Speedo correction - how?

Agreed, Marks Adapters have a 'match box' speedo corrector. I have one and it works perfectly and no guesswork involved or remembering to add/subtract the difference. Works for all tyre combinations on the vehicle. Check their web site.

You can also buy a kit from Jaycar for about $50 against Marks $150 or so

Thanks Graham,
Their web site says they're out of stock, but I might pop in to see if they are getting more in. Sounds the deal.
Cheers
Gerry

Jarcar kit (later versions) are good to use now Gerry, if you don't mind building electronics things.

These types of things cause about a 1/2 second time delay in reading your speed but the versatility is worth it.

Try one of the shops they may have one when the warehouse doesnt
To see if you actually need one use this It will let you set all sorts of settings and give you the spped difference

http://www.rims-n-tires.com/rt_specs.jsp

Tasrat
Without going into a major explanation about tyre size and
% aspects, the answer is Yes the diameter will have changed..

To explain a bit

If, say, your tyre size is 245.75.16, 245 is tread width, 75 is the aspest ratio (also known as tyre depth), (this is the percentage of the tread width) and 16 is the rim diameter.

So if you upsize to 285.75.16 from 245.75.16, the diameter will be increased (75% of 285 is greater than 75% of 245).

However, if you lower the aspect ratio to 65 or 70 the tyre diameter will not be changed by as much.

Hope this helps

Disco.

Thanks for that disco, I probably should have known that but always thought the 65/70/75 etc was just mm height.But when you think about (which I obviously didn't ) it makes much more sense.
Thankyou for your clarification
If you know anything about airbags as coil replacements I would be happy to hear it.
Cheers Paul