You can do without buying new rims.
Carlisle makes a few different tread patterns in the 4.80/4.00-8 (16.6 x 5.0 x 3.75) that will fit. When you buy new tires, It is a good idea to get the tubes also and change them.

absoluteblock, check out these guys for your tires and tubes. I have had excellent experiences with their service.

http://www.jackssmallengines.com/tiresearch.cfm

Thanks for the advice.

Apparently I wasn't thinking straight last night...Why can't I just add tubes and use the existing tires? Despite being 27 years old, they've only had maybe 100-150 hours of use. They won't retain air pressure because the sidewalls are cracked but the tread is still excellent.

I suppose I could use rubber goo of some sort to fill in the cracks, can't I? Obviously this would be to preserve the tire a while longer, not to patch the holes so it will retain air pressure.

Also, this wheel/tire combination that Troybilt sold as one piece...is there anything I should watch out for when removing the tire from the rim? I wasn't sure if this was a different design (hence the one piece notation) or merely a money making technique by Troybilt.

I dislike changeing tires on small dia. wheels especialy if they haven't been changed in 10 years. They have a thick rubber tube you could try, Maybe try and remove one of the tires and if it's not difficult and the inside wheel is in good cond. try that type tube. I plan to one day fill a tire with expanding foam,and see how that works. but i have no idea that it will.

I'd replace the tires since they's nearly 30 years old and rotten.

The other issue with old tires is the loss of flexibility which translates to a loss of traction. I had this happen on my GT-18 after 22 years and reluctantly replace the tires. Wow, the grip is back! I also like to use a local tire shop who can not only change these ornery buggers but steer ya toward a good tread pattern. Besides, you build some bridges which can be useful when you really need a quick favor at the shop (the favor being, 'quick').

Just put tubes in the existing tires, then not worry about it for another 20 years.
Getting the rims off of the shafts is usually the main
problem, so I just change the tires or install tubes while
the wheel is on the tiller.
Fish

My tiller tires seem to tall from the looks of the pics of others. 480x8 is on it now and the engine can barely pull thru in low at full throttle. Last week I bought 400x8 tractor tires and they are taller than the ones on it. I know the wheels were changed before I bought it and was wondering if maybe mine came with 6 inch wheels. This is tiller number 80738 does anyone know if this is right?