My rear tube is 120/90-17 I've looked up the inch equivalent ... 4.50/4.75 (http://dr650.zenseeker.net/Tires.htm) Motorbike Shop did not have it, but said 4.50/5.10 will work fine Will it ? Are the inch tubes always 100% aspect ratio so 4.50/5.10 is a range for the section width ? Or is it similar to the metric where one is the Section Width and the other is the aspect ratio ? ie 4.5 is section width and 5.10 is height ? Apologies if this is a really dumb question. Been googling and trying to work out the answer, but not getting anywhere.