inaccurate speedometer

Originally Posted by texomawaves

My GPS says I'm doing 70, but my Boxster says I'm doing 75 (5 mph difference). Is there a way to have it calibrated?

The tires are Pirelli Pzero 205/50 ZR-17 on the front, and 255/40 ZR-17 on the back. I feel pretty sure these rims are original stock 17 inch.

I have the same exact problem only my PZeroes are on the stock 16" wheels. Seems it has to be something with the tire, perhaps the very low profile? I'm curious for an answer as well because not only does my GPS indicate a 5 mph difference but so does everyone of those speed warning devices I go through.

[Burg Boxster]

it's consistent w/ others, not only Porsche but most mfgs. Might be a "reduce the liability" issue so people can't come back and say they were caught speeding due to low reading speedo - erring on the high side of course

Anyhow, yes I'm pretty sure can have it calibrated to read correctly. Incidentally, you'll note it's not a 'linear' over-reading. At least on my '99 I never found it to be. For example, if I was really doing 40, it would say 44. If 70, it would say 74MPH...

YMMV - pun intended of course

[Bob O]

I too have the exact same readings. GPS says 70, speedo says 74. The difference varies from roughly 3-5% depending on speed. Don't think it has anything to do with tires, at least standard size tires. My 16" snows and summer 17's give the same error at the same speed. Somewhere a while back I read that Porsche deliberately overstates the speed for safety and because there's some sort of huge penalty to the mfg if the speedo's read slow. (Can you believe anything you read on the internet?) I certainly don't know that for sure but this issue does seem to be common.

Oddly, when I was at Boxstoberfest last week, I was talking with a guy who had an 07 S. We were following him from Dallas to FB and when we stopped for a quick break several of us were comparing what our speedos were reading. He said he had just compared his speedo with his GPS and it was dead on accurate (or at least the two agreed). Maybe Porsche more accurately calibrates the speedos in the newer models???

Bob

[schnellman]

The topic of over optimistic speedometers has been discussed here several times. Some give a constant difference of 3 to 5 mph and others believe it is a 10 per cent difference. Either way, Porsche's answer is that it's a safety margin.
Not sure which model years this applies to, but on early models you can use the diagnostic settings on the HVAC display to accurate speeds in KPH. Yes, you can also find that little tidbit here on the forum. Enjoy!

[texomawaves]

Originally Posted by schnellman

The topic of over optimistic speedometers has been discussed here several times. Some give a constant difference of 3 to 5 mph and others believe it is a 10 per cent difference. Either way, Porsche's answer is that it's a safety margin.
Not sure which model years this applies to, but on early models you can use the diagnostic settings on the HVAC display to accurate speeds in KPH. Yes, you can also find that little tidbit here on the forum. Enjoy!

I have a 98 and reading in the owners manual where I know can change from mph to kph. It's not on the hvac, but one of the instrument cluster knobs, the right one (I think). Are you saying that it is more accurate in the kph mode or I can adjust it somehow? Sorry, I'm a newbie (10 day Porsche owner actually) :dance: maybe there is some hvac diagnostic to compaer to, but I've only read thru the first 1/3 of the owners manual.

[Bob O]

I too have the exact same readings. GPS says 70, speedo says 74. The difference varies from roughly 3-5% depending on speed. Don't think it has anything to do with tires, at least standard size tires. My 16" snows and summer 17's give the same error at the same speed. Somewhere a while back I read that Porsche deliberately overstates the speed for safety and because there's some sort of huge penalty to the mfg if the speedo's read slow. (Can you believe anything you read on the internet?) I certainly don't know that for sure but this issue does seem to be common.

Oddly, when I was at Boxstoberfest last week, I was talking with a guy who had an 07 S. We were following him from Dallas to FB and when we stopped for a quick break several of us were comparing what our speedos were reading. He said he had just compared his speedo with his GPS and it was dead on accurate (or at least the two agreed). Maybe Porsche more accurately calibrates the speedos in the newer models???

Bob

[Bob O]

How did that happen?? Sorry about the double post

Bob

[voltairesb]

I have a '97 box, and I can't seem to get the hvac into diagnostic mode. Is there a trick?