Was away for a couple of days this week, and took my brother's GPS radar detector with me. This tells you your speed a lot more accurately than the actual speedo.

I found that I was pretty much exactly 10% out, right across the range - when it showed 100k I was actually doing 90, 50 indicated was 45, and so on. I also drove past a couple of those "you're going this fast" signs, and they matched the GPS both times.

The rolling radius of my wheels is pretty close to stock, and I haven't done anything to my drivetrain, so I assume this error is factory.

It's sort of handy overstating it (I can now cruise at indicated 120, and still be pretty safe from cops, or overtake at indicated 140 and still keep my license if I get caught).

I'm not sure if the odometer is also out, but I think it is. This means my fuel economy is actually 10% worse than I thought, and I do oil changes 10% more often than I need to. It also means I'm thinking of spending ~$800 to change my cam belt when my car has actually only travelled 82,000k, not 91,000 - I could leave that for another year or so until my car shows 110,000 and still get it done before I've actually travelled 100. Also, would my speed limiter kick in at 162kph? And is Nick's car actually dinging at him when he's just doing 108?

I stumbled across this site - http://www.hondaodometerclassaction.com/. Basically some Americans found Honda were intentionally making their cars read 5% higher than they should have, and as a result they got 5% more services done, the warranty lasted 5% less, and so on. Honda got in trouble for that...

So has anyone else checked theirs accurately?

Mines 6Kph out..

I also use my GPS to gauge my speed, look at the OD and notice its out of sync...

I use kilometer posts at the side of the road and a stopwatch.. done it for 30years now. The set down Highway 27 are great, you can hold exactly 110kph for several km at a time.

6% I would expect in a modern car, 10% is at the top range. I check all the cars I drive when I can.

It makes the cars seem faster than they are, and which manufacturer would make them under-read and get into trouble for their owners getting speeding tickets.

There is a lovely SF story about a man who is frozen & awakened 100years in the future. One little point made was how the cars sounded really fast, had lots of dials and lights, but only seemed to be going about 80kph flatout at 200kph... :D

mine is 7kph out to Garmin /TomTom GPS speed

Mine is out by about 6%. i.e. at an indicated 100km/h I am doing 94

ditto here , hmmm problem / trend.

if we were all paying road user charges that could get interesting !

not sure about mine but u have a uniden radar detector ... would i b right spoon?!

ditto 100k = 94 etc etc - 6% seems common - tyres about 50% worn - Garmin Streetpilot 2610, with remote control

Mine is only out by 3k. My digital speedo reads 100 but im only going 97.

All cars do it. Trend rather than problem. They even note it in the likes of wheels mag when they give vehicle specs.
http://www.wheelsmag.com.au/wheels/site/ar...;file/SPECS.jpg
http://www.wheelsmag.com.au/wheels/site/ar...;file/SPECS.jpg


I've also found a RSM will read spot on just like GPS

Mine used to be reading 4% low at 100kph but since i fitted 245/45/17 (rear) 225/45/17(front) for the bad roads it now reads spot on.... mines the 5AT so gets it's speed from the ABS AFAIK

You can get the speedo recalibriated.

Robinson's Instruments in Akl do it

According to Keith (Greeneyes) when he tested mine using the marker test.
My one is .5km out @ 100kph :D

Thats on 18's

You surely would! GPS9000-NZ from memory