Wouldn't it be better and more consistent to adopt veganism to avoid the climate change, pandemic, resource intensity, and animal cruelty risks rather than "make internal combustion engines more efficient"? There are many other livelihoods.
I dont see how any of those things would be helped by veganism, except for animal cruelty. That is if you don't count pest animals, and humans. I would 100% get behind outlawing factory farming of animals, and maybe that would lead to more vegans if prices rise, but not the opposite.
I know the argument for vegans helping climate change, but I'm pretty sure it only holds up if you eat local plants that are grown and processed in a way that is safe for the environment. That is to say most common vegan diets are worse for the environment. This is because the food is traveling farther, weighs more per calorie, uses more pesticides, and in many cases more water and land. With shipping of the food being the worst part of the pipeline for any diet. So making engines more efficient, or finding a better mode of shipment, is a much better use of resources regarding climate change.
Edit: sorry I see now you didn’t mean for eating. But for dairy. Yes, access to a cow for dairy is definitely a good thing. :-)