just got back from singapore with my car.
to get autopass, no need to peel out road tax.
after showing ur passport at sg immi, u ask the officer where to get one, it's at block b, it's along the way only.
park ur car and walk up to the counter, less than 10 meters. ANYONE can buy the autopass. they'll come up to ur car to confirm ur plate number.
they 'll give u a small chit, then go to next counter, side-by-side only, and fill out one form.
they proceed to get the autopass (which is infront the counter as well)
pay S$10, get user manual and the card.
Inside got map to autopass top-up locations (as the card only got S$4 only, but any 7-11 can do)

the card is valid for 5 years, and not transferable to other cars. every checkpoints got camera pointing at ur number plate.
side note: when oming back into malaysia, need to pay RM1.20 (if not mistaken) using touch n go, which i didnt bring. simply press the buzzer and tell them u havent got any tng card, the'll come over, and u pay cash. this one i dunno if i missed the sign saying need to hv tng ready on the lanes, maybe other lanes got cash only lanes, 

plan ur route and get a map  though parking can be hard in non-touristy area, cant simply pull over like in malaysia