Slightly different, but DH was away a lot for a few years so we dropped to 1 car. We only had 1 (shared, but mostly "his") until DD was born, and dropped again when DD was about 6 for about 5 years until his DF died and he was commuting weekly to DMIL 250 miles away to sort things out afterwards and I still needed a car here for Scouts (Leader) and DD's activities.
Plan ahead on the grocery shopping - plan your meals, taking into account various activities and who will be late or evenings you need fast meals v evenings that you can do cheaper slow cooking meals from scratch. (Fast meals can mean making the sauce the night before or peeling/chopping veg to just turn on pots when you get in - not necessarily relying on frozen pizza).
But also keep a well stocked cupboard to allow you to change in a hurry if necessary. Snack things (especially for a teen - hollow leg syndrome!). Storecupboard things that can be turned into meals easily. A few long-life things in the fridge like cheese, eggs, bacon pieces etc. Onions and potatoes. And a few key ingredients or batch cooked meals in single portion sizes in the freezer.
The reason for that is so you can survive with only getting a bottle of milk and some bread or other small things midweek if necessary when DH is away with the car.
Do the DCs have activities that you need to drive them to?
Do any activities currently (or look likely to becoming) clash with each other?
Could you offer a lift to another DC for 1 activity in exchange for a lift for yours on a day you don't have the car to another activity?
Are there often events around school or other activities that require "one off" lifts or being collected somewhere?
What about in bad weather?
Can DCs still get to school walking, or bus?
Can you still get to work on those days?
(In your budgeting, add in the cost of DECENT weather gear for everyone - proper rain coats, wellies for smallies and decent walking shoes or boots for older DCs/adults, and warm layers, to be out and about in all weathers).
And be prepared to do things like re-waterproofing coats and shoes at least annually so that they will continue to keep the rain out.
Apart from regular, everyday things (school, works, groceries), what about other less frequent outings?
Do all the DC fit into the other car for family outings?
How often do you need to do things like shopping trips to "town" for clothes, furniture and furnishings, or Ikea for 1,001 "must haves" etc?
Can you manage all of these in the time that DH doesn't need it for work?
Doctor and dentist trips etc
When we dropped the car, we had similar conversations around using taxis and hiring cars on occasion.
We have pretty decent public transport to get around locally and to get to the city centre, but sometimes I needed to get home fast (if DD was sick in school for example) or the public transport system fell apart on occasion.
We looked on it that the cost of public transport, occasional taxis, and the cost of hiring a car if we both needed one for a weekend on occasion etc, was a lot less than the overall annual running costs of a second car. So there was no need to feel guilty about using them.
But we also had to have a couple of conversations about one person not just disappearing with the car - that we needed to check in advance about our plans to make sure they didn't interfere with the other and the family.
Neither of us play golf, but if one did, they would need to make sure to either get dropped off to the course by the other person around their planned activities, or get a lift with someone else, or have agreed they could have the car for the 4-6 hours involved that would likely be over a weekend day, for example. (Or make arrangements for a taxi/hire car ....themselves). Or not just "I feel like a pint with the lads as it's Friday, I can get a taxi home and leave the car at work" kinds of thoughts - preplanned pints were fine, but not just not having the car available on Saturday for everyone. That kind of thing.