Speedo issue

[sleah]

Since having my van I've noticed the speedo is reading low.
So if it says 30mph, my gps speedo shows more like 33/34mph, which is worrying as it should be the other way round! Roadside speed signs agree with my gps speedo so the van is definately wrong..

I was told when I bought the van that the clocks had been changed (the mileage reading is wrong - but noted on the MOT ) however they seem to be correct for the van (2005)
They are the type with the little LCD displays showing time & mileage.

The only thought I've had, is that my van has 16" steel wheels, what if the speedo came from a van with 15" wheels. Would that make that much difference?

Thoughts??

Should I find 'yet another' second hand speedo?

[Mad Husky]

or someone has had the clocks out to repair the solder joint on the back and put the needles in the wrong place.

[knobby1]

or someone has had the clocks out to repair the solder joint on the back and put the needles in the wrong place.

+1...or changed the diff ratio without telling the PCM..!!

Lord Knobrot

[Mashee]

Yes, changing from 15" to 16" will make a huge difference. The math is high-school-level and easy to confirm.

Circumference = pie * diameter (or 2pie*radius)
15" circumference = 47.12"
16" circumference = 50.26"

16" is 7% MORE or FASTER.

Keep in mind this is the RIM ONLY, not the actual circumference of the MUCH bigger tyre which is fitted on said rim, and this makes the ~10% difference you're seeing in your speed vs GPS.

Don't waste your time imagining something so much more complicated like a different diff ratio given the probability of a wheel swap vs diff swap. Doesn't mean it's not an option though...just far less likely...

[sleah]

Yes, changing from 15" to 16" will make a huge difference. The math is high-school-level and easy to confirm.

Circumference = pie * diameter (or 2pie*radius)
15" circumference = 47.12"
16" circumference = 50.26"

16" is 7% MORE or FASTER.

Keep in mind this is the RIM ONLY, not the actual circumference of the MUCH bigger tyre which is fitted on said rim, and this makes the ~10% difference you're seeing in your speed vs GPS.

Don't waste your time imagining something so much more complicated like a different diff ratio given the probability of a wheel swap vs diff swap. Doesn't mean it's not an option though...just far less likely...

Thanks Mashee. Rim size was my first thought. I've no reason to think anything else has been changed apart from the clocks which the seller told me about.
Apart from swapping to 15" rims what's the fix?

[knobby1]

Apart from swapping to 15" rims what's the fix?

Tell the PCM you've got bigger wheels/tyres...there's options in IDS & Forscan....although I'm not too sure with Mk6's.

Lord Knobrot

[sleah]

Apart from swapping to 15" rims what's the fix?

Tell the PCM you've got bigger wheels/tyres...there's options in IDS & Forscan....although I'm not too sure with Mk6's.

Lord Knobrot

I've got the full-fat version of Forscan and sadly there is nowhere to enter anything to do with tyre size that I can see

Any other ideas? Could a main dealer sort the problem?

[twin--turbo]

I would ignore the wheel size, and find the original tyre size and check the rolling radius against the fitted tyre.

[sleah]

I would ignore the wheel size, and find the original tyre size and check the rolling radius against the fitted tyre.

The tyre/rim size fitted is exactly the same as printed on the drivers' side door column plate.

So is it not possible to correct the issue short of putting smaller wheels on or finding a secondhand speedo from an identical van?