Speedometer Check

[Blue MK3]

How accurate are the apps that can be downloaded to a phone or IPad for checking the speedometer? I guess part of the answer depends on phone coverage in the area where the check is being made. The guy that I bought my car from suggested that the speedo was OK up to about 45 and then read too low at higher speeds. It would be nice to know HOW low.

[michael everson]

I have found them to match all my vehicles speedometers.
Mike

[Blue MK3]

Mike, I gather from your answer that all your vehicles' speedometers read the same and that the GPS app agrees with them. I take that to mean the apps are accurate (or else everything is off by the same amount).

[michael everson]

I think the apps are accurate.
Mike

[Blue MK3]

Thanks, Mike. I'll get my wife to bring her IPad along and go for a ride.

[WIS89]

Blue MK3

If you have a GPS or a friend with a GPS, you should give that a try. I actually use my GPS unit's speed function more than my car's speedo when I am driving. Just a thought...

Regards,

Steve

[Mike N]

Blue MK3

I actually use my GPS unit's speed function more than my car's speedo when I am driving.

For longer trips I do the same.

My cars are all very slightly off compared to the GPS. The worst is my Touareg where the speedo reads about 5% higher than the GPS speed.

[avgjoe]

Almost all cars from the factoy have a 3-5% error in actual vs indicated speed. That's to be expected with all the variables involved. I think Honda got in trouble a few years ago due to theirs being off more than the accepted amount. Not a big deal until you count up all the lease returns that were over their mileage limit. :eek:

In my '03 330, 80mph indicated is a fat 75mph GPS, but in my '99 F250 75mph indicated is 73mph GPS.

You should try out a GPS unit to at least see if the error is consistant and if not where and by how much. If you have a straight, flat piece of road you can time your distance between two mile posts and then do the math.

Joe

[Ultimate Track Car]

I have a Jerico transmission, so I don't have any input for a regular speedo. I spent a lot and bought a GPS speedo. Works great, but it's too bad that obscure iphone apps weren't as prevalent a few years ago, as I could have just zip tied mu iphone to my steering wheel hub or rear view mirror. LOL

[Blue MK3]

To follow up on this thread, my wife and I took the car out last weekend, she with her IPhone and IPad. She had the same GPS app loaded on both devices with the purpose of checking the car speedometer against the "true" speed. Turns out that the two devices read different from each other AND the speedometer. The differences varied from one to four mph. We stopped for gas and while parked, both accurately displayed a satellite image of where we were. The IPhone read "accurate to 32 feet" and the Ipad read 16 feet. So we used that to check the car's speed vs. what the speedometer displayed. In general, the car speedo read (on average) 1.5 MPH slower than the GPS app. But the question is which GPS app (and device) is accurate? And are GPS speedometers like the Speedhut unit inherently more accurate than phones and pads?

[Big Blocker]

I know for a fact that my speedo reads approximately 5-6 miles slow at 75mph . . . checked with numerous iPads, iPhones and two different GPS devices. I guess if the "posted" speed states 45, I do exactly 45 on my speedo so I know I'm just 5 over in any area I travel. Most LEO's won't write for 5 over in the Vegas area unless you're in a school or construction zone.

Just my 2¢

Doc

[rich grsc]

To follow up on this thread, my wife and I took the car out last weekend, she with her IPhone and IPad. She had the same GPS app loaded on both devices with the purpose of checking the car speedometer against the "true" speed. Turns out that the two devices read different from each other AND the speedometer. The differences varied from one to four mph. We stopped for gas and while parked, both accurately displayed a satellite image of where we were. The IPhone read "accurate to 32 feet" and the Ipad read 16 feet. So we used that to check the car's speed vs. what the speedometer displayed. In general, the car speedo read (on average) 1.5 MPH slower than the GPS app. But the question is which GPS app (and device) is accurate? And are GPS speedometers like the Speedhut unit inherently more accurate than phones and pads?

That is pretty darn accurate unless you want to spend a lot of money on a GPS.

[Bob Cowan]

You need to understand how it all works. The unit queries as many satellites as it can see, and accurately locate your position on the planet. A pretty amazing feat when you think about it. So it checks you location and marks. Then it checks your position again and marks that. Then it calculates how long it took to get from A to B.

In order for a GPS speedometer to be accurate, it needs to be able to do a few things.

It has to get data from more than 3 satellites. 5-6 gives you better accuracy than 2-3. For that you need a big antenna with wide exposure that can find as many satellites as possible. The more satellites it can get data from, the greater the accuracy of location. pads and phones have a tiny little antenna.

It needs to refresh that data often. If you get a data read at the entry to a 180* turn, and then another at the exit of that turn, the computer will not know you went the long way around the corner, and think you went in a straight line from A to B. A good GPS unit has a 5Hz cycle rate. A really good GPS has a 10 hz rate. smart phones and pads generally have a 1hz rate.

[Blue MK3]

Thanks for the feedback, Bob et al. I understand the principles of how GPS works but hadn't considered the small size of the antennae on popular mobile devices and the lack of adequate coverage of the constellation. I guess my car speedo is reasonably accurate, what with it being only 1-2 mph off the GPS.