Actual speed

[Rincewind]

Read my splurge a page back about how speedos are cal'd.

Doing similar test tomorrow out here in India on a new car and I'll have to test the car on 4 sets of tyres in order to establish if their speedo cal is correct for the various tyre sizes. Sometimes manufacturers will have specific speedos for a specific gearbox or set of wheels/tyres.

Don't rely on satnav's to give you accurate speedo readings as they're prone to delays and what not and it also depends on who's using the satellites too. If the military are mega busy, your sat navs will be affected!!

[baxlin]

Originally Posted by Mike

My speedo under reads by circa 10% so at 60mph indicated actually doing around 71/72. I always go by my TomTom.

Haven't tested it on the V8...yet

Mike, which car is that on? It seems a heck of an under-read, when everyone else's over-reads........

malcolm

[Jürgen]

Originally Posted by David3807

All above sounds about average for a 75. My speedo reads 10% fast throughout the range ie True 30 indicated 33. True 70 indicated 77.

Same here, +10% at any speed.

Originally Posted by Mike

Haven't tested it on the V8...yet

Now I'm curious about the real top speed, Mike.
Well, my test on the German Autobahn got a spot on result (TomTom = MGR technical data sheet).

Originally Posted by cjmillsnun

Mike please don't go by your Tom Tom. GPS being what it is, it is in itself inaccurate.

Yes and no. You won't get an accurate position unless you're a member of the American military . So the software of the satnav system matches the measured position with the nearest street on the map. But if you're measuring your speed, all that counts is the difference between two places. This will be always correct! Professionals are using differential GPS, thats a standard GPS device combined with a stationary GPS receiver at an exactly known location. So the variation of the mobile device is corrected on the fly.

Originally Posted by Rincewind

Don't rely on satnav's to give you accurate speedo readings as they're prone to delays

Just cruise at a constant speed for a moment and the satnav will be spot on.

[grossebaby]

My 04 zt is spot on on new tyres up to 100 ish then about 1mph out, on old tyres it over reads by between 1 and 3mph increasing as speed increases up to about 90mph.

The most accurate car I have ever owned.

[Keith]

I think in my case it was a relative observation unless the same sat navs displayed speeds can vary on a day to day basis, and all things are possible

Same Sat Nav at 70mph
Rover way out, Citroen C4 pretty near spot on
Obviously there are three variables probably all three are out a bit but the Sat Nav and Citroen at least seemed to agree with each other.

I will have to investigate some more
What will be a laugh is checking the speedo on my mini as I know that is miles out!

[JohnDotCom]

My mini speedo use to move between 25MPH and 50 MPH when driving at a constant speed.
Never knew what I was doing.
Fun to watch bounce up and down.

We use to have a Calibrated Speedometer on the Traffic Cars separate to main Speedo.
Don't know if they still do that with modern technology and cameras etc.

[Alhumphrey]

my speedo used to read 10% over until I got it checked when I put new alloys on. now my speedo is right I'm doing 10% less milage and my brim to brim fuel economy has increased. I think thats what they call cooking the books. lol

[crofts]

Well I set 78 on the speedo with cruise control and for miles and miles my satnav does not vary from 70mph ( & I have not yet got a ticket !)
It's amazing how many drivers obviously think the limit on a 2 lane A road is 60mph. On the A505 from Royston to Baldock there are 4 cameras within about 5 miles which I go through on 78 indicated, on cruise, and have never had a ticket. Very rarely is another car doing the same speed !

[stevemac]

Originally Posted by cjmillsnun

Mike please don't go by your Tom Tom. GPS being what it is, it is in itself inaccurate. However unlike a speedo which always overreads the difference is + or -. Unless you spend on surveying or aviation grade kit (and on an expensive D-GPS subscription) then you have a chance of it being up to 10% out in either direction, depending on the orientation of the satellites at any given time (GPS satellites are not geostationary)

Edited to add, those with High Line Nav have a fall back in that they also have a speed reading from the ABS system (speedo feed) which is plumbed into the Nav system and provides some error correction.

This info might help. GPS is manufactured to be no more than .5mph out.

http://gpsinformation.net/main/gpsspeed.htm

[stocktake]

Speedo on my company focus is 10% fast showing 77 on speedo and 70 on various sat navs
Rover on 18" rims is showing 73 on speedo when 70 shows on various sat navs.