inaccurate speedometer

[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.

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!

[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

[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.

[Topless]

Trust your GPS. Our 986 speedos are overly optimistic.

[texomawaves]

Interesting... I just talked Boardwalk Porsche service in Plano, Tx. They don't calibrate speedos, and the service mgr acted surprised that mine is 5 mph off from my GPS. He said "well, you haven't got any speeding tickets yet, have ya? ...then that's a good thing." (that was a bull**************** answer)

He asked me if I had non-factory sized tires. He seemed to act "unaware" of this issue, and didn't even question the accuracy of my GPS hmmmm. He then referred me to some other business in Richardson, Tx called Zebco (not the same fishin pole guys)

I think Porsche knows about it, and they don't have a fix for it. Now I get the runaround to a 3rd party business. I'd sure like to know exactly how fast I'm going and not have to add/subtract before I hit the cruise control, I only use my GPS when I need it. Everything about this car is nice, but the speedometer is crap!

[schnellman]

My apologies to those of you have been through this before. But, for the uninitiated, go here to see just what your HVAC controls can do for you.

http://www.cb-racing.com/boxster_AC.html

[Bob O]

Unfortunately, as noted, that HVAC feature only works through MY2000. Seems to be a pretty useful tool. Wonder why Porsche stopped letting us access that info?

Bob

[texomawaves]

Hey guys, the HVAC trick worked on my 98... pretty cool, and thanks. So I set my GPS to kph drove down the road and compared... still a difference, but doesn't seem as much. I forgot to note the difference, but will do it again later today. I will then calculate the error and compare (HVAC vs. speedometer). I will assume my GPS is accurate (of course). Chris

[jmatta]

Everything about this car is nice, but the speedometer is crap!

The speedo has more accuracy than the coolant temp gauge...

I've learned to live with the inaccuracy and just factor the difference into my speed. When I first bought the car, I wondered why everyone on the road was always passing me or riding my a$$.

[texomawaves]

I may get radical and change out my wheels to 18" if that's what it takes The speedo may be designed around 18" wheels. Then anyone with 16" or 17" wheels get the benefit of the Porsche factory "safety-margin" feature (sarcasm) You'd think with 3 different wheel size options there would be and adjustment for all three. Maybe there is???????? I could only hope. However I love my 17" wheels with the cool lookin Pzeros.

[texomawaves]

Results from my lunchtime test drive to get a Sonic burger:

7.1% error against the speedometer (75-70)/70 * 100 = 7.1%
2.9% error against the HVAC diagnostic (106-103)/103 * 100 = 2.9%

I am not sure what to conclude from this other than the HVAC diagnostic is closer to the GPS reading than the speedo, duhh. So perhaps my Pirelli Pzero's are too low of a profile. I guess I might consider bigger tires. The ones I currently have are 205/50 ZR-17 on the front, and 255/40 ZR-17 on the back. Perhaps a "taller" tire will shave off a few percentage points on the error.

So guys... what size tires are you running? ...and what error are you getting? Let's compare.

Chris

[gregdacat]

Sonic Burger Mmmmmmmmm

Originally Posted by texomawaves

Results from my lunchtime test drive to get a Sonic burger:

7.1% error against the speedometer (75-70)/70 * 100 = 7.1%
2.9% error against the HVAC diagnostic (106-103)/103 * 100 = 2.9%

I am not sure what to conclude from this other than the HVAC diagnostic is closer to the GPS reading than the speedo, duhh. So perhaps my Pirelli Pzero's are too low of a profile. I guess I might consider bigger tires. The ones I currently have are 205/50 ZR-17 on the front, and 255/40 ZR-17 on the back. Perhaps a "taller" tire will shave off a few percentage points on the error.

So guys... what size tires are you running? ...and what error are you getting? Let's compare.

Chris

I'm running the same tires as you but on 16" wheels and my speedo is exactly 5 mph faster than my GPS or the radar guns they use to warn you of your approaching speed. All this time when I was doing 75 and thinking my spoiler was up it wasn't. Dang!

My bad, 17" wheels with PZero's.

[jmatta]

The speedo may be designed around 18" wheels.

Not; I have 18s and as I post earlier, my speedo is off at least 4 to 5 mph.