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Old 10-30-2013, 10:39 PM   #1
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Inaccurate speedometer

We have several signs that show your speed and after going by several I noticed that my speedo shows about three to four mph over actual speed. I also did a side by side comparison with my buddy driving his car and I again came to a three to four mph difference. My question is there something that can be done to correct this? What is causing this. I have stock turbo look wheels with factory recommended tire sizes so I am pretty sure this is not the issue.

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Old 10-31-2013, 01:44 AM   #2
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I believe all Boxsters do the same thing. There was something about a "Safety factor" in some other posts about this issue.
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Old 10-31-2013, 06:04 AM   #3
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GPS tells the story. The 8-9 cars I have had since getting GPS, only 2 have been accurate. The Boxster is the most inaccurate @ 5-6% high.
That means that those quick romps to triple digits have to build in a buffer of + 6-7 mph to make it official
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Old 10-31-2013, 06:45 AM   #4
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Are you running stock tires, if not the speedo will be off. I notice that I have a small difference between the analog and digital speedometer on my 2002 the analog showiung a higher speed. Could it be that the road sign is adjusted to show a higher speed ??? Just a thought.
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Old 10-31-2013, 06:52 AM   #5
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Mine reads roughly 7 kph high at 100 kph, verified by gps and roadside radar. Could be a German thing as my buddy reports almost the exact same variance on all of the 6 or 7 BMW motorcycles he has owned.

I don't see a need to correct it once the actual variance is identified. I know that the Mounties don't generally ticket unless you are doing better than 117 in a 100, so I just keep my displayed speed at or under 124 and confidently blow through radar traps. Hasn't failed me yet. I hear the OPP are a bit stickier so I am more careful when I'm on their turf.
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Old 10-31-2013, 08:44 AM   #6
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Quote:
Originally Posted by Mark_T View Post
Mine reads roughly 7 kph high at 100 kph, verified by gps and roadside radar. Could be a German thing as my buddy reports almost the exact same variance on all of the 6 or 7 BMW motorcycles he has owned.

I don't see a need to correct it once the actual variance is identified. I know that the Mounties don't generally ticket unless you are doing better than 117 in a 100, so I just keep my displayed speed at or under 124 and confidently blow through radar traps. Hasn't failed me yet. I hear the OPP are a bit stickier so I am more careful when I'm on their turf.
mine too are 7 khp faster, so the odometer shows 100k actually is 93k.....?
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Old 10-31-2013, 09:26 AM   #7
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I believe that it is an established fact that the speedos read n% faster than real speed. The correlation between speedo and odometer is an issue as ilikeching mentioned. So is the car devalued because the odo reads 100k when there's only 93k on it? Just wondering.
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Old 10-31-2013, 09:27 AM   #8
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In the past, I clocked my car at +3mph versus GPS and road signs. It is annoying but has probably saved me from many tickets. However, I would have an issue with +5mph assuming what you are testing the speedo against is accurate.

Today, I clocked my speedo at +5mph from a road sign. I am wondering if the police set it to show you a lower speed than your actually speed so they can ticket you.
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Old 10-31-2013, 09:30 AM   #9
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mine too are 7 khp faster, so the odometer shows 100k actually is 93k.....?

You make a good point. In my case that means the odometer is showing 10K km more than is actually on the car. Somehow I don't think it's going to have much impact on my resale value though...
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Old 10-31-2013, 09:34 AM   #10
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I am wondering if the police set it to show you a lower speed than your actually speed so they can ticket you.
I'd really like to shrug that off as paranoia, but now you've got me thinking. With all the red light cams and photo radar, traffic tickets have become a major revenue source. It's not that much of a stretch for them to pull a stunt like that.
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Old 10-31-2013, 09:41 AM   #11
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It might be paranoia, or it is simply be more inaccurate than it should be.

I only raise that point because I have two by my house. One is +3mph and there are rarely cops near it. But the other sign that shows +5mph, is on a curvy (aka - fun) road which always has cops waiting on it.
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Old 10-31-2013, 01:14 PM   #12
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In the manual it says that the speedo is showing you faster than you are, but that the odometer is correct with its readings of mileage travelled.
Porsche did this to help with the speeding tickets, you are not actually travelling as fast as the speedo says...
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Old 10-31-2013, 01:19 PM   #13
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My speedo indicates -3mph as compared to GPS and police radar.

A speedo repair shop might be able to adjust for whatever error you're seeing but its likley not worth the co$t.
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Old 10-31-2013, 07:08 PM   #14
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Mine shows 3mph high, according to the radar boxes on campus. Set it to 38, says 35, and 33/30 accordingly.

Better to show you're going a little faster than slower.
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Old 10-31-2013, 10:21 PM   #15
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So with the Boxster it appears to be pretty common and maybe even the norm that it shows 3 to 4 mph above actual. That being the case I am not going to worry about it. Thanks everyone.
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Old 11-01-2013, 05:19 AM   #16
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It's not the calibration of the "side-of-the-road" signs as they match GPS. Mine reads roughly 4.5 mph faster.

So to be safe I just do 10 over the limit on city roads and 15 mph over on the highway.

OK. It's not to be safe. It's just to get to McDonald's before they stop serving breakfast. They'll usually still serve you if you're late, but it's the old stuff and I want fresh preservatives and faux meat.
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Old 11-01-2013, 05:25 AM   #17
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Would 17" vs 19" tires make a difference?
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Old 11-01-2013, 07:25 AM   #18
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Interesting. My speed is accurate according to my radar detector. At most, it is a 1mph deviation. I am running 18s. However, I heard the detectors get their speed from car wheels sensors, not gps, but my detector is equipped with gps.
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Old 11-01-2013, 08:32 AM   #19
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My 2002 has stock wheels but is no longer running on the original tires. The tires are of stock size. The speedometer reads between 3-4 mph over the actual speed.

I also wondered if there was something I could do to show actual speed. So I plugged in my OBD reader a scan gauge 2 and found out that the car actually knows what the accurate speed is, so your odometer is correct but some how our speedometers actively convert the speed a percentage up.

I now run my smart phone with the app "torque" and GPS speed shown.
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Old 11-01-2013, 08:59 AM   #20
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Would 17" vs 19" tires make a difference?
Probably not, but it can. When going to a different wheel diameter or tire size, the recommendation is to stay within 1% of the circumference of the stock tires. I've heard of autocrossers using a smaller tire for the perceived torque advantage.

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