Impreza speedo accuracy

[Animal]

Hi All,

Does anyone know how accurate these are? I know that by law a speedo can over-read by up to 10% but cannot under-read.

Cheers,

Animal

[Riknos]

Borrow a satnav, then test it against that. Or go through one of those digital speed indicator things they have on highstreets in town, only real way of telling.

[edwardsje]

Mine ('07) overreads pretty consistently by c5% but no substitute for doing what the previous poster has suggested and getting a GPS reading.

[ScoobieWRX]

MY02 WRX Wagon:
On 215/45/17 tyres mine read c.5-6mph too much at 55+mph, below that it was 3-4mph out
On 225/45/17 tyres it reads 3-4mph too much at 55+mph and 2-3mph out below that.

My next set of tyres will be 235 or 245/45/17 to see if i can get the speedo reading as true as poss and a bit more grip of course

The above readings we're done using a 51channel Bluetooth GPS receiver, lap timing software on a PDA and a very steady right foot!!

I would recommend using Performance Box or Drift Box from RaceLogic as that has an integral GPS receiver that monitors your position 5 times a second(1/20th) as opposed once every second or half second

[Animal]

Don't think I'll get the chance for a while - I was at Bruntingthorpe and saw an indicated 150. Just wondered how fast we were actually going!

Cheers,

Animal

[ScoobieWRX]

Probably 145mph.

But then that's what the manufacturer builds in. If Subaru wanted to they could make the speedo pretty accurate and indicate a true speed to within .5 - 1mph at the top end but they don't.

You really need the GPS system to give you an accurate reading.

[MilnerR]

Mine over-reads by around 4-5 mph at all speeds. When I'm doing 30 the GPS will read 26 or so, when I'm doing 120 (on a track) it'll read 115 or there abouts. My elise speedo is absolutely spot on though, the GPS agrees to within the thickness of the needle.

[Nicol@]

When my speedo reads 75mph, the satnav reads 70.
When my speedo reads 53mph, the satnav reads 50.

Can you tell I was bored on the M1 on Monday.

So I would guess that 150 speedo reading is just under 140mph.

Edited by Nicol@ on Wednesday 16th July 15:41

[ScoobieWRX]

Nicol@ said:

When my speedo reads 75mph, the satnav reads 70.
When my speedo reads 53mph, the satnav reads 50.

Can you tell I was bored on the M1 on Monday.

So I would guess that 150 speedo reading is just under 140mph.

Edited by Nicol@ on Wednesday 16th July 15:41

Your speedo won't be out any more than 5-6mph max at the top end so 150 on the clock won't be any less than 144-145mph true speed.

[_Mechanic]

ScoobieWRX said:

You really need the GPS system to give you an accurate reading.

Make sure you test it on the flat as satnavs do not compensate for hills. Thats if you want to get it spot on.

[ScoobieWRX]

_Mechanic said:

ScoobieWRX said:

You really need the GPS system to give you an accurate reading.

Make sure you test it on the flat as satnavs do not compensate for hills. Thats if you want to get it spot on.

Ordinary Satnavs maybe don't have that built in but i was talking about GPS software, not SatNav. Some of the GPS software i use measures your altitude as well as speed, direction, acceleration, lap times etc... but even more accurate is the Racelogic standalone kit. Most UK circuits have parts of a track at differing altitudes to other parts so this equipment or software on the road will be accurate whether you're going up or down a hill.

[liner33]

ScoobieWRX said:

Nicol@ said:

When my speedo reads 75mph, the satnav reads 70.
When my speedo reads 53mph, the satnav reads 50.

Can you tell I was bored on the M1 on Monday.

So I would guess that 150 speedo reading is just under 140mph.

Edited by Nicol@ on Wednesday 16th July 15:41

Your speedo won't be out any more than 5-6mph max at the top end so 150 on the clock won't be any less than 144-145mph true speed.

Any discrepancy will be magnified by the faster you go ie a 5mph error at 70mph is likely to be more than 10mph at 140mph thats why the standard is set as a percentage. Even GPS isnt 100% accurate, I've seen many bikes with 180mph+ top speeds displayed on their GPS and the same guys have seen 200mph indicated on the speedo but struggle to hit 175mph through a proper speed trap

There are so many factors which effect speedo accuracy the design of most cars means their speedo will never read accurately and if they do at one point when the tyres have worn down another 1mm or the components have covered a few thousand miles the reading will be different.

[ScoobieWRX]

So you're saying that GPS timing equipment maufactured by companies like Racelogic and others, that are used globally by all the top automags and car manufaturers, having spent £hundreds of thousands designing and developing this kit are manufacturing equipment that isn't accurate at those top end speeds??

And are you're also saying that radar/laser speed traps like those used by the police and certain UK race tracks are 100% accurate, bearing in mind how many of these systems have been proven inaccurate in UK law courts.

I appreciate your speedo will read different speeds when tyres wear, specially at higher speeds but equally your tyres circumference gets bigger the faster you travel thereby cancelling out effects from wear rate when pressing on.

I'll put my trust in GPS equipment being pretty damn accurate to within an inth of a degree bearing in mind it's widely used in aircraft to measure velocity, distances and location very accurately, travelling much faster than any car i can think of, and my understanding is that even speed traps have their speed and accuracy limitations!!

Read this..... http://www.physorg.com/news111482871.html

P.S. the most accurate speed traps going can be found at drag racing venues where they use the trusted and very simple ligthbeam switch at both ends of a measured distance, and the more complex RF based systems used in F1 with sensors placed all around an F1 circuit and one in the car.

[liner33]

GPS is reliant on how many satellites it is in contact with , so it has varying degrees of accuracy especially when its placed inside a metal box even if its accurate to 1 metre that is still a high degree of error, most people arent talking about something like a Racelogic box which measures speed every 1/10th of a second but a TOM TOM or Garmin system these all have degrees of accuracy but none are 100%

Drag racing beams and other types of speed traps will only record an average speed over the set distance they cant tell you how fast your entered the beam or how fast you left it , only the average