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Speedo Accuracy and Speed Cameras

It's not just down to your speedo. Depends on rolling radius of your tyres. If you want a more accurate measure, use satellites.

You can go as fast as you want without prosecution.......the trick is not to get caught :wink:
 
If want to know your actual speed, suggest you use Waze or a similar app which shows GPS speed. Typically the speedo is a few % optimistic on speed to account for the full range of tyre wear etc. without showing speed which is less than your actual speed.
 
We have a lot of active speed measurement signs over here to help people understand their speed: my Porsche over-reads quite significantly (even on part worn tyres with 5mm of tread). You really need to use a GPS to see true speed (on the flat).
 
Not sure about the Porsche speedo , my company supplied work bus reads about 2-3 mph above a gps reading, difference increases as speed does.
 
I was quite impressed with my porsche speedo. I use Waze for every journey and that has the GPS speedo on, and my porsche speedo is pretty much spot on. Once you get over 50mph its about 1 or 2 mph over actual speed.

Comparing that to the wifes Mini, that says you are doing 85mph when you are really doing 78mph!
 
You must also factor in grandient with GPS as it doesn't compensate. I've had GPS read even slower than normal on a steep hill when set with cruise control. It caught back up on the flat.
Much in the same way when you go through a tunnel, the GPS will assume you last speed as constant and then readjust when you pop out the other side.
Nothing is going to be perfect
 
Porsche are German. Their speedometers are pretty close.

I was beam timed at 196mph on a runway (still climbing). My speedo was reading exactly 200mph. So that's 2% over actual. I think my Ducati is ~8%
 
The most accurate speedo I've ever experienced is the Land Rover Discovery3. The 9972 speedo over reads, and the speedo reads more than the kph display on a Scanguage plugged into the ECU.
 
The only car I've ever owned with an accurate speedo was my 997 it was spot on .Wife's BMW X5 is 10% over not checked my M2 .
My Noble M12 is also slightly optimistic but not by much.
 
was always told there was a 1% plus 3 rule, but a quick google says it's actually 10% plus 2mph, and it's not law, its a rule, so might not be used.

So, 70mph limit, 79mph should be ok. But isn't law. Think there have just been new laws passed which mean you can get done for being 1mph over the limit, but that is ridiculous......
 
In my 997 (and the mk2 Megane RS I had before), plugging in an OBD reader to an app such as Torque will give a reading directly from the ECU which is lower than the digital speedo by about 3mph, pretty much bang on with my GPS readings.

It seems the ECU is calibrated accurately, but you're given a bit of slack on the car reading. Many people theorise that it's better to err on the side of caution hence why speedos tend to read less than your real speed.
 
My Border Terrier is calibrated to 70 m.p.h. I discovered today. Top down
as soon as you hit 70 she started complaining, ears flapping about too much I think, worked everytime no need to look at speedo.
 
My Cayman is the closest I've seen out of all of my cars I have owned. 70 is only 1 or 2mph out.
 
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