wheel tyre change effect speedo?

I have read that a few folks have changed there wheels and or tyres for smaller/larger.
can anyone tell me what if any effect this has on the speedo,
with the outside circumference changed how can it still read true?

The speedo reads slightly faster than actual anyway for "safety", so with standard tyres when the speedo is reading 30 you're actually doing something like 27.

My tyres are 215/70/16 and my speedo is now only a little bit above what my GPS shows.

Not sure but isnt it to the gearbox anyway and thus the gearbox still turns at the same speed? It was on some of my previous motors..

WLJayne said:

The speedo reads slightly faster than actual anyway for "safety", so with standard tyres when the speedo is reading 30 you're actually doing something like 27.

My tyres are 215/70/16 and my speedo is now only a little bit above what my GPS shows.

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:behindsofa:

I have a '02, TD4 with factory fitted 16" alloys and 215/65-16 tyres.
Like the post above my speedo reads approx 5 mph more than the speed indicated on my GPS....e.g. 70mph on the speedo is shown as 65mph on the GPS.
Thing is-which is correct ?????

My 225/75's have a 7% difference to standard, and my speedo now reads bang on to gps

mine reads 4 mph faster

myotherworld said:

I have read that a few folks have changed there wheels and or tyres for smaller/larger.
can anyone tell me what if any effect this has on the speedo,
with the outside circumference changed how can it still read true?

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it will change speedo reading by how much depends on difference from lrs nominal size

jamesmartin said:

it will change speedo reading by how much depends on difference from lrs nominal size

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Does it go to the axle/diff on the freebie then ratber anan the gear box?

thequeenscheese said:

Does it go to the axle/diff on the freebie then ratber anan the gear box?

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Speed is sensed by the ABS system.

Nodge68 said:

Speed is sensed by the ABS system.

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My old mk1 Lseries will be a speedo cable wont it? As far as i know i dont have abs (it deffo doesnt work lol)

thequeenscheese said:

My old mk1 Lseries will be a speedo cable wont it? As far as i know i dont have abs (it deffo doesnt work lol)

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Ah
No ABS means speed is sensed by a transducer in the gearbox.

My speedo reads 34mph, the Scangauge 2 reads 30mph and the GPS about 29mph.....take your pick.

I'm running slightly smaller tyres-215/60 R16's.

The analogue speedo is quite a bit out compared to the digital readout on the Scangauge 2. Surprising or not, as I would expect it to use the same input......

Nodge68 said:

Ah
No ABS means speed is sensed by a transducer in the gearbox.

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So im right in thinking that wheels n tyres wontaffect the speedo then?

thequeenscheese said:

So im right in thinking that wheels n tyres wontaffect the speedo then?

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no ,they will have effect ,one turn of drive shaft will carry you further or less depending on outside circumference of tyre

What he said.

changing from 215/65/16 to 205/80/16 now makes my speed read correct with gps

This site will work out your tyre differences....

Tyre Size Calculator

Yr right doh i was being thick of course it does..

dave21478 said:

This site will work out your tyre differences....

Tyre Size Calculator

Click to expand...

thanks Dave