Changing wheels, affect speedo?

Yes. It will read slower. It'll probably be more accurate though, as speedo's are allowed by law to read up to 10% over, but are not allowed to read under. In practice, they do read 5% over or thereabouts, and that is the difference your 17's will make.

Also if you put 225/40/17's on instead of 205/40/17's then your speedo will under-read further.

 

One side effect is a reduction in fuel economy and possibly a worse ride....

 

as lippo said,i changed to 17s with 205/40/17 tyres from 205/45/16s and my speedo is just about spot on now maybe reading 1mph slow at most.

ride will be harsher with the 40s not that ive noticed as its rock hard anyway!, not sure about fuel consumption being different though as the rolling circumference is about the same what with the bigger wheels but thinner tyres.

 

Running on 215/35 18s my speedo is 3mph ut at 60mph. Ride isnt 'too' much worse. MPG is actually better on the motorway but about 4mpg down around the doors. You pays your money...........

 

This is a useful calculation website, definitely one for the bookmarks:

http://www.kouki.co.uk/utilities/visual-tyre-size-calculator

 

Haha the alloys used on this site resemble those of the Leon Starfish!

 

I think the mpg gets worse due to the combined weight of the new, larger wheel and the tyre. This unsprung weight can have an adverse effect. I lost around 5mpg when I put VRS alloys on. I did use cheapo tyres so they may have caused extra drag as well.

The 17" wheel/tyre combo felt much heavier than the 16"s. If you use 215 then they have aarger surface area that creates drag.

 

They aren't This is my Brothers website and Tyre calculator - He based the Alloys on Momo Arrows that he had on his S13 years ago.

The calculator is pretty handy though!

 

Good website and calc. I used it when getting my 17"s

 

Credit to your brother, it's the slickest calculator of it's type I've seen. Some are horribly confusing or require an enormous amount of data.

 

Yeah, it's been up for ages, feel free to click a couple of the ad links It pays for his hosting!

He's been coding an Offset calculator (ET) for a while

He was also thinking about a stretch calculator too... but wasn't sure as he doesn't want to be responsible for tyres pinging off cars and people killing themselves after putting 165's on 9J wheels

 

Haha, a disclaimer on the website front page will cover any legal troubles. Moral ones, however, are a little harder to ignore!