Speedo accuracy

my2cta · Post by **my2cta** » Sun Apr 18, 2004 1:11 pm

I checked out the MX5's speedo accuracy with a GPS today. Not very good I'm
afraid. At an indicated 70 on the speedo the car is doing 66, at 100 it is
doing 94.5 and at 110 it is doing 104. I guess it makes it easier not to get a
ticket when you're always going slower than you think. The trip meter is out
too: when it said I'd gone 13 kms the distance covered was actually 12.6. The
car is a 2000 (NB) model on the original tyres, tread depth is down to 4 mm
from whatever it was new. That wouldn't make that much difference in the
accuracy though.

When I checked the Honda Jazz is reads 4 kph too fast at all speeds - implies
the speedo is accurate but the zero stop isn't set right.

Friend of mine with a 1997 (NA) model says when he got a speeding ticket
saying he was doing 114 he reckoned he was doing 114 according to his speedo
as well. Wonder if NA speedos are better than NB, or just if it varies from
car to car for some reason?

Tim
MY2CTA

Bill Rehm · Post by **Bill Rehm** » Sun Apr 18, 2004 2:29 pm

Thanks Tim, I'm still thinking about fighting the speed camera ticket
--- if only out of principle --- some people still have them.

Regards
Bill

Bill Rehm · Post by **Bill Rehm** » Sun Apr 18, 2004 2:29 pm

Thanks Tim, I'm still thinking about fighting the speed camera ticket
--- if only out of principle --- some people still have them.

Regards
Bill

Grant · Post by **Grant** » Sun Apr 18, 2004 6:32 pm

Those results I would think are quite normal for any vehicle. Quite close to
what I have had on a number of vehicles. As for the distance covered, I
wouldn't put too much store in what the GPS reckons. I have found with mine
that it can be quite inaccurate for distance traveled, especially if the
number of acquired satellites is small and you are in windy and hilly
terrain. Do your tests on straight roads in clear terrain for accuracy.
Some motoring magazines have speedo accuracy in their road tests and even
very expensive exotics are often worse than your tests.
The accuracy will vary from car to car as well. At least it errs on the safe
side for tickets.
Some of us from Marlborough had a great run in the Marlborough Sounds today
along with the Nelson club. 17 cars in all and wonderful MX5 weather.

Grant in Blenheim.

Grant · Post by **Grant** » Sun Apr 18, 2004 6:32 pm

Those results I would think are quite normal for any vehicle. Quite close to
what I have had on a number of vehicles. As for the distance covered, I
wouldn't put too much store in what the GPS reckons. I have found with mine
that it can be quite inaccurate for distance traveled, especially if the
number of acquired satellites is small and you are in windy and hilly
terrain. Do your tests on straight roads in clear terrain for accuracy.
Some motoring magazines have speedo accuracy in their road tests and even
very expensive exotics are often worse than your tests.
The accuracy will vary from car to car as well. At least it errs on the safe
side for tickets.
Some of us from Marlborough had a great run in the Marlborough Sounds today
along with the Nelson club. 17 cars in all and wonderful MX5 weather.

Grant in Blenheim.

my2cta · Post by **my2cta** » Mon Apr 19, 2004 7:14 pm

The test was on a straight road, SH2 between Upper Hutt and Lower Hutt.
Should be pretty accurate there. I use this GPS system almost every day when
I'm walking (which I do for a sport) and it has proven itself to be accurate
within about 0.5% for distance measured so long as you are in the open.
Going through forest or very close to tall buildings can throw it off
though.

Glad to hear exotics can be as bad or worse. But why? Surely it's no great
problem to get an accurate distance measure (within 1%) in a car? Anyone got
any theories?

Tim

Bill Rehm · Post by **Bill Rehm** » Mon Apr 19, 2004 8:40 pm

Well, I sent my letter to the NZ Police stating that my speedo is
accurate and their speed camera has a margin of error = I wasn't doing
61Km/h and I deserve to pay no/ a lower fine, i.e. less than $80.

Stay tuned, if nothing else their response will be amusing -- or more
likely dogmatic

Regards
Bill

Bill Rehm · Post by **Bill Rehm** » Mon Apr 19, 2004 8:40 pm

Well, I sent my letter to the NZ Police stating that my speedo is
accurate and their speed camera has a margin of error = I wasn't doing
61Km/h and I deserve to pay no/ a lower fine, i.e. less than $80.

Stay tuned, if nothing else their response will be amusing -- or more
likely dogmatic

Regards
Bill

Lou Girardin · Post by **Lou Girardin** » Mon Apr 19, 2004 9:20 pm

Why don't you just pay the fine. You know that you're criminal, we're told
that everynight on TV. You might kill someone if you continue driving at
those insane speeds.

Lou (tongue firmly in cheek)

Lou Girardin · Post by **Lou Girardin** » Mon Apr 19, 2004 9:20 pm

Why don't you just pay the fine. You know that you're criminal, we're told
that everynight on TV. You might kill someone if you continue driving at
those insane speeds.

Lou (tongue firmly in cheek)

Grant · Post by **Grant** » Mon Apr 19, 2004 11:03 pm

The figures of 12.6km for an indicated 13km is about the same error as the
speedo reading has. Actually it's slightly better at a bit more than 3%
fast. Your speed figures are about 5.5% out.
I had to fit slightly larger tyres to my 4WD recently due to the original
size not being available. My speedo accuracy changed from about 5% fast to
within 1% now. Does mean the odometer is now 4% lower indication. The
10,000km diesel road tax now lasts a bit longer though, so there are some
benefits!

Grant.

Grant · Post by **Grant** » Mon Apr 19, 2004 11:03 pm

The figures of 12.6km for an indicated 13km is about the same error as the
speedo reading has. Actually it's slightly better at a bit more than 3%
fast. Your speed figures are about 5.5% out.
I had to fit slightly larger tyres to my 4WD recently due to the original
size not being available. My speedo accuracy changed from about 5% fast to
within 1% now. Does mean the odometer is now 4% lower indication. The
10,000km diesel road tax now lasts a bit longer though, so there are some
benefits!

Grant.

John Forsyth · Post by **John Forsyth** » Tue Apr 20, 2004 7:40 am

Not quite correct Lou, if you're a 40 tonne B-train then speed is OK.

JF
BCG191

John Forsyth · Post by **John Forsyth** » Tue Apr 20, 2004 7:40 am

Not quite correct Lou, if you're a 40 tonne B-train then speed is OK.

JF
BCG191

Biff · Post by **Biff** » Tue Apr 20, 2004 6:10 pm

Hell yes, hung drawn and quartered I say.
Ship him off to the colonies!
And his family, and friends, and ...well anyone else we feel
like!
For generations the scars will be visible around their ankles, they will
naively call it a birthmark.

Feel bad yet?
teehee
Taxes Taxes, one form or another they will get ya in the end.

Biff
ZOOM1N (very carefully

Biff · Post by **Biff** » Tue Apr 20, 2004 6:10 pm

Hell yes, hung drawn and quartered I say.
Ship him off to the colonies!
And his family, and friends, and ...well anyone else we feel
like!
For generations the scars will be visible around their ankles, they will
naively call it a birthmark.

Feel bad yet?
teehee
Taxes Taxes, one form or another they will get ya in the end.

Biff
ZOOM1N (very carefully

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