Cows are not the problem and lettuce is not the fix.
The confusion is from focusing on animal and vegetable foods when the problem is the way they are produced and brought to market. The industrial food system is extremely fuel intensive and uses the worst possible methods for raising both vegetables and animals.
Cows and lettuce are not the issue.
Eliminating industrial mega-growers and replacing them with SMALLER, educated local growers would be a great solution. ADM, Monsanto, Cargill, et al just need to be OK with profiting somewhere else.
Therein lies our real problem. The mega farms will figure out a way to pump methane from a feed lot before they do away with the problem - the feed lot system itself. They'll figure out a way for lettuce to withstand even more toxic chemical inputs before they do away with destructive growing practices that mandate those inputs.
The biggest thing anyone can do to change the world (even if you only consider fuel use) is to grow one's own food - at home. Whether it's animal or vegetable. Even if you can only grow one thing.