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?:confused: |
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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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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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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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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97 Base with 17s stock. 6-7 KM difference. or 3-4MPH.
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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. |
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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My speedometer shows about 3MPH faster than the actual speed, very common.
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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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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 |
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. |
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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When going slower like 60KMH my gps may read 63KPH |
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) |
My digital speedo displays 3 MPH faster than actual, per varying speeds at the mobile radar on campus at uni.
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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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