I do not recommend liquid fert in a ride-on, mixture too hot. Not much agitation. So you are thinking about adding 17 pounds of urea to your 8 gallon tank? Really?
I am thinking...too concentrated, too hot...not enough volume to prevent burning the grass.
True, I have used urea solution from my large tank to fill my Permagreen at times when I ran out of water, but it only contained about 5 pounds of urea per tank. And about 14 percent of the proper herbicide concentration--so herbicide had to be added.