DME reporting higher MPH than the speedometer

Looking for a little wisdom, 1/98 C2 cabrio, 60 K, I recently change rims from what I believe to be are FRH Hartmann 3 piece twist, (only stamp is FRH INTL), 8 x 18 et 49ish fronts and 9.5 x 18 rears et 52 ish, 7mm spacer front and 15mm spacer rear. Tires are 225/40/18 and 285/30/18. I recently procure a set of BBS Sport Classic II's from a fellow Rennlister, thank you Nick, 8 x 18 et 52 and 10 x 18 et 65, re-added the spacer and tires from the above. Rides and feels much better, however my obdlink MX+ is reading 8 MPH more at 60 MPH than the speedometer is showing, which is a ticket waiting to happen, from what I read, it's usually the other way around, the speedometer showing more than the DME. With the old rim set-up, the speedo was 1 or 2 MPH higher at 60 MPH when compared to the OBD2 . Thoughts?

Quotes source post: 20122104

Try using Waze to see what is says.

Quotes source post: 20122128

my redenso radar detector does a readout of my speed through its built in gps. My lcd mph readout is missing so many lc�s that I can�t tell if I am doing 130 or 80 mph, so the redenso helps maintain my speed at or under all posted limits. I swear. My Carrera is the first car I have ever owned that is dead nuts accurate on speed v speedometer.

Since you�rE using a smaller diameter tire than the stock size you expect the Speedo to register higher than actual speed. Agree to compare it to gps speed or time it going 10 miles on the highway at 60.

Quotes source posts: 20122128, 20122274

Thanks for the advice, I'll give Waze a shot for the trifecta, won't be until Sunday though. I thought I was using the correct tires sizes for my Sport Classic II wheels?

Yes, your tyre sizes are according to stock (for 996.2) sizes. 996.1 had 265/35R18 as factory fitment but that won't make a difference.

Was able to take a ripped today, speedo 60, waze 55, obdmx+ 68. Thought it maybe a ABS wheel sensor, however, no DTC's showing. No other changes than the wheel swap, Just trying to rule out it being symptomatic of a different issue I should be looking out for.. Otherwise, I don't mind living with it. Thanks again, Paul

Quotes source post: 20122623

You are incorrect to say a smaller diameter tire won't make a difference.

That is right. Tire diameter effects speedometer accuracy. Does it read from a front wheel or rear wheel? Width, offset, etc do not matter.

You are both correct, obviously. I worded it badly. What I meant to say was: both rear tyre sizes are factory fitment sizes and are therefore all considered in the so-called k-factor that porsche programmed inside the speedometer to ensure that with any of those sizes the speedometer reading will never be lower than actual speed. Speedo reading lower than DME speed is what the OP observed and my response above was a sore attempt to express that he shouldn't be seeing that with any tyre size that porsche fitted as factory equipment. I cannot explain why the dme readout changed when changing wheels* nor do I know how it is calculated. But as long as the speedometer does not read lower than actual vehicle speed, it is of no traffic law relevance that it reads lower than the speed output given by the DME. The waze measurement of actual speed showed that speedometer is reading as intended, so all good for OP. ETA *. The 285 on a 10" wide vs 9.5" wide rear wheel probably yielded a slight difference in rolling radius.

To put a pin in this one for me, mine was set at 1.1, changed it to 1.0 and now waze and the OBD2 are on the same page. Thanks to all who chimed in, much appreciated

So this was an error in your Scanner ...( and not the DME output )