from the same manufacturer. Finally, this is also varied by the pressure of the tires which impacts the actual diameter or revs/mile.

*edit: Forgot to mention, cruise control should not be used as a measurement and is not accurate at all. All that is, is a target speed you would like to be going and the vehicle will do what it can to hold that speed. It is then dependent on variables I mentioned above like tires selected, tire pressure, etc.

 

[Hurricane]

Forgot to mention, cruise control should not be used as a measurement and is not accurate at all. All that is, is a target speed you would like to be going and the vehicle will do what it can to hold that speed. It is then dependent on variables I mentioned above like tires selected, tire pressure, etc.

While traveling on a dead flat highway road with zero headwind or tailwind, my cruise control will read X miles per hour, but my speedo will usually read two or three miles per hour higher. That's with me leaning over to the right as well so I'm looking at it straight on. I agree that with the margin of error on an analog needle, I'd trust the nGauge or the cruise control first. Naturally, GPS would be the most accurate as others have mentioned.

 

[VinnAY]

Not only margin for error in a mechanical speedo, they've ~always, been calibrated on the high side. Some of my owners manual have had charts showing that at lower speeds the indicated speed of say 30mph is actually 28 and as indicated speed increased increased it got much closer to 1:1...just as an example.
Its much of the reason cops won't write tickets for 5 over the speed limit as they're easily fought in court; mechanical variances or error. The old rule of "9 you're fine, 10 your mine"...10 over indicates you're not even trying to abide by the speed limit.

I set my speed by the cruise indicator, five over is about all I'll do, but sometimes it seems like I'm smoking past other cars doing 75 in a 70.

 

[HoosierDaddy]

Officer, I was going 100 MPH because that's the only speed my speedometer is accurate.

 

[Blue Horse]

Travelling down the interstate the other week, I noticed that I had three different MPH readings. My spedometer showed nearly 90, cruise control was set to 85, and Nguage showed 84. Which reading do you guys think is the accurate one?

The only one that counts is # 4,the speed shows up on the Police radar gun. LOL.

 

[Hurricane]

Its much of the reason cops won't write tickets for 5 over the speed limit as they're easily fought in court; mechanical variances or error. The old rule of "9 you're fine, 10 your mine"...10 over indicates you're not even trying to abide by the speed limit.

In my early driving years, I was busted for speeds like 60 in a 45 and 72 in a 55. I stick to between 5 and 7mph over now though. I'll do 50 in a 45 zone or 77 in a 70. Haven't had a speeding ticket in 13 years.

I set my speed by the cruise indicator, five over is about all I'll do, but sometimes it seems like I'm smoking past other cars doing 75 in a 70.

It feels like the opposite to me. If I'm in the mid-70s on a 70mph highway, people frequently fly by me doing probably 85 or 90. Why would you beg for a ticket like that? Speed traps are EVERYWHERE these days.

 

[Chameleon]

I only trust the speedoemeter. The gauges show too many inconsistencies.

 

[GT Pony]

Officer, I was going 100 MPH because that's the only speed my speedometer is accurate.

LoL ... yep, mine was dead nuts on at 100 MPH (post # 3).

 

[ElAviator72]

Not only margin for error in a mechanical speedo, they've ~always, been calibrated on the high side. Some of my owners manual have had charts showing that at lower speeds the indicated speed of say 30mph is actually 28 and as indicated speed increased increased it got much closer to 1:1...just as an example.
Its much of the reason cops won't write tickets for 5 over the speed limit as they're easily fought in court; mechanical variances or error. The old rule of "9 you're fine, 10 your mine"...10 over indicates you're not even trying to abide by the speed limit.

I set my speed by the cruise indicator, five over is about all I'll do, but sometimes it seems like I'm smoking past other cars doing 75 in a 70.

I'm sure that our speedo is digital-I don't know of many cars that had (failure prone) mechanical speedos since the 1980's ended. With hall effect sensors, it's just too easy to make them all electronic now. I haven't seen a high mileage vehicle with a wobbly speedometer needle in years, and if I had, it's a relic from before about the year 1990 :lol:

That said, yeah, I've wondered how the speedo and cruise control could be that far off. I am aware of the parallax issues on the speedo (way to go, Ford! :tsk: )

On the highway, you can use an external GPS to verify who's "more right".

 

[Road Dog]

Hmm,

Looking at your signature I noticed you have upgraded the tires on the car. Did you factor in the percentage difference in the speedometer? I would think the speedo would be the least accurate. But since the Cruise control gets its reading from that system also it wouldn't be accurate either. I'd suggest a GPS app on your cellphone to check your actual speed.

 

[HoosierDaddy]

LoL ... yep, mine was dead nuts on at 100 MPH (post # 3).

Yep. I plan to call you as a witness when I go to trial. ;)

 

[ElAviator72]

Looking at your signature I noticed you have upgraded the tires on the car. Did you factor in the percentage difference in the speedometer? I would think the speedo would be the least accurate. But since the Cruise control gets its reading from that system also it wouldn't be accurate either. I'd suggest a GPS app on your cellphone to check your actual speed.

You know, the PCM can be reprogrammed so the speedo/odometer read accurately when you change the tire size...just thought I'd put that one out there

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