mattm

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

rb5 286

iTrader: (2)

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

hope this helps

Puff The Magic Wagon!

iTrader: (2)

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

Chip

Legally how accurate does a speedo reading have to be?

Chip.

ThrustSSC

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...

RB5_245

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

dr_ming

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.

Ken 275

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

macka

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!

RB Simon

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