what difference will this size of wheels affect spedeo

[rugbymad]

http://cgi.ebay.co.uk/18-ALLOY-WHEELS-VOLKSWAGEN-GOLF-MK5-225-40-18-TYRES_W0QQitemZ350119098166QQcmdZViewItem?hash=item350119098166&_trksid=p3286.c0.m14&_trkparms=66%3A4%7C65%3A1%7C39%3A1%7C240%3A1318
HI folks

i'm intresten in these wheels for a golf gt sport 07, these are 1 inch bigger than my standard wheels, will this make a difference to my spedo reading and mileage.  i.e the wheel size.i am away from home at the moment and unable to check my standard size of tyre i know they are 17 wheels.

YOU ARE BUYING A BRAND NEW SET OF 18" CRUIZE 258 ALLOY WHEEL AND 225/40/18 PERFORMANCE EXTRA LOAD TYRES FULLY FITTED AND BALANCED

the size of tyres given will be
thanks
i can upgrade to 235/40/18 as an option

[bacillus]

Using the 225/40/18 tyres will keep your speedo readings unaltered from at present.

The 235/40/18 tyres will make your speedo under read.

[markymark]

I think your 17's will be 225/45/17, my wheels are 18's and the tyres are 40 profile so they wont make a difference to your speedo as the rolling radius will remain unchanged!

[Greenouse]

As 18's are an option on both the GT Sport and the GTI I cannot forsee a problem. I have recently upgraded to 19's and the GPS readout on my Tom Tom is the same as it was on the 18's!   HTH

[markymark]

Using the 225/40/18 tyres will keep your speedo readings unaltered from at present.

The 235/40/18 tyres will make your speedo under read.

But the 235 only relates to the tyres width, I cant see how this will change speedo readings.

[mortygt140]

Off the top of my head at about 70mph the gps reads 63mph, if Im going through a 40mph gatso gps reads 35mph.

Im on 225/40/18 dont go for the 235 tyres cos you'll just get more tram lining than you need, and also if the offset is lower than std then you may get some rubbing on the back arches?

Darren

[JC]

http://www.tyresave.co.uk/tyresize.html

stick them in there, will tell you expected % you will be out

[mortygt140]

Sorry should have said im running a GT diesel so std rims would have been 16",  by the way is there anyway of changing this with my vagcom?

Darren

[Hurdy]

There will be no noticeable difference between using the standard 16" wheels and tyres you have now, to using the 225/40/18's in the advert. If you want to go to 235/40/18 then there will be a small difference in rolling circumference, but nothing to worry about. I thought about this too, for the slightly extra grip you would get. I don't think you would get any rubbing issues either as long as you went with the correct offset.

225/40 will give you a sidewall that is 40% of 225mm width = 90mm

235/35 will give you a sidewall that is 40% of 235mm width = 94mm

so the difference in sidewall is 4mm = 8mm smaller diameter with 235/40 section tyres.

Rolling circumference is 2002mm stock and 2010mm with 235/ section tyres which means an error of less than 1%

[bacillus]

But the 235 only relates to the tyres width, I cant see how this will change speedo readings.

The profile of the tyre (in this case 40) is a ratio of the tyre width and it's height.
The 235/40/18 tyre height will be greater will be greater than the 225/40/18 one. This in turn will mean that rolling circumference is greater hence the speedo will under read.