I thought 30mph felt especially slow in the range I bought a few weeks ago.
I passed a few of those show your speed things on the road yesterday, 30 mph on my speedo shows as 24mph on the road display. Why would it under read so much ?

Has someone fitted undersize tyres to the car ? Paul,
2001 Discovery 2 TD5, 211,000 miles & climbing
2006 FFRR TDV8 Vogue 145,000 miles & climbing
Member of Midland (Land) Rover Owners Club, www.mroc.co.uk

i wouldnt take for granted that the 'road speedo' readouts are that accurate...get an aftermarket gps that shows the mph..

Your car speedo is not necessarily reading the actual road speed. As Pawl and Stan say, tyres and sign accuracy come into it. I found this bit some time ago which seems to indicate that the regs give some leeway to overreading, but do not allow the speedo to under read. Again, that will depend on the tyres and anything else the manufaturer uses in their design.


http://www.legislation.gov.uk/uksi/1986/1078/regul...

There's this bit in Section 19

"1. The vehicle shall be fitted with a speedometer capable of indicating speed in mph at all speeds mph. up the maximum design speed of the vehicle.
2. For all true speeds up to the maximum design speed of the vehicle, the true speed shall not exceed the indicated speed.
3. For all true speeds of between 25 mph and 70 mph (or the maximum design speed if lower), the difference between the indicated speed and the true speed shall not exceed—
V / 10 + 6.25mph
where
V = the true speed of the vehicle in mph."

http://www.legislation.gov.uk/uksi/1996/3013/sched...

Thus at a true speed of 24 your speedo could be indicating 32.65. Cheers
Richard

(there are no such things as an ordinary FullFat, an ordinary cat or too many tools)
2011 TDV8 Vogue Santorini Black
Gap IIDBT

I appreciate that the speedo is not totally accurate , but I have had several people overtake me on a 30 limit . 6mph seems too much to be out. I have a transit and a freelander which are both accurate to the display mph at 30.
I bought the car with the wheel tyre combo it has but cannot say if they are correct in size. I will upload a pic maybe someone will know.
Tyres are 255,55 r20
Car is 2007 ajv8

I had that size on my 2007vse and my 2011vse and the speedo was very accurate according to the App i use ( Speedometer) Also road side speed warning signs that i pass everyday agree with the App  2015 4.4 AB
GSXR 1000 K5
R1 1998

My 2008MY 3.6TDV8 on 255/55/20 Wheels and tyres reads about 2 to 3 mph under true speed. Thus at 50mph on my Sygic satnav the speedo is showing around 52. Same difference at 30 and 70. Previously a 2008MY Tonga Green Vogue SE 3.6 TDV8

Switched to 22" wheels from 20's.

So used my IID Tool - great thing and well worth buying if you don't have one - to set my tyre sizes up.

All the road side warning signs agree with the speedo within +/- 2mph What do I put here then?

So it's likely these are non original wheels and it needs an adjustment then via someone with the correct equipment ?

you need to have a look at what's on first and go from there....post a pic or the size ..

All posted above

wheels look ok, tyre size will make the actual mph to speedo slightly more accurate...get or borrow a sat nav and compare your speedo to it..

Busterbulldog wrote:

So it's likely these are non original wheels and it needs an adjustment then via someone with the correct equipment ?

What is this tool I need and where would I find it and how much will it cost ? Or should I just book into a dealer for adjustment ?

Buster,

I run the same wheels and tyre sizes -- and all is well with my setup.

The IID tool is a way to make changes to the car and diagnostic -- a very useful tool all round -- if you want a new one there's a good price from Duckworths on the forum here!

However, for the time being, Stan is absolutely correct -- get a sat nav that displays your speed and go and do some comparisons, if your speedo is readin slower than the display on the Nav you can be pretty sure it's not the rims or tyres -- you'll be needing to look at another issue

Bushy Current: 2011 FFRR Vogue SE 4.4 TDV8
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