What is a true speedo reading?

[rctempire]

Any idea how a manufacturer works out the correct MPH reading on the speedo?
Eg. If a new tyre has 8mm tread and a uk legal tyre has 1.6mm that is a difference in diameter of 12.8mm which is approximately 50mm difference in the circumfrence on an average tyre.
Also a big difference would be seen for example on a LandRover Defender/Discovery, new and old tyre. The speedo reading cannot be correct in all situations.

As a side issue could you argue in court that due to new tyres or just road legal tyres the vehicle speedo reading was inaccurate?

Debate.

 

[Beef-Mc]

Been caught speeding bud?

 

[.GAZ.]

:S

 

[rctempire]

No, was having a discussion with my dad today about different profiles and sizes of wheels.

 

[MillerLight]

police give a "margin of error" before they ticket you anyway to take into acount such things.

 

[.GAZ.]

how interesting :O

 

[rctempire]

Its not about tickets, is the actual speedo reading from manfacturers how they determine the speed on what type of tyre.

 

[rctempire]

BUMP. Anyone else add info?

 

[Munson]

I'd imagine it's with new OEM tyres at the recommended pressure.

 

[B16DOE]

is there one

 

[.GAZ.]

roy where do u live in germany?

 

[Alastair.]

dont worry about speedos, just drivvvvveeeeeeeeeeeee

 

[Munson]

roy where do u live in germany?

Cologne

 

[rctempire]

Anyone else, its not for whos right and wrong, im genuinely interested in this.

 

[Ex-User (19928)]

I'd imagine it's with new OEM tyres at the recommended pressure.

Anyone else, its not for whos right and wrong, im genuinely interested in this.

As above, Renault aren't likely to calibrate a speedo to be accurate on 22s...

The difference is also less than you might think with different tyre profile.

Take a read on this site http://www.carbibles.com/tyre_bible.html

The bits on profiles is on page 2.

 

[allyrs]

Assuming that the speedo is 100% accurate (which it isnt but ill get to that) there will be a small change in registered speed between brand new and totally worn tyres.

My wheels are 195 45 16.

at 65 mph with new tyres at the correct pressure i will be doing 65mph as it is with new tyres when a speedo is calibrated for.

if i wore the tread down to below leagal limits. say 9.75mm (figure picked because its the difference in height between a 45 profile and a 40 profile tyre- showing what can happen if you run differing profiles) then when the speedo said i was doing 65 again i would infact be traveling at 62.882mph.

so a difference of 2.118mph (slower) by dropping a tyre profile/having excessively worn tyres. the speedo is 3.4% out. At a low speed, up to say 70 its pretty hard for you to see the difference on the speedo of 2mph. im mean even at absolute top speed if the speedo says 140 your actually doing 135.24mph

a speedo is fairly accurate but will usually be set slightly behind the real speed for safety and comeback. speedo accuracy is within a couple of %.

I mean, your never going to wear your tyres down this much the inaccuracy at the speedo and with worn tyres sould never really exceed 3-4% ever!

 

[.GAZ.]

roy where do u live in germany?

Cologne

lol..

i travelled from Fallingbostel nr Hannover to Cologne a few years ago to watch WWE lol..

i won the tickets on forces radio (BFBS) Think it was 2003 tbh!