There are far more effective "low hanging fruit" to tackle than lowering the speed limit when it comes to reducing emissions.
- Transporting goods by rail instead of truck or plane. And while we're at it, cargo ships are huge polluters because they can save money when using low grade dirty fuel. Disallowing that practice would have a big effect, but that would require the global community to act (good luck with that).
- Increasing the use of public transport and other ways to lower the need to even drive a car, like (partial) work-from-home schemes, and political incentives driving their adoption.
- Increasing the efficiency of roads, first and foremost by getting smarter traffic flow control that reduces congestion (nothing in transport is more wasteful then a car just idling in a traffic jam). But even the quality of the street and type of pavement you use in roads can have a rather significant effect on fuel efficiency (regardless of whether the fuel is stored in a gas tank or a battery).
Reductions of emissions is one important piece of becoming carbon neutral, but not the only piece. Renewable energy sources and other technologies that will take carbon out of the atmosphere again play another important role in the complex solution that is needed. And e.g. with renewable sources Germany might not look too bad internationally, but we're greatly lacking behind our own goals, and mostly for political reasons.