Tyre / Speedo Query ...going Up In Size

[Stagea_Neo]

I just changed from 205/55R16 Stag stock wheels to R34 GTR 245/40R18 wheels and my speedo got more accurate.

I have a 98 s2 auto 4wd Stag.

With the 16's if the speedo showed 60 i was actually doing 57.

With the 18's if the speedo shows 60 im doing 58.5.

I've verified this against those smiley face trailers that show speed they place on side streets and my gps.

Get the same readings every time.

Havent tested 100 yet, might go down to the monash now and see.

This is normal yeah?

Ive heard others going up in size and speedo got less accurate.

(dont recall sizes quoted)

p.s. i thought the 16's were good around corners.....these 18's are waaaay better

[Daleo]

Seems to be pretty common, most cars over read to a greater or lesser extent, going up in rolling diameter, generally minimises the error. Mine used to over read by 10%, down to 3% with bigger rolling diameter.

[Stagea_Neo]

hmm my nokia gps doesn't like the bad weather, kept cutting in/out around 76kmh.

wait till its clear skies.

[GoldZilla]

I just changed from 205/55R16 Stag stock wheels to R34 GTR 245/40R18 wheels and my speedo got more accurate.

I have a 98 s2 auto 4wd Stag.

With the 16's if the speedo showed 60 i was actually doing 57.

With the 18's if the speedo shows 60 im doing 58.5.

I've verified this against those smiley face trailers that show speed they place on side streets and my gps.

Get the same readings every time.

Havent tested 100 yet, might go down to the monash now and see.

This is normal yeah?

Ive heard others going up in size and speedo got less accurate.

(dont recall sizes quoted)

p.s. i thought the 16's were good around corners.....these 18's are waaaay better

I've got the same size tyres as yours, Pete, but on 18x8 wheels (as opposed to the 18x9's on the R34 GTR). My speedo is almost dead accurate.

[iamhe77]

Rims 'n' Tires will tell you everything you need to know Pete.

[lachlanw]

lol at title

more accurate is not the word

[Stagea_Neo]

did my little test on the Monash.

Speedo 60kmh - GPS 58.5kmh

Speedo 80kmh - GPS 76.5kmh

Speedo 90kmh - GPS 86.5kmh

Speedo 100kmh - GPS 95.5 - 95.8kmh (had to keep an eye on traffic:) )