A large family creates many future taxpayers. The state has an interest in subsidizing or at least not discouraging the creation of large families

That seems like an oversimplification. Rural states receive more in benefits than they pay in federal tax. Large families by necessity are going to favor rural areas and states. It is not clear to me how a large family is a net tax benefit, there are too many other factors to consider.

Having more people pay tax does not mean there is a net increase in tax collected.

> It is not clear to me how a large family is a net tax benefit

Some of the children from a large rural families will move to cities and make careers that pay lots of taxes. Cities almost always have lower birthrates than countryside.

Yes, we need children. That's not a question. The question is if a single couple needs to produce so many children they drive a vehicle so massive is earns tax breaks.

In this context, I assume a "large family" is 3 or 4 children (a 5 seat sedan cannot physically, legally fit 4 children and two parents, and due to the size of modern carseats it can be dicey or impossible to fit 3 of them in compact or midsize sedans even for a 3-child family).

Even just maintaining replacement rate fertility requires an average of 2.1 children TFR, which means some families would need to have 3 or 4 children to get there since other families will have only 2 or 1 child.

In France, you end up paying a lower tax on your vehicle the more children you have, so a family with four children might pay the same taxes on a large diesel as a childless couple with a tiny gas car. I was trying to register my 3-cylinder 1.0 liter vehicle, just to find out I’d have to pay 1500 Euro tax for my gas guzzler. Ugh.

Most things I need, I need because I chose to need them.