Trying to work out if one of the sizes makes the bike higher off the ground, appreciate the chart.
Found this somewhere...
Up until the start of the 1971 season, BSA (and Triumph) fitted US-market models with a 4.00x18 rear tyre (with a 3.25x19 front) and models for all other markets (including GB and Spain) with a 3.50x18 rear (with either a 3.00x19 or 3.25x18 front).
From the start of the 1971 season, BSA (and Triumph) fitted all models (except triples) with 4.00x18 rear and 3.25x19 front tyres. Also, from 1968, 3.50-width tyres were superseded by 4.10 (very similar overall diameter but greater overall width) so tyre makers gradually stopped making 3.50x18, 3.50x19, 3.25x18 and 3.00x19, but carried on making 3.25x19 and 4.00x18.
So, depends what tyre size your bike has on the front - 3.25x19 front would be better with 4.00x18 rear; 90/90x19 front would be better with 100/90x18 rear (modern equivalent of 4.10x18).