speedo inaccuracies

boxy · Post by **boxy** » Sat Jan 27, 2007 5:35 pm

I thought I`d do a little experiment whilst I was bored today and decided to see how accurate my speedo is. Luckily the Pioneer Avic -X1 has a GPS Sat nav with built in speedo. So I switched it on and off I went. Pretty accurate up to 50mph then it all went hay wire after that. Seems to be that 60mph is really 55mph and then every ten mile an hour increments loose a mile an hour of the actual speed i.e.

70mph really 64mph,
80moh really 73mph,
90mph really 82mph.

so on and so on.

How can this be, surely if it`s 5mph out at 60mph it should be 5 mile out at 70, or 80. How bizarre!

duncan · Post by **duncan** » Sat Jan 27, 2007 5:54 pm

they are generally accepted as being 10% optimistic... mine is

IWANT1 · Post by **IWANT1** » Sat Jan 27, 2007 5:59 pm

3mph out from 0-150mph

Didnt think it should change as its measured after the output of the gearbox!

cannit · Post by **cannit** » Sat Jan 27, 2007 6:09 pm

Very rare to get a completely accurate speedo, as Dunc said, accepted 10% for inaccuracies

Have you changed your wheels boxy? This can be another reason for them being out..

Cannit

boxy · Post by **boxy** » Sat Jan 27, 2007 6:21 pm

Yeah I understand and accepted that it would be out, just find it hard to belive that the inaccuray can increase along the spectrum, supose it isn`t though if it`s 10% accross the board. The wheels are 18inch instead of standard yes.

IWANT1 · Post by **IWANT1** » Sat Jan 27, 2007 6:29 pm

like you say it should be the same at any speed, Size of wheels doesnt make a difference to how much it changes at different speed just how far it would be out across the whole range :?

Iuse my road angel and it struggles to keep up and doesnt like going up 1mph at a time and usually jumps by 2mph!

boxy · Post by **boxy** » Sat Jan 27, 2007 6:32 pm

can`t drive everywhere looking at my GPS though as I`ll crash.

rolty · Post by **rolty** » Sat Jan 27, 2007 7:44 pm

The difference between your actual speed and the displayed speed gets larger as the speed rises as the size of the inaccuracy is directly proportional to the speed.

Say the speedo over-reads by 10% for example:

@ 50mph ground speed speed, the speedo will be reading this 50mph + the 10% over-read which would be 5mph.

As such the speedo would be showing your speed to be around 55mph.

Now say you're doing 90mph, the speedo will still over-read by 10%, but as 10% of 90mph is 9mph, the speedo could be reading 99mph, despite your ground speed only being 90mph.

Post by **Stuart** » Sat Jan 27, 2007 8:19 pm

Mine indicated 170 and a bit but was actually 155.6mph according to speed gun (at Honington RAF base

)

Driftbox shows accuracy to within 10% so I'm happy :thumb: