Okay,

Prompted by a discussion of engine revs at a given speed.... just how accurate are most scooby owners speedos?

Real road speed is easy to read if you have got a GPS, otherwise you will probably need a rolling road... I used a GPS receiver.

my MY03 WRX isnt too far out... the error appears to be linear between the two legal readings that I have calculated.

indicated 32.5 = 30mph actual
indicated 75 = 70mph actual

Matt

the dial reads around 50mph, my SECS unit read 45, at 160mph on the dial, the secs unit reads 150mph (airstrip)

hope this helps

Further to that...

At Elvington the other day, I was achieving terminal speeds at the line of 97. Say that I accelerated through the line & backed off after, then maybe 98/99 over a measured distance, 100 tops. My SECS recorded a maximum of 104. I reset each run, but the results were roughly the same - @ around 100, SECS reported +5mph over true.

Bear in mind that SECS reports ideal r/speed based on final gearing, so if tyres are new/worn, then there will be a difference based on what the "optimum" setting was, but it shouldn't account for more than 1 or 2 mph. At some point it will be at optimum.

Again, further to that, my GPS receiver would generally read around 2 or 3 mph below that of the SECS, so at an indicated (speedo) of 70 mph, SECS is 67 & GPS 65 or thereabouts.

However, not a good way to base your "true speed" in real life in case your estimates are wrong ;)

Legally how accurate does a speedo reading have to be?

Chip.

If you want to check, find the nearest set of calibration disks to you on a handy motorway or dual carriageway - the ones the police use to calibrate their speedos. They're easily spotted, circles with increasing numbers of quarters blacked out.

Just time yourself and do some simple sums...

end of 1/4mile, speed 95, speedo reading around 97

Legal limits are -0%, -0MPH and +10%, +2MPH, so for a true speed of 100MPH, speedo can legally read anything from 100MPH to 112MPH. Most are designed to be 3-5% over with new tyres.

Fair enough, but is the speedo reading affected when changing wheel sizes?

Depends on the wheels/tyres but potentially yes. My speedo is also out by about 5mph at 70mph indicated (true 65mph). I think that's quite a bit really!

ThrustSSC or anybody else.
Have you seen these discs, :confused: if so can you give some locations?
I've seen a pic of some, but never seen any on the motorway.