i have a road angel 2, whilst travelling i have noticed that the spedo on my car is 2mph faster than the roadangel.at 80mph on the clock it read 78mph. because the road angel is very accurate at constant speeds that means my spedo is out. doesent this lead to more milage on my clock that i have not done, i have tried this in my car, my dads and mates , all 2mph slower than the spedo

Very few if any speedos are accurate there are just to many variables. Also a speedo usually gets more inacurate the faster it goes.

My octy 2 is out by about 4 mph usually!

I drove a Toyota Prius a while back. The speedo was reading 8% too high at motorway speeds!

As Lummox says, this is very common and the variation changes according to your speed. Analogue instruments have their limitations.

Theres a diagnostic function on climatronic unit in some VAG cars. I think this can give you a readout of actual speed. It would be interesting to know how they compare.

I caught a programme on Sky recently (police camera stop type thing) where they were talking to a motorway cop with a Volvo T5 using a radar gun on cars from a bridge. He said they didn't bother until they spotted a car doing at least 90mph. That means you could be doing nearly a ton on the speedo before you'd risk getting pulled!

The 'actual' speed on the climatronic only comes from the same sender source as the dash speedo.

Fair enough but we are talking a discrepancy of only 2 mph between the analogue speedo and road angel. At least the climatronic gives an actual figure for comparison instead of relying on your eyesight, the assumption you are directly facing the dash (ie not too tall or too short) and the calibration of the speedo which is a partly mechanical device anyway.

When the clock says 120, Tom Tom Nav says 111 in my Octy. It's the same in every car I have been in with sat nav to various degrees. Two different Astravans, one 02 reg and one 03 reg actually read differently. At 80 miles an hour one said 75 on Tom Tom and the other said 77.

Is it not something to do with tyre tread depth ?? as this changes, so does the wheel rotation per mile, so they have to build in an allowance for it or something :confused: :confused:

Speedo's have an allowed tolorance +10% -0%.

Mine appeared all-too-accurate. I know the Furby vRS has the stupid 90 - 110 thing going on, but you can pretty much know which 5 mph zone you're in.

Got done doing 105 and when the officer asked me what speed I was doing that was the one I mentioned. It was 105.1 so that's as close as it's gonna get.

Please note this is not 'for bragging purposes' - it is purely for the accuracy side of things.

Speedos are supposed to be accurate or overread. Most that are basically accurate will overread by a few mph at motorway speeds. This is so car manufacturers can cope with incorrect tyre pressures and not get loads of complaints when people get 'done' for speeding on the basis of doing say 62 mph in a 60 mph zone. In reality in the UK that won't happen anyway, but that's another issue.