tyres and rims

Depends if the tyre is a dedicated rear tyre or a universal (front or rear fitting) A dedicated rear tyre usually has a greater tread depth and so gets more miles per tyre. 

A quick look shows up more choice for 4.00x18 tyres than it does for rear fitment 3.50x18's.
For what its worth,  most older bikes upto the mid 1960's usually had the difference in width of a 1/4" front to back, ie front would be a 3.25", rear a 3.5". This gradually increased during the 1970's  as low profile tyres became the norm then and later chassis geometry was able to allow vastly different front - rear sizes.
Generally the wider the tyre, the greater load it can carry, so this might also be a factor for you also it may well last longer too.

Me? If it will fit onto your bike without fouling anything , I'd go with the wider ​​one.

Regards, 
George 
 

John
I have just fitted a 4.00-18 to the rear of my 750cc Commando on a WM3 rim. This is a dedicated rear rather than the universal 110/90-18 it replaces. All Avon Roadriders.

The 4.00-18 has a tread depth of 7.5mm versus the 5.6mm of the universal. It weighs in at 6.8kg versus the 5.1kg of the universal. Width is the same at 116mm for both tyres.

On the front  is a partly worn 90/90-19, also a Roadrider, on a WM2 rim.

Having just ridden about 50 miles with the new rear, there is to my mind a loss of nimbleness in the handling. The bike feels too firmly planted on the road. It's slower to chuck into the bends.
Of course my perception may change as I get used to it and the tyre wears.

In fact I had considered replacing the 110/90-18 with a 100/90-18 but Roadriders in these sizes are out of production until January 2016 which pushed me to buy the (available) 4.00-18.
 
Perhaps your decision should be influenced by your riding preference; chunky and wide for straight line stability, load bearing, longevity versus a slimmer, lighter universal if sharpness of handling matters more.

Robin

John, how did it go with the 4.00 x 18 tyre?

I would go for a dedicated rear tyre on the 18'' rear as it makes sense, just make sure the width is not too wide as the tyre will struggle to get warm. WM3 rim in 19'' is a good choice for those intending to use Roadrider MK2 universal, but again the choice in tyres in 19'' rear are very limited so the 18'' makes sense but I notice that even this size of tyre though plentiful is not as great as it once was.  
The Roadrider MK2 will start manufacture on the 26th April 2026 in Turkey (confirmed by Goodyear global sales manager) not France as the French Dunlop production unit we believe no longer belongs to Goodyear as they sold Dunlop to Sumitomo Tyre in January 2025. 

BMW R100/7 sizes are 3.25x19 front and 4.00x18 rear. Decent range of options and widely available. Should work well on a Commando. My tyres of choice are Metzeler Roadtec 01 X-ply.

(My Commando runs 100/90x18 front and 120/80x18 rear.)

Have these modern tyres got wider size for size to what we where used to in the sixtes ?

Wheel rims have certainly got wider. That 1981 BMW has a 2.15 (aka WM3) front with a 3.25 Roadtec measuring 94mm wide. Rear rim is a 2.75 with a 4.00 Roadtec coming in at 115mm wide.

If you are splashing out on new rims then 2.15x18" or 19" front and 2.50x18" rear are good choices for a Commando.