Thats right!And the 17 to 19s rolling circumference is 99.9%, the same.
Its only in 20s ,that you get this slight difference(-1.31%) because 100% equivalent tyres are not commercially available.:)

There can also be a slight size difference between tyre brands, and tyres within that brand as well.

yep checking speedo against GPS'

just come back from a drive to some country markets and noted the same indicated speed differential, on my new 20 inch wheels

the car indicated 100 kmph and the GPS indicates 95

across the board my car speedo is 5kmph faster that actual

ive had the same experience with my two previous cars, mazda 6,s they showed the same error, i was of the opinion it was by design, to effectively give us a greater margin, to allow for speed traps etc.

because the next question would be about warranty and km,s travelled before it expires.

but as best as i can tell even though the indicated speed is incorrect the indicated distance travelled seems accurate

i was told by my local dealer (not much confidence in them) when i put 17's on for winter that they were pretty much exactly the same size as the stock 19" and would make no difference. It seems to me though going 85 with the 17's i get passed every so often but not doing 85 with the 19's. you could do 20's and if you picked the right size tire it could match up very close to your 18.s

The factory sizes, ie:

235/65R17
255/55R18
255/50R19
285/45R19
275/40R20
315/35R20

all provide the same effective rolling circumference. It's as simple as that.

understood completely Fraser but what about the overall inaccuracy do other see the same level of error

Dealer is right:)

But you misunderstood the thread completely.This is what the OP asked
"Does anyone know much about the effects of changing wheel size on the speedo reading ?
"
This has been answered effectively!
Why you going off topic!Its not about GPS versus Speedo!
Frazer also already stated that all cars speedo over read, and yet ,you last post is asking same question again:(

i'm a little late to the game i see, but i wanted to chime in with this tire size calculator as well. it has side and front profiles to show the difference in stock vs new tire that is in question.

Tire size calculator

Use the formula on post #4..its easier and you dont even have to turn your pc on :)