New rims throwing out my speedo?

[MEANVY]

Afternoon guys,

Well, with my tax back i finally get my sexy new rims. i havn't decided yet but i'm thinking about either 18 or 19 inch SS or GTS rims, to keep it all holden. My current ones are 16 or so i think, whatever stockies were with the VY
Taking on board what people have been saying, the bigger the wheels, the less rotations they do and therefore this impacts on the speedo readings, correct?
So my question is will going from 16s to 18/19s effect my car badly? and if so how do i get this sorted out? with a programmer or something?
Judging by other peoples experiences with rims, it throws it out maybe 5ks or so? I'm just being a worry wart....
any help will be appreciated!!

Cheers guys & Gals

[wingwang99]

If you don't go above 19s you should be able to fit them with lower profile tyres which won't change the overall diameter of the wheel, will not affect speedo that way...

[1991_Vn2nV]

As long as you use legal tyres in terms of overall diameter (within 15mm of standard) you shouldn't see the speedo out by any more than 2km/h when travelling at 100km/h. Most of the time less.

Bigger wheels means smaller profile tyres which keeps the same overall rolling diameter so no big change in rotation.

[Philthy]

STOCK
15” 205/65/15
Diameter: 647.5mm
Circumference: 2034.2mm
Revs/Km: 492.0
When your Speedo reads 100Km/h, you are actually travelling at 100Km/h
Stock ride height

REPLACEMENT WHEELS
16” 225/55/16
Diameter: 653.9mm
Circumference: 2054.3mm
Revs/Km: 487.0
When your Speedo reads 100Km/h, you are actually travelling at 101Km/h
3.2mm higher then stock ride height

17” 235/45/17
Diameter: 643.3mm
Circumference: 2021.0mm
Revs/Km: 495.0
When your Speedo reads 100Km/h, you are actually travelling at 99.4Km/h
2.1mm lower then stock ride height

18” 235/40/18
Diameter: 645.2mm
Circumference: 2027.0mm
Revs/Km: 493.0
When your Speedo reads 100Km/h, you are actually travelling at 99.6Km/h
1.15mm lower then stock ride height

19” 235/35/19
Diameter: 647.1mm
Circumference: 2032.9mm
Revs/Km: 492.0
When your Speedo reads 100Km/h, you are actually travelling at 99.9Km/h
0.2mm lower then stock ride height

20” 225/35/20
Diameter: 665.5mm
Circumference: 2090.7mm
Revs/Km: 478.0
When your Speedo reads 100Km/h, you are actually travelling at 102.8Km/h
9mm higher then stock ride height

Stolen from another forum so I won't take the credit for it. These are based on stock wheels being 205/65/15, but compare the 16" to 18" & 19" and you will have a rough guide.

EDIT: Standard VY exec were 15" weren't they?

[1991_Vn2nV]

^ Why is the 20" wheel a 35 profile? 35 profile is legally suitable for a 19" rim, not a 20".

[ephect]

Tyre Size Calculator -> Tire size calculator

drop your standard size in (whats on ur car) and then drop on the sizes your thinking about, and it will tell you how much ur out, awesome little utility, bookmark!

^ Why is the 20" wheel a 35 profile? 35 profile is legally suitable for a 19" rim, not a 20".

i dont know exactly, but I think its becoz the 20s are 225/35 and the 19s are 235/35

[1991_Vn2nV]

i dont know exactly, but I think its becoz the 20s are 225/35 and the 19s are 235/35

That difference in width accounts for a 3.5mm reduction in the size of the tyre wall. Not nearly enough to cover the extra inch of the rim size.

Its an illegal wheel and tyre combination. 225/30 R20 is the appropriate size.

[LowVp]

^ Why is the 20" wheel a 35 profile? 35 profile is legally suitable for a 19" rim, not a 20".

because the tyre profile is a percentage of the tread width, notice that the 20" is on 225 where as the 19 is on 235

[MEANVY]

EDIT: Standard VY exec were 15" weren't they?

Crap, i've never even bothered to check before. Yeah they are! is there any problem going up to 18s or 19s from 15s? or as long as i make sure the wheels are sorted i wont have a problem?
I remember someone saying you can only legally go up two sizes from your originals, ie i could only go to 17s on the commodore, is this true?

[hsvpgts]

once you've got rims and tyres on pay a visit to holden dealer and ask them to reprogram the PPK's in the instrument cluster to rectify the speedo. they should have a list of different tyre sizes and correct PPK's to suit to get speedo right.
All cars from VT on with bigger wheels should have this done.