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Old 11-02-2016, 10:56 AM   #1
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Exclamation Shake down cruise - 5 mph over ?

Sunny and 65* couldn't resist - played hooky for 2 hours from work

Oh what a glorious day totally enhanced by 986 fun . I put the top down to see how wind felt without the screens - wasn't bad but screens do help

Noticed that I'm moving along at a good clip but cars are passing me . Well I'm not racing just cruising but I felt my speedo was not accurate - I'm doing what I read as 75-80mph and being passed ? this is a 55mph zone and plenty of police around .

So I paid particular attention going by a street with radar read out on the street
30 mph it will say slow down and give speed - my speedo was at 40 the machine was at 35 .
Not sure whats throwing the calibration off - 17" wheels came from factory . tires seem standard height
I had a similar thing happen when I put taller tires on my 4 runner but it was the reverse - I was going faster than speedo showed

any ideas ? Cheap thrills if nothing else
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Old 11-02-2016, 10:57 AM   #2
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Mine reads about 2-3 over when doing around 70-75, 2000 S with 17's and OEM sizes/diameter.
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Old 11-02-2016, 11:29 AM   #3
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All manufacturers set the speedos to read about 5 MPH high to avoid being sued for speeding tickets. The tactic did not work so well for Honda, who got hit with a class action suit claiming that the speedo was set high to run out the warranty faster.
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Old 11-02-2016, 11:43 AM   #4
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All manufacturers set the speedos to read about 5 MPH high to avoid being sued for speeding tickets. The tactic did not work so well for Honda, who got hit with a class action suit claiming that the speedo was set high to run out the warranty faster.
Not that way across the board. My Touareg is spot on while it seems typical of 986 cars to read 4MPH high. My old Expedition was 2MPH high.
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Old 11-02-2016, 11:48 AM   #5
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My 2003 S speedometer reads high whether its with the standard size 18 inch wheels and tires it came with or the standard size 17 inch wheels and tires I bought later. I asked my independent Porsche garage about it. He said they all read high. He also said Porsche does this intentionally because there is a law in Germany that would subject the manufacturer to a fine if even one car's speedometer said less than the actual speed. The fine is an amount per car that is then multiplied by the total number of cars of that model produced. I'm not sure I buy that. But I have put the Speed Box app on my iPhone - there are lots of free speedometer apps for smartphones, and it shows my Boxster's speedometer is a bit optimistic! Here is a screenshot of the highest speed I've ever recorded -



If you're still reading this screenshot was taken on the Bullet Train in Japan, not in my Boxster.
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Old 11-02-2016, 12:01 PM   #6
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Not that way across the board. My Touareg is spot on while it seems typical of 986 cars to read 4MPH high. My old Expedition was 2MPH high.
Point is that most are set high, never low.
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Old 11-02-2016, 12:22 PM   #7
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My 2001 S reads 3.5 higher than true speed. Checked with GPS.

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Old 11-02-2016, 12:31 PM   #8
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Interesting another Boxster "secret"?

I should forget about this and take the speedo at face value - probably save my a$$ from some tickets
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Old 11-02-2016, 01:54 PM   #9
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Mine always indicates 4 mph over (at least above 35 mph, not sure below that).

Confirmed via GPS, Radar, and a friend in Law Enforcement who paced me.
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Old 11-02-2016, 02:55 PM   #10
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My portable GPS will tell you your actual speed. Mine speedometer reads about 2 mph faster at 60 mph than you are actually going.
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Old 11-02-2016, 03:08 PM   #11
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double post. oops.
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Old 11-02-2016, 03:17 PM   #12
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Even though the speedo reads high, the odometer on my cars is spot on.
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Old 11-02-2016, 03:48 PM   #13
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I concur

My odometer is accurate too, based on GPS and known trip distances traveled in different cars.

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Even though the speedo reads high, the odometer on my cars is spot on.
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Old 11-02-2016, 04:11 PM   #14
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speedo is very close to gps on mine, perhaps a mile or two over, odometer is exact with gps
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Old 11-02-2016, 06:14 PM   #15
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Originally Posted by JFP in PA View Post
Point is that most are set high, never low.
As a matter of fact it's by German law.

Among other things, not every brand tire marked the same size is actually the same size. The point is it's considered a safety issue should a speedo indicate less than actual speed.

In the BMW motorcycle world, we've seen a couple tech bulletins about "speedometer advance." They claim the odometer is accurate and there's obviously no mechanical reason why not.
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Old 11-02-2016, 06:39 PM   #16
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My Garmin for Car reads 3 over, Garmin handheld Nav for sailing is almost spot on
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Old 11-02-2016, 08:15 PM   #17
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My speedo is 3-4 mph over what the GPS data system logs.
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Old 11-03-2016, 03:26 AM   #18
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My last two cars were spot on.
I am getting so tried when driving with the Wife
"Watch your speed - You'll get a ticket"

And I always have to correct her
"No we aren't doing 95, were only doing 90 - Look at the GPS"
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Old 11-04-2016, 04:27 PM   #19
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I use Waze (mostly for the alerts - police and debris in the road) and it showed the speedo is 3-4 high at 80(+) on my 03 S.
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Old 11-04-2016, 06:35 PM   #20
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Downloaded a speedo app onto phone today. Car came in at 4mph at 50,60,70,80 mph
Top speed indicator is pretty neat, 97 today and it looks like I'll be needing to set the Alarm feature !
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