1975 Commando Mk3 18'' rear wheel

Why are you wanting to go back to a 19" tyre? The OD of a 4.00 x 18 Avon road rider is the same as the 100/90 19" avon road rider, plus you have more tread depth as it is a dedicated rear tyre (the 4.00 x 18 RR has  7.5mm tread depth whilst the 19" tyre has 5.6mm as it is a universal fitting tyre) The 100/90 x 18 RR you have is a universal fitting tyre as well so its tread depth is 5.6mm and its OD is 642mm Where as the 4.00 x 18 RR has a 671mm OD so you would get 1K to 2K more miles per tyre and the speedo will still read correctly.

A no brainer in my opinion

Hello Chris. I have a standard size 19" Mk 3 rear wheel with Akront alloy flanged rim , st st spokes if you are interested in a straight swap, my wheel is in great condition. Sam 

If the right 18" tyre is used the total wheel size will match the 19" so changing tyres to one with the same OD as the 19" would fix the centre stand issue if this is solely the issue. However worn holes in the centre stand, cradle and worn stops can all give the same problem. So before jumping in and wasting good money find out what the real issue is. I fixed my centre stand problem by welding and dressing the stops on the stand and the worn matching part of the cradle. 

I used a 110/90-18 on the rear for a while and there was considerably more daylight underneath when on the mainstand than there is now that I have returned to a 100/90-19. Avon's widths, and therefore the height are a bit on the nominal size. I prefer the skinny-tyre feel. It suits the style of the bike.

Stand wear is indeed the most likely cause of extreme difficulty though. They wear on the feet too and this makes them harder to roll back.

I'm on the Wirral, Chris. Across the river from Liverpool

Worn holes in the center stand will make it easier to get on the stand as the stand is effectively shorter (by the worn amount) conversely the bike will be harder to roll off the stand as the feet pads and stops will be worn so making the stand lean back further. The Mk3's are notorious for both wheels touching the ground when on the center stand. Running the bike when on the center stand is what wears the stand out. Indeed,  if you rev the engine, like when strobing the ignition, the whole machine will move around backwards!     Other stuff that can make it difficult to get on the stand are worn/saggy front and rear suspension. Rear suspension on its softest setting and poor stand technique.   

Fit a 4.00 x 18" Road Rider tyre

Your current tyres outside diameter is too small. The 4.00 x 18" is bigger in diameter so helping reduce the stand effort   

Visit the Avon tyres website, all the current tyres dimensions are listed 

Put the bike on the sidestand and rope/support it so it cannot fall over, then get down on the ground and look at the stops on the centre stand and the marks they make on the cradle. Any wear will be seen on close examination, pulling and pushing on the centre stand will tell you is the any slack in the bolts and bushings.

If both wheels are on the ground when the bike is on the c/stand. the stand and or cradle needs work. 

I make new bushings to suit the fitted stand and cradle. if the holes are very worn (visibly oval) i ream the holes in the stand and make sure the stand bolts clamp the bushings tightly.

You can either weld a pad on the cradle where the stand abuts to or weld the stops on the stand to build them up. Every MK3 i've come across that hasn't had this issue addressed has this problem.

Welding up the stops on the stand is easier as you can take it off to work on.    

The side stand is another problem area and to correct that does mean welding the stop block on the frame. 

Not sure if this helps, but:

I had difficulty lifting my 58 model 50 onto the centre stand.  When on the stand the rear wheel was a good 2" off the ground. I found that by rolling the rear wheel onto a 2" block of wood then lifting onto centre strand made the whole job a lot easier.

The Bridgstone Battlax 45 is a great choice too. They can be used with inner tubes (confirmed by the factory) and have identical diameters. The handling is superb. 

Front: 100/90 - 19 57V

Rear: 120/90 - 18  65V

PS. My bike has never had a center stand since I bought it in 1980. Its never been a problem. I just use a paddock stand if I need to work on the bike or store it.  

The tyres look good too...