Newer cars have cap-less fuel tanks that lock the opening and prevent siphoning.  To refill such vehicles from a gas can requires a special adaptor to open the interior lock, but those adaptors work poorly with fuel cans, spilling a lot.  Although this funnel does not replace that unlock adaptor; if so it would rate 6 stars, it fits perfectly into my Ford adaptors and all spilling stopped.  I bought more to have on hand as spares, and to tie on my main gas cans to have handy.  An additional challenge in filling from fuel cans is slow flow rate and air burps.  I have no problems with either challenge using this funnel; I think its now faster than any filling device I've used.