Originally Posted by stylinexpat
You can get them with 18" or 19" from the factory and an inch must make some sort of difference. Between size and weight I can't see them as pretty close if the weight savings were 5-7 lbs. per corner. If one went to a lightweight forged 18" wheel and saved 5-7 lbs. per corner I would be surprised to see no performance gains..
I think it is important to separate the benefits of:
1.) reducing wheel weight,
2.) tyres, and
2.) changing wheel size
There are wheel/tyre combinations with 18's that are lighter than 19's and the same can be found the other way around. It is a generally accepted fact that lighter is better for wheel/tyre weights so comparing a heavy set of 18's against a light set of 19's (or any way around) may give misleading results. Depending on the test, the weight benefits may outweigh anything you gain from side wall/construction or any difference in tyre quality/suitability.
Again, I don't know how the mentioned magazine test was done (not citation) but it really should be done with the same type of tyre. It would be silly to test an Advan AD07 on 18's against, say, RE-01R on 19's and look at the difference in lap times.
I feel that for the sake of the topic of this thread and to answer the OP, things should be kept strictly to having the same wheel/tyre weight and the same type of tyres and just considering the difference between the two sizes.
To add to what I posted earlier, I think that it is cheaper to find a light 18"+tyre combination than an equally light combo with 19's. This is perhaps a reason why people who track their cars tend to use 18's. Also there is, or perhaps was cos I've not look at it for a while, a problem finding R-compound tyres in 19's or larger. So the 'advantage' here may be one of cost.
Another side issue is on of brakes. In truth this is not strictly on-topic, but anyway... A bigger wheel will allow for larger rotors. This has benefits (better cooling) but also drawbacks (larger mass + inertia). We'll be even more confused if we put this in the mix...