thanks PJ133
Don't worry Gaz, very common mistake!
Hence why most people do a double-take when you tell them that completely difference "profile" tyres can be fitted to the same rims and have an almost identical rolling radius (although NOT close enough to run different spec across the same vehicle, especially if it's a quattro!!!!!)*.
235/45/17 (-0.29%), 205/50/17 (-1.3%)
* although personally, if I had a FWD, I'd be tempted with putting the narrower ones on rear, which is how the new Polo GTI is apparently setup... likewise, I always used to run nice deep, new tyres on my old Golf GTI's fronts and borderline legal ones on the rear... as the fronts got "dubious", I'd shuffle them back and put a new set on the front
Back to the tyres: Because of the price difference, lots of people end up with the wrong rolling radius (and real-world lower fuel economy) because they go for narrow tyres (either in an attempt to keep tyre costs down or to improve fuel economy)... what actually happens, which only exacerbates the issue is that the INDICATED economy goes UP, even if you compare fuel usage versus the odometer readings, this will also show an improvement.... but the actual amount of fuel used on any given A to B journey will actually be higher!
The reason being that the very slightly smaller (narrower) wheels are actually recording more miles for that same A to B journey!
But the car is actually geared very slightly lower, so certainly for a long motorway haul, your actual economy will be slightly worse!
And on the other economising aspect: narrow tyres = more chance of wheelspin/skid/drift, which although more fun, will mean you wear tyres out a lot faster!
Personally, I go for the other extreme and save my summer tyres from going brittle and wearing prematurely* by never using them when the ambient temp is <4deg C (I normally change as soon as it's consistently below 10deg C since winter compound is better than summer below 7deg C). Conversely: Winter tyres are not to be used over 20deg C.... those tyres are like sticky warmed up track tyres when they're sat on your drive at -10C, so they get REALLY soft when the weather warms up!!
*Summer tyres apparently wear 40% faster in winter than they do in summer due to being so much more brittle and due to the increased risk of any wheel spin / sliding / skidding - although I think that stat is North American, so probably not as extreme in the UK!
edit: I always find is slightly funny that I post those using the "Post Quick Reply" button
