Toro Timecutter SS4250

Recently purchased the Timecutter SS4250 down here in Australia. Its got the 24 HP 708CC Toro engine with the ZT2100 drive system.
It came with turf tires 18x7.5 -8 on a 5.5 rim.I have around 2 acres of some flat some sloping grass (not seeded lawn) in the country.
I am having trouble with it sliding and want to put some lumpier tires on it. I have been looking at some bar tread types and am not concerned with
damaging the natural grassland. I am concerned however these may damage the drive system,and also the only size tire I can get here is 18x8.5.
Would these go on a 5.5 rim. and what thoughts would you folk have on the effects to the hydro's.

Won't hurt the hydros so long as you are not 300Kg and towing a heavy trailer. :laughing:

Fitting the slightly narrower tyres should not be a problem but you might need to keep them harder to stop them rolling off the rims.
You can also get weights for the front which can help side slip a little.

The optional wider tire you stated 18x8.5 is only 1 inch wider, they will fit fine. Inflated they might be a tad taller in the middle.