best gearing

[bonny]

What is the best primary drive gearing for the a10 for modern roads. The bike has an ariel rear wheel, so there is only one size sprocket afaik, the clutch chainwheel is likewise fixed in the number of teeth it has, so that leaves only the engine sprocket and gearbox sprocket that can be changed.

Traffic is faster now then when these bikes were new, but i don't want the bike revving its head off to keep up with the traffic , or too intractable when travelling in congested areas.     

[Greybeard]

I had a 19 tooth gearbox sprocket on my Plunger; presumably from when it had a sidecar. The engine always felt like it needed a fifth gear. I've recently changed to a 20 tooth gearbox sprocket. That change has made the machine much more enjoyable in traffic.

[Rocket Racer]

You don't mention what state of tune the bike is in, what sort of load you expect the bike to carry (passengers/luggage) or the type of roads you frequent (motorways or goat tracks).
The A10's were built before the advent of the motorway so standard gearing is fine for typical A or B roads. Lets face it motorways are not nice places to ride a fifty year old bike.
I usually start with standard gearing for the model in question as this was intended to provide the appropriate compromise for town and hill use.

Overgearing is a common mistake, it actually makes the bikes slower (acceleration and top speed) and you'll spend more time in third which is hard on the gearbox.
A healthy A10 will have plenty of oil circulating so won't mind a few revs on. In fact better than slogging along at no revs with lower oil pressure and more load on the big ends.

For the BSA group triples, the Americans typically stuck massive rear sprockets on as they figured who needed 130mph when acceleration to the ton was more important.