Probably because Avon haven't made Road
runners for the best part of twenty years ... You should be looking for Road
riders ...
You are being misled.
"100", "3.25", "110", "4.00" are 'section widths', the "/90" after two of them indicates the 'section height' is 90% of the 'section width', that 3.25 and 4.00 don't have a 'slash number' indicates their section height is the same as their section width.
I couldn't tell you what a 'section width' is but I do know they are not overall widths and, in tyre sections alternative universe, something like a 4.10 TT100 is
narrower overall than a 4.00 K70 ...
Also, even though they might have the same numbers moulded on the sidewalls, 'new' tyre designs like Roadriders are wider overall than older tyre designs like the K70 and TT100; for example:-
. A 3.25 Roadrider is wider overall than the 3.25 K70 Triumph fitted to your bike originally, the 3.25 Roadrider is about the same overall width as a 4.10 TT100; a modern-design 100/90 is wider again. Conical-hub fork legs' centres are only 6-3/4" apart:-
.. a 3.25 K70 will fit between them;
.. a 4.10 TT100 or 3.25 Roadrider will
probably fit between them (not tried it myself so I'd be inclined to pick a tyre supplier that won't mind changing it if it doesn't fit);
.. a modern-design (e.g. Roadrider) 100/90
might fit, but you might have to resort to fitting the wheel with the tyre deflated, inflating it when fitted; then, given the greater (in tyre terms) overall width compared to the original 3.25 K70, you might not be happy with the steering 'feel' on a light bike like yours ...
. At the rear:-
.. A 110/90 Roadrider is only a little wider overall than a 4.00 K70, and a 4.00 Roadrider is only a little wider again. Fwiw, ime the widest part of the tyre rubs on the chainguard and leaves an arc-shaped mark, I use that to cut a triangle shape out of the back of the chainguard with tin-snips, the tyre then clears the chain itself; however, check carefully as YMMV.
.. Be aware Avon has had some unspecified issue with specifically the new 110/90x18 Roadrider Mk.2, you might find it not available and, if you go for a modern design by another maker, for given section widths, Avon's overall widths are comparatively narrow compared to some other makers'.
Continental tubes seem to have the best rep., second are Michelin, either are better than no-name or Chinese cheapie.
When fitted and inflated, do
not screw a nut down the valve to the rim. If the tyre 'creeps' on the rim, as it pulls the tube with it, you see the valve leaning in one direction or the other,
before it rips out of the tyre,
Not mentioned so far are rear tyre security bolts (= US "rim locks"?). Your bike's rear wheel was originally fitted with two; if still present, the tyre bead fits between them and the rim, when the nuts on their central studs are tightened, they clamp the tyre to the rim. Popular fiction has it they were to prevent 'tyre creep', Triumph engines never generated enough power unless the rear tyre pressure was allowed to fall very low. However, in the event of a rear wheel puncture, ime they are your new best friends ... it's still one of the best natural laxatives (surpassed only by a front wheel puncture ...) but the tyre won't come off the rim.
If the security bolts are still present on your bike, fit them between the rim and the new rim tape; when fitting the new tyre (or subsequently removing it), only loosen the bolts' nuts to the ends of their studs, the bolts can be manoeuvred when fitting or removing a tyre bead but will otherwise stay in place.
