When Norton came out with the 750 engine in the Atlas in 1962, they advertised that it could be ridden from 10-100 mph in top gear. I had a 63' Chevrolet that would do the same when I was younger which had an almost identical overall drive ratio of around 4.5:1. Yes I did drag race this car, but on the highway that gearing coupled with it's high volume oil pump let me watch the engine run out of oil at 70 mph because it was not draining back to the sump as fast as it was being pumped to the engine, not to mention the wear and tear the poor thing was seeing.
So suitable gearing on a Norton twin will depend a lot on whether the rider is using it for.
If the bike is a toy for thrills and "bike night" events around your local area then the stock gearing is going to be fine, just as overall gear ratios in the 4.x:1 are suitable for children driving 60s and 70s American muscle cars around "cruising" main-street.
However if you happen to own a practical automobile manufactured in the last 30 years that is equipped with a tachometer, you will notice that on the highway cruising at 70 mph the engine rpm is in the 1500-2500 range and the final drive ratios are high, such as the roughly 2.9:1 in my wife's Toyota Camry which turns a bit over 2K rpm at 70.
This is why back in the 70s and earlier if someone had a family sedan with 100,000 miles on it, it was definitely clapped out and due for an engine overhaul, but when high ratios and overdrive became popular became common to drive automobiles 200,000 miles and more before the engines were worn out.
The stroke of a Norton 650-850 is the same as a 305-350 Cubic Inch small block Chevrolet, or my wife's Toyota Camry, a good stroke for mid-range torque which will suffer longevity problems if constantly buzzed at high-rpms because of high pistons speeds, not to mention the flawed two-main bearing crank and camshaft, oil breathing problems etc..
However, tuned for mid-range and geared up to significantly lower rpm at cruising speed, most all the design shortcomings of the Norton twin are side-stepped. Now your crank and cam are not flexing, your piston speed is down along with breathing problems and oil consumption, and mechanical wear and heat is cut way down because it is an exponential function of speed.
So children driving/riding around any sort of vehicle with with a 4x:1 overall drive ratio, on the throttle and revving the engine to 6K up through the gears constantly are going to see very short engine life, they are going to be taking the engine apart in as little as 30,000 miles, 50,000 if they are lucky enough to have a better design.
The adults though, using moderate throttle and shifting at or a bit under torque peak are going to get much more use out of their engines, especially if like recent automotive practice they have an overall drive ratio that keeps the speed of their engines low for most of the time they are riding. And on main highways traveling at 70mph average, I would say that the largest gearbox sprocket offered for the twins in their day, a 23T for 650 and larger Dominators and a 24T for Commandos is the highway gear that makes sense.
This gearing is not too bad in the city either even with a four-speed box as long as there is an adult in the saddle with no drag-racing plans. The as-built torquey Norton engine will pull away from a stop in traffic like a locomotive with minimal clutch slipping and throttle.
When Norton came out with the 750 engine in the Atlas in 1962, they advertised that it could be ridden from 10-100 mph in top gear. I had a 63' Chevrolet that would do the same when I was younger which had an almost identical overall drive ratio of around 4.5:1. Yes I did drag race this car, but on the highway that gearing coupled with it's high volume oil pump let me watch the engine run out of oil at 70 mph because it was not draining back to the sump as fast as it was being pumped to the engine, not to mention the wear and tear the poor thing was seeing.
