Help needed re non standard rims & tyres

hi all,
I have a 2010 Thruxton which I bought pre owned, and it has Morad rims both 17" but 5" rear and 3.5" front, shod with 150/60/17 and 120/70/17 Michelin pilot power tyres.

I need to replace the tyres and I just realised I have non standard rims and that the diameter of both wheels is probably 2" less than standard.

Handling seems ok but a bit twitchy, but the bike also has Norman Hyde ace bars which are very sharp drop compared to standard bars so I wonder if I can improve handling/cornering with taller tyres?

IS the combo I have a typical mod? I see the point of fatter tyres for stability and grip but has it compromised the handling too much? Is the smaller diameter a big issue?

Any help much appreciated.
Steve

 

Thanks crispey, I've also found a tyre size comparison website (link below) which tells me my current fitments are smaller diameter by about 1" on the rear and 1.5" on the front compared to standard. Also that means that whereas the standard bike has a slightly taller front wheel than the rear my situation is the reverse, which must affect the handling?

I'm wondering if a previous owner dropped the sizes to accommodate short legs?

My bike steering is definitely not as sweet as it should be and just doesn't seem to corner smoothly - it feels as if you cornering requires differing steering input as you turn, a bit on/off if that makes sense, having to adjust as you go round?

I think having read the advice on that other thread I'd be tempted to go back to standard tyres but I have a 17" front rim now so that's not possible without changing back to an 18".

I suppose the alternative is to try taller tyres to compensate?

url=http://www.tacomaworld.com

 

Photo of bike for reference, note ace bars - do they require a smaller front wheel than standard for any reason?

 

yes I will try to do that as it will make more sense if the objective was to lower the bike..