GPS speed accuracy Vs speedometer?

StanStill · Regular Visitor Joined: Feb 27, 2005 Posts: 81

What nobody has mentioned is that tyre wear plays a part in the accuracy of your speedo reading. A new tyre has a greater circumference than a worn down one. I think this is one of the reasons that manufactures have to build in a bit of slack in their speedo readings.

if1977 · Posted: Tue Jan 15, 2008 1:46 pm Post subject:

OK, Here goes.

I took a brand new car out for a test drive about an hour ago and it was a mid-range 5-door with 195-55-15 tyres on it (alloy wheel 5 stud). I did as i said in the previous post and the findings were. Just for re-assurance another college was driving so i could consentrate in the screens.

At 30mph (According to the cars speedo) the actual speed of the car(according to the diagnostic machine in parameters) was 28mph and the speed according to the satnav was 33-34 mph(changed between these about 4 times in a 20 second stretch). We done the same test at 70mph (according to the cars speedo) the actual speed (according to the diagnostic machine) was 66mph and the speed according to the satnav was 75 mph. These were carried out when it was raining and on straight(ish) roads and at a constant speed for 20-30 seconds.

A bit of technical jargon.

On the same make of car wether it has 13'', 14'', 15'', 16'' wheels and does not matter what size of tyre it is the speedo drive( which is in the gearbox and measures the speed of the cars drive shafts) then sends the information to the cars ECU ( Electronic Control Unit) which then adjusts the speedo reading to legal % adjustment for the road and then that reading is the one that gets sent to the speedo head in the car which you see. So this is why if you change the wheel or tyre size of the car it will change the revolutions of the driveshart which will give the ECU a different reading and that will give the speedo head a wrong reading also.

jspencer · Posted: Tue Jan 15, 2008 3:28 pm Post subject:

How does the diagnostic machine arrive at its readings?

Jon

Robin2 · Posted: Tue Jan 15, 2008 4:28 pm Post subject:

if1977 · Posted: Tue Jan 15, 2008 5:12 pm Post subject:

I did not do the test in my Astra Robin2. I just added that at the end to tell people what my car was like. My Astra is a 1998 model and has about 80k on it so it is not the newest car and will have some wear and tear to it and would say my satnav is OK because it has been checked with others.
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mikealder · Posted: Tue Jan 15, 2008 5:54 pm Post subject:

Ian, from what you describe the car you took for a test drive is illegal, I would not expect the GPS speed to be inaccurate enough to explain the difference you observed, had the car been modified in way by the garage - Mike

if1977 · Posted: Tue Jan 15, 2008 6:39 pm Post subject:

Andy_P · Posted: Tue Jan 15, 2008 6:51 pm Post subject:

Andy_P · Posted: Tue Jan 15, 2008 6:55 pm Post subject:

tgold · Posted: Tue Jan 15, 2008 7:00 pm Post subject:

tgold · Posted: Tue Jan 15, 2008 7:05 pm Post subject:

mikealder · Posted: Tue Jan 15, 2008 7:07 pm Post subject:

With a GPS speed of 70 mph I would expect the vehicles speedo to read between 70 to 77. What is not acceptable is for the vehicles speedo to read under the actual speed the vehicle is travelling at, so where you had a speedo reading of 70MPH and the GPS speed was indicating 75 the speedo is illegal - it is enticing you to break the law - Mike

Andy_P · Posted: Tue Jan 15, 2008 7:17 pm Post subject:

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