[Bill J]

I have a 2019 ATAS. I have been looking everywhere to find the correct size of the rear tire inner tube. There is nothing listed in the Honda Service manual. I have never seen a 150/70 18 tube. I have seen people say to use a 140/110 18 tube which is fairly close in tire size and seemingly the only thing available that is close in size. Also I have seen a 4.00/4.50 used on the rear which is kind of small and would match the rim but not the tire. The rear rim is 4.00 x 18 but the tire ,150mm, is 5.90 or closer to 6. I know that tubes can stretch but the tube gets thinner doing it and you don't want too large of a tube or it could fold and then you get a flat anyway.

Front: Tire 90/90 21 Tube 2.75/3.00 - 21 (This size was with a Honda parts number)
Rear: Tire 150/70 18 Tube ??

Also do put in new tubes with new tires every time or only replace them if needed? Do you put any talcum powder in for friction negation between the Tire and Tube? I am only used to dirt bikes and am much less finicky with the tubes in that application where I only replace as needed and use talcum power, but tubes are readily available in those sizes if needed, cheap, and I am not doing 80 MPH down the highway with them, well rarely anyway.

I have been toying with doing a 3M tape tubeless conversion on the rear wheel and leaving the front tubed just because of this rear tube size issue. Maybe I am overthinking it. I just want to have everything on hand when I change out these tires and not have a week down waiting for a tube. I guess I could just order the OEM tube from Honda and see if the thing is marked in size but another $40 just for a tube when tires are already going to cost over $300 seems a little ridiculous especially with the mileage I am racking up.

I checked the original tube for the rear tire on my 2019 ATAS. The markings on the tube are below.
Dunlop M/C11J85521
140/10R18
150/70R18 SNI

I kept the original tube as a potential spare if needed. It still looks good and I would be comfortable using it in a bind. I just prefer to replace the tube when I replace the tire. Rim bands are not very expensive and can be handy to have on hand if the one in use is questionable. The tube is made by Dunlop. Unless it is produced and sold only to Honda, there should be a reference to it in someone's parts list.