SJB993

This post was prompted by a couple of other posts about fuel economy and tyre/wheel combinations. In the post about fuel economy, the poster said he had found his speedo to be highly accurate.

In my previous 993 I was suspicious about my speedo - I never felt as though I was going as fast as it said I was ( the MPH in the OBC display always tallied with the speed shown on the dial). I compared the speed shown on my GPS with that on the speedo and found a difference of around 10%. I recently did the same test with my current 993, and got a similar discrepancy. I have done the same thing in my daily driver (VW Golf GTI), and its speedo matches the GPS speed very closely.

I am wondering whether there is some subtle difference in the way KPH are converted to MPH for the UK and American markets (i.e. there may be other things that are different in the ECU, and other electronic stuff, between UK and US cars, and maybe the guy who implemented the US conversions did a better job than his colleague who did the UK ones).

So, I'd be interested to hear from US and UK drivers to hear how accurate they have found their speedos to be, to see if a UK/US split develops.

(Both cars have standard (stock) wheel/tyre combinations although my previous car ran on 17" wheels and the current one runs on 18" wheels - but I assume the outside diameters of the tyres will be the same, thereby covering the same distance per revolution).

TIA, Steve.

jimbo3

My 993 digital speedo is spot-on, based on several of those roadside radar speed displays. (Stock 17" wheels and tire sizes.) Mechanical speedo reads a couple MPH faster (<5%), though. My 914 and (former) SC's speedos read 10% high and virtually everyone who had an SC around here had about the same error factor.

GabiOsz

I found mine to be about 7% out, based on the GPS reading on a road angel. The gauge is hard to gauge But the OBC display is just not right.

Bruce SEA 993

I posted on the tire diameter thread about the accuaracy...I checked mine with my gps and it was within one or two mph at 60 and 70 mph

dhicks

Every time I drive towards those roadside speed boards I try and assess my speed with that displayed...I am always within 1-2mph of the reading on the display.....but my Valentine hates these things....hahahaha

sidned

My Road Angel (GPS/speed detector) shows about 8-10% above my speedo. I'd rather my speedo show a slightly lower discrepancy than the other way round!

SJB993

So, maybe we're seeing the start of a trend here?

US speedos pretty accurate, UK speedos pretty crap.

Keep the inputs coming please!

TheOtherEric

All 3 of my 911's (two have been 993's) indicated higher than actual speed. I checked all against GPS. My current 993 speedo reads about 3% - 4% high. I've checked it against the GPS of my DL1 data logger several times.

97Targa

I've been told by a PCNA rep. that our speedometers are a bit "optimistic". That is that they read higher than the actual speed.

GabiOsz

Every german car I have owned has been out between 5% and 10%.

BMW, Merc & the Porsche. My father has a Range Rover and that speedo is spot on.

SJB993

Gabi, thanks for the input. Interestingly my Golf GTI (German of course) is more or less spot on. It does seem as though most of the American guys feel there speedos are pretty accurate.

I really don't see why they shouldn't be close to 100% these days, now that they are counting the revolutions of the wheels at the actual wheels (via the ABS sensor), and the dials are driven by electronics, instead of spring wire cables from the gearbox as they used to be.

I just need to keep on top of the maths. Assuming I want to do 80 MPH on the M4 (folk lore has it that the plods never do anyone for less than that), allowing for the 10% + 2MPH the plods are supposed to allow, plus the 10% exaggeration in my speedo, then if the speedo says 98 MPH, I should be OK

Take care, Steve.

911nut

My 993 and 964 with new tires are within 1 mph compared to a handheld gps. For all intents, the same. When stopped for speeding, the speedo was the same as the radar (oops) My 87 3.2 reads 10% higher than the gps.

Chuck W.

Using a GPS I found my 993 dead on. However, both my Boxster S and my brother's Boxster S read 3 MPH across the board.

BobbyT

It isn't a coincidence that these things read fast. It skews performance figures on the positive side, mileage-based service intervals come up sooner, and the mileage warranty runs out sooner.

epj993

Is there anyway to get these recalibrated to accurately display the speed, i.e. an instrument shop rebuild? I wonder if this is ultimately due to wear of some internal gauge parts that could be replaced.