I'm running 295/30 rear and 235/40 front PS2s on the Turbo. According to the speedometer function on my 9500ix radar detector, the car's speedometer is reading about 10-11% too high. This doesn't seem right, especially since the overall tire diameter is slightly larger than stock

 

I check mine with a GPS. The 993's have always been pretty much right on. The Cayenne, after I put slightly larger tires reads low by about 1%.

Not the wife's '09 BMW (and her previous '07) read WAY high, about 7 - 8% and BMW wants them that way. Their acceptable error is 10% high, it's outrageous. I won't buy a new BMW because of it (and the fact they removed the oil dipsticks).

 

Interesting post as I am preparing to run the Nevada Open Road Challenge next month, and I was closely monitoring mine against GPS this weekend driving all over Nevada. I found pretty much the same result you've posted. I'm running the same tires/wheels, and against gps, speedo was about 10% too high.

Denny

 

Stock NB wheels on my 95 and my 9500ix says my guage is 1mph slower than the 9500IX gps reading up to about 95mph then it's ~2mph difference.

Nick

 

'97 NB 18" turbo twists, Kumho tires. 40mph on speedo = 46 on Garmin GPS.

 

Not to derail the thread...but does this unit show speed?

 

Hi

Mine is fine at lower speeds, but higher up it can read as much as 10% over. Silly really.

Berni

 

Yes

 

The 993, Wifes SL and the Trooper all read right on.

The '02 BMW 540 reads 8 -10% high and really bugs me. It is a known quirk of the BMWs. I am going to go with larger rear tires next time and may take out most of it. It is a nit but I have to spend brain power thinking about it and correcting.

Cheers!

 

It isn't really a "quirk" of BMW's, they intentionally do it that way. Go on any BMW forum and you'll read about it and most likely someone has posted BMW's Tech advisory sheet on it. It just flat out irritates me (in case that wasn't obvious by now).

 

True...it is intentional. There is a way to fix it by moving the speedo needle the amount it is off to correct it.

Ciao

 

It can be programmed to show speed or voltage (I think it's silly that Porsche's don't have a voltmeter when even cheaper Japanese and American cars do). If you don't program it to constantly show speed, it will still show speed as soon as an alert goes off. One of the most advanced detectors out there. Pricey but well worth it, and I would fully switch if it had two additional features: (i) directional arrows like the V1; and (ii) a better remote display for stealth mounting.

 

I went to 35 series tires in the rear (and can without issue since disconnecting my front driveline) and my speedo to GPS was right on the $$. It used to read 3 mph fast with 30 series tires.