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Old 12-30-2014, 03:29 AM   #1
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GPS mph accuracy

I notice that when I use Garmin in my Boxster that there's a 4 mph difference. When the GPS shows 75 mph the cars speedo shows 71 mph. Always 4 mph...

I'm wondering which is accurate? Or....possible my 99 boxster might have come with standard 16's? I purchased it used with 17's. It is the "base" model. Maybe that would be the reason for the difference...if the math works?

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Old 12-30-2014, 03:46 AM   #2
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I have 3 Porsches and all 3 do the same. Somewhere from 2 to 4 MPH less than the gps shows.
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Old 12-30-2014, 03:50 AM   #3
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Interesting. My Boxster always shows faster, which is apparently quite common. I set my cruise at 105 km/h which actually means I am going about 98 km/h. I don't think 16 vs 17 tires would make any difference. Something isn't calibrated right. Get a buddy to ride behind you and track his speed....see if it's your car or your gps.
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Old 12-30-2014, 03:54 AM   #4
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And Fyi - German cars are never designed to show a lower speed than what you are going. Due to European law, their speedos are always to show a slightly faster speed than what you are doing. Google it up, lots of info on it.
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Old 12-30-2014, 03:55 AM   #5
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Auto manufacturers typically calibrate speedometers to read a small % faster than actual, to make sure they comply with the law and (I suppose) to avoid liability relating to speeding tickets.
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Old 12-30-2014, 04:13 AM   #6
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97 Base with 17s stock. 6-7 KM difference. or 3-4MPH.
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Old 12-30-2014, 04:32 AM   #7
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Quote:
Originally Posted by brjak View Post
97 Base with 17s stock. 6-7 KM difference. or 3-4MPH.
16 vs 17 inch tires does not change the overall tire diameter. Only changes the size of the sidewall, therefore, no affect on speedometer readings.

That being said...if he changed out the tires and DID change the diameter by installing completely different type tires - the speedometer calibrated to show fast would still not be reduced to show less speed than actual.
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Old 12-30-2014, 06:25 AM   #8
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GPS speed should be pretty spot on. The speedo with factory size tires and wheels will reed a little low. In both my Boxster and our 996, the speedo reads around 3 mph fast.

I thinking it as my safety margin when driving fast. Probably saved me from a ticket or two
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Old 12-30-2014, 06:36 AM   #9
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Quote:
Originally Posted by Giller View Post
Interesting. My Boxster always shows faster, which is apparently quite common.
Agreed. All of the Boxsters seem to read 3 to 4 MPH higher. I have never heard of on reading higher.
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Old 12-30-2014, 06:48 AM   #10
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My speedometer shows about 3MPH faster than the actual speed, very common.
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Old 12-30-2014, 06:59 AM   #11
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Quote:
Originally Posted by mountainman View Post
I have 3 Porsches and all 3 do the same. Somewhere from 2 to 4 MPH less than the gps shows.
Must have had a senior moment, or I have sniffed too much of the paint I am brushing today. Meant to say the speedo reads more than the gps on all 3. It is hell getting old, but there is only one alternative...
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Old 12-30-2014, 09:46 AM   #12
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Like everyone else, my speedometer reads 3 to 5 miles per hour faster than the GPS. I had a deputy sheriff clock me on radar one day just to make sure. The GPS is right, the speedo is not.
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Old 12-30-2014, 09:58 AM   #13
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Yes, but the OP is having the opposite problem, unless he mistyped.
His speedo is reading slow, so he is speeding! Never heard of this one, so may be a typo.

Speedo = 71 mph
Actual (GPS) = 75 mph
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Old 12-30-2014, 10:07 AM   #14
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Now I have to double check.... I could of sworn my GPS shows a faster speed than my speedometer. I will check on my way to work tomorrow and confirm.

Either way it's no big deal. It was just a general observation and question for other owners. Thanks for all the input.
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Old 12-30-2014, 12:07 PM   #15
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I'm on my eighth Porsche through the years, and they all showed me going 3 mph faster than the actual speed measured by roadside radar signs, and also on GPS in later years.
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Old 12-30-2014, 12:18 PM   #16
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Yes, but the OP is having the opposite problem, unless he mistyped.
His speedo is reading slow, so he is speeding! Never heard of this one, so may be a typo.

Speedo = 71 mph
Actual (GPS) = 75 mph
My 97 is same. it shows me doing 114KMH when GPS shows 120KMH.

When going slower like 60KMH my gps may read 63KPH
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Old 12-31-2014, 03:24 AM   #17
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Most of the time I look at GPS for Speed and trying to read Analog while driving is like texting
Porsche should have used that dial space for Oil Pressure and temp and just used a Digital display (Sure there is some regulation that a dial be there)
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Old 12-31-2014, 03:44 AM   #18
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My digital speedo displays 3 MPH faster than actual, per varying speeds at the mobile radar on campus at uni.
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Old 12-31-2014, 03:50 AM   #19
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Originally Posted by brjak View Post
My 97 is same. it shows me doing 114KMH when GPS shows 120KMH.

When going slower like 60KMH my gps may read 63KPH
Yep! I confirmed....My 99 boxster shows slower than my GPS. Guess some Boxsters are more special than others..ha ha..

BTW - This morning was frosty and chilly (21 degrees) here in Chester County, Pa. Boxster ran smooth as ice in the cold. I will be driving it in much colder weather in the coming months. Will see how she performs.. The only time I won't be driving her is in rain and snow.
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Old 12-31-2014, 04:07 AM   #20
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Quote:
Originally Posted by SJEFFB986 View Post
BTW - This morning was frosty and chilly (21 degrees) here in Chester County, Pa. Boxster ran smooth as ice in the cold. I will be driving it in much colder weather in the coming months. Will see how she performs.. The only time I won't be driving her is in rain and snow.
I'm basically across the river from you and when I drove my Box during the winter cold weather was never an issue, even single digits. If you have decent tires even snow is not really any worse than any other RWD car

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