I currently lift share with the lady down the road. We've done it sucessfully for 4 years now. We have another 3 years to go, with our youngest who are in the same class.
I do mornings, they do afternoons.
Next year, I will do school X and they will do school Y - juniors and seniors.
We also make sure neither of us every feel burdened, so when there is a disaster - like me breaking my arm - there was no ill will about them doing both runs for a month. Next month she goes in for an op, and I will do the mums taxi bit for the rest of the year, no problem.
They key is making sure everyone does their bit and no one feels they are being taken for mug.
Id tell both families that you are not happy to continue the current arrangements as your TIME is precious too - its not just about the money, but having to delay your morning by at least 1/2 hour and spend your afternoons clock watching. Suggest a rota where you do one run in three...how your or ganise it is up to you, but eg you do Mondays and tuesdays pickup and drop off and every third Friday. Or week by week, as previously suggested.
Maybe make some discrete enquiries with other mums about the possibility of a more equitable lift share, and if your current families dont want to play fair, just jump in with someone who will.
If they dont even want to give you a decent anmount of petrol money, they are hardly your mates.