is there a fix for speedo reading too high?
 

I have a 1998 986 base tiptronic and a 1999 986 base 5 speed.
The 98 has 17" tires and the 99 has 18" tires.
They are BOTH reading too high on the speedo.
At 50 mph they both read about 54 mph on the speedo.
I've tested this several ways and believe this to be accurate.

Anyone know how to fix this?

Your 17s and 18s likely have the same overall diameter which is why both cars are over-reading by the same amount. Input your tire sizes into this calculator to confirm.

https://tiresize.com/calculator/

By design, most cars over-read by some extent.

Yes, the difference in sidewall makes the overall measurement the same.

The issue remains, the speedo is off by 8%+ and that throws off mileage and makes using the odometer for navigation more difficult.

My other 4 cars are about 100% accurate.

So there is no speedo gear that is easily changed or something like that?

Both of my Boxsters and my BMW readabout7% high. I heard that the odometer reads the correct distance. It must be a German regulation.

Manufacturers don't want to be in legal trouble if you get a ticket. Going back 40ish years, all my cars have been the same. Except one, a Ford. I got out of a speeding ticket in it once by having the speedometer checked and handing that certificate to the judge. Funny thing: it wasn't the speedometer that was at fault, it was my right foot. But the judge saw I had spent money on the test and I was young and poor so he cut me a break.

The OEM's set them to run high for two reasons: Prevent lawsuits over what mike mentioned above, and to run your warranty out just a little bit faster. Most cars are this way.

I would bet your odometer is still very accurate, years ago automakers got in trouble for that as warranties would expire sooner and gas mileage would look higher. If you really want to find out if something can be done, try contacting North Hollywood Speedometer or Palo Alto Speedometer. If something can be done, I'm sure it won't be cheap!

Good luck with that; the Boxster uses one of the ABS/PSM wheel speed sensors to determine ground speed, and the instrument cluster gets that information from the DME. Considering Porsches penchant for incomprehensible logic in the DME software, you would need to reprogram the DME, which is anything but easy, even with a PIWIS. And the government gave automakers considerable leeway on instrument cluster accuracy a long time ago, which is why these cars have idiot lights that move instead of real instruments.

8% high compared to what? If comparing to GPS....GPS isnt a good test down to low %. Best method....have a cop friend run radar on you, that will be accurate and compare to what your speedometer says. I doubt you are off by much to even notice it.

It isnt like the old 944/911's where you could change the speedo gear in the trans. As Jeff points out...it's an electrical signal.

If you want it reading the way it left the factory....put factory tire sizes back on.

Thanks for that, now no one has to call!:cheers:

Got it. Now I understand. Good to know the odometer is correct. Not worth taking this further.

FWIW I was using radar as one of the speed measurements vs my speedo.

Thank you everyone for the info :0)

I also see the displayed speed being high. I would prefer it to be on spot, or maybe just a tiny bit higher, as with the knowledge I am doing 50 mph when I see 55 mph gives me stupid ideas and sometimes I am lost in the math.

I will be OK going 72 mph in 65 mph zone, so it means I need to drive 77 mph... you see the point?

I get the general idea, but almost 10% is too much.

Oops my bad, I posted the below before seeing there was a thread. Sorry.
Not familiar with a fix, but can verify that you are not alone. Have owned several Boxsters and they all have run about 3 mph slower than the speedometer shows. Confirmed by roadside radar.

a slightly taller tire would "fix" this