ninety

I would personally only say "fitty" when joking. (An', Trav, you know you still owe me fitty dollah.) ;)

<<thirty: ["TR=i:], ["TR=4i:] (I may sporadically have [t_d] rather than [T] here in informal speech)>>

That ["TR=i:] sounds an awful lot like "three" to me. Has it ever caused confusion? You saying "thirty" and people thinking you were saying "three".

I pronounce "twenty" like "twinny".

No, I haven't ever had any confusion from such that I know of. It should be noted that ["TR_o=i:] for "thirty" and ["Tr\_-i:] for "three" differ both in stress placement (as in "thirty" the [R=] is stressed whereas in "three" the [i:] is stressed) and in the rhotic used (as in "thirty" I have a uvular approximant whereas in "three" I have a postalveolar approximant).

(I do not normally mark [R_o] and [r\_-] as narrowly when writing in X-SAMPA due to it not being concise, but I am doing such here to be specific.)

<<No, I haven't ever had any confusion from such that I know of. It should be noted that ["TR_o=i:] for "thirty" and ["Tr\_-i:] for "three" differ both in stress placement (as in "thirty" the [R=] is stressed whereas in "three" the [i:] is stressed) and in the rhotic used (as in "thirty" I have a uvular approximant whereas in "three" I have a postalveolar approximant).>>

Don't they also differ in that "thirty" is disyllabic and "three" is monosyllabic?