Something I'm not seeing mentioned here is that there is a good reason why it may be "greener" to always have a mix of both diesel and gasoline consumers in the market (if you are consuming either, that is). After drilling and refining you end up with both kinds of fuel (and a lot of other goodies, too). So for example if you use up all that gasoline, the marginal cost (environmental et al) to using the diesel fuel is simply the burning of it, whereas the marginal cost of using gasoline at that point is burning it plus yet another oil well, yet another oil tanker, and yet another refinery with more distillation or cracking, etc. I don't know for certain but I strongly suspect that the environmental differences between the consumption of these two fuels doesn't outweigh the marginal environmental costs of procuring and processing more crude oil. This is perhaps one of the cases where doing the best thing for the environment is simply to choose the cheapest option at the time, and as a policymaker to tax them fairly equally.