If your wheels both have security bolts then 15 would be OK on the road and IMO a tad high for trailing. However, for mostly road work I would run mine nearer 20 front and 16 rear. Be aware, knobbly tyres are not best suited to lots of tarmac work and will square off in short time.
On the trail I run mine about 10 back and front to give just that bit better grip in the gloopy stuff. I am also very careful not to "snake bite" the inner tube which happens when quickly attacking "steps" in the trail. I also avoid big, sharp rocks like the plague.
If I was using my TTR mainly on the road then I would prefer to use supermoto tyres like Avon Distanzia.
