I've ben given wrong info then. I wasn't aware you could get different profiles for the GS (assuming OEM wheels)
It's about
tyre profiles. A pure road tyre, say a Michelin Roadsmart, will have a slightly different profile to say a Conti trail attack.
When we talk about profiles, we're talking how pointy or how flat the tyre contact patch is on the road. By and large, sports road tyres are quite 'pointy', that way they'll give you a nice fat footprint when you're cranked over. The down side to a very pointy sports based tyre say, is that in a straight line, you're now riding on quite a narrow V (unlike a U) so it can feel a tad flighty. The benefit is that it will fall into a turn easily and then, when on its side, as we said, have a big foot print.
A Pair of tyres will share the same profile, so front and rear will be V V or U U, but if you put different makes on front and rear with different profiles, you may well be riding V U so the front is trying to fall and the back is doing something entirely different, or what ever.
I agree with the others that a pair of Tourances, albeit one standard and one EXP, will almost bound to have exactly the same profile, as would say a Conti RA2 and the original version. (Pilot ... Pilot 2 etc etc..)
I also agree with Sgt Bilco that we're not talking cutting edge race technology, so even if you did mix makes of tyres, it's not exactly going to throw you off your bike, but, if we're going to be picky, sharing similar
profiles is probably best practise. This is why when you replace your knackered, squared off rear, but keep the same slightly worn front, it suddenly handles again
Mixing different compounds is another topic that comes up from time to time. Food for thought would be having an ultra grippy, confidence inspiring tyre on say the front (Maybe it's raining and you've got a Roadsmart or a Pilot road 3..) and something made of Bakelite on the back. Not ideal ....
