She’s not stupid or oblivious, and she knows exactly what she’s doing.
Rounding down the ticket price says it all. You supplying lunch and her not offering to buy a pot of tea or ice creams etc is extremely tight.
I would speak to her. There are lots of ways you could say it kindly, but you must use the opportunity to get your point across load and clear.
But it’s all how you say it!!
My approach would be to say with the cost of living crisis bla bla bla, things are pretty bad, would you possibly mind paying half of your share on cups of tea, food, ice cream or whatever.
With the petrol (and wear and tear of your car), a total of two 2-hour round trips has got to be £20(?), so it’s got to be £10 at the very least.
If she genuinely holds the friendship in high regard, she will understand and she will want to maintain the friendship.
If she’s just a tight-fisted piss-taker, you will get your answer.