You might have some luck by obtaining one of those three piece funnels that are available in discount stores.
Take the nozzel part that won't quite fit into the filler pipe and slit it along its length for about 4 cm back from the end. This lets it collapse onto itself and provides the "expansion" force you neeed to push all of the "fingers" in the fill pipe neck back so that the "shutter" can release to admit the nozzle. Make the slit so it ends up on the top of the nozzle when inserted.
Works like a charm for me in a diesel fill neck, but might need some experimenting on a petrol engined car. (Then I found the adapter that allows you to use high rate pumps
: )
Hope this helps
Cheers
Hope this helps.