I seen arguments that the emissions of Electric cars is higher than current if you factor in what has to produce the electricity in 1st place. So while cars themselves might be "efficient" the means of powering them isn't.
I did calcs based on the EPA mileage figures for the Nissan LEAF and the Versa hatchback (same body as the LEAF so aerodynamics should be identical, though I suspect the LEAF may have more energy-efficient tires). The numbers I got were:
Gasoline ICE car is about 20% efficient (20% of the energy in the gasoline goes into moving the car)
Based on that, diesel ICE cars are probably about 25% efficient.
EV charged with electricity generated from coal is about 19% efficient.
EV charged with electricity generated from natural gas is about 25% efficient.
The primary reason EVs are cheaper to operate per mile is because gasoline costs roughly 10x more per Joule than coal or natural gas. Notably, the fuel taxes on gasoline are more than the cost of coal or natural gas needed to power an EV. This is going to become important as EVs begin to comprise a significant fraction of vehicles on the road. Fuel taxes pay for constructing and maintaining roads, and right now EVs are getting a free ride. That will have to end eventually if we don't want our roads falling into disrepair. A few states are experimenting with a surcharge of about $300 when you renew the registration on an EV, to make up the fuel taxes you aren't paying.
https://tech.slashdot.org/comments....old=-1&commentsort=3&mode=thread&cid=56040159
Do note that which is better changes depending on what you mean by "emissions". CO2 emissions are lowest for natural gas. Diesel is next (it generates slightly more CO2 than gasoline, but the higher efficiency of diesel engines means you'd use less diesel than gasoline to travel the same distance, resulting in lower CO2 emissions). Then gasoline. Coal is the worst by a fairly large margin. So that's probably why you read that EVs could emit more CO2 traveling the same distance as a gasoline car.
https://www.eia.gov/tools/faqs/faq.php?id=73&t=11
For other emissions though (particulates, sulfur dioxides, nitrous oxides), diesel tends to be worst, followed by gasoline. Coal is actually the worst by a large margin again, but the fact that you burn coal in a factory means you can put extensive filters and scrubbers on it to remove most of the particulates before they enter the atmosphere. (Of course then you end up with mountains of coal ash, which causes its own problems.) Natural gas is the cleanest - it should produce only water and CO2 unless something is wrong with your flame.
As for EVs being powered by nuclear or renewables, that simply doesn't happen. Right now, nearly all our flexible electricity generation comes from coal and gas (hydro too, but its generation in a given year is fixed independent of consumption - depends on how much rainfall there is). So if you trade in your gasoline car for an EV, and plug the EV in to charge it, that increases the electricity the grid needs to generate. The power companies need to somehow supply that extra electricity, so they run a gas or coal plant a little harder. So pretty much 100% of the electricity used by EVs comes from coal and natural gas.
The only things that might change that are increased nuclear power (enough to absorb the additional load of EVs charging overnight), and battery storage of wind or solar. But battery storage makes no sense unless fossil fuel plant use during the day drops to 0%. If you're still running fossil fuel plants during the day but have excess wind/solar power, then it makes more sense to shut the fossil fuel plants off during the day and use the wind/solar power immediately, then turn the fossil fuel plants on during the night. Basically shift the fossil fuel plant's on-time from day to night, and use the wind/solar power immediately with no losses. If you run the fossil fuel plant during the day, and store the wind/solar power for use at night, you end up losing about 20%-40% of the wind/solar energy in battery charge/discharge losses. Meaning you have to run the fossil fuel plants more to make up those losses.