there are 2 ways to look at
1.) $.30 per gallon for the people doing this is not a whole lot of money compared to all the money comming in for road tax across the state. $3.00 per gal for all the people doing this is alot of money. I talked to a guy who works for exxon mobile a few months ago ( not a gas station person, but someone higher up that works in Fairfax VA), and he said gas would be about 18-22 cents higher per gal. if it were not for peple running their cars on home made fuel. now take that 3 bucks from above and do some simple economics with supply and demand and you see where that 18-22 cents comes from. Even though gas is a unitary elastic good, it is effected by alternative fuels. Now think about what happens when gas is high, the economy does worse, this sounds like a stretch, but it is true, by allowing all the svo users to run their cars, it keeps gas prices lower, which boosts the economy, and the economy boost far outweighs the the fall in road tax. This works alot better with more numbers, and stuff, but I don't want to type for an hour to illustrate it.
2.) I can see why some think it is unfair, many people don't mind paying the tax, it's the process of keeping a record, and if you had to run on diesel for a while, and the milage didn't add up, it's alot of hassle. This process is what keeps so many people from doing it. I think they should charge a flat fee based on the model vehicle you drive. For a voltswagon rabbit, just pay 5 bucks a month, if you have a 7.3 powerstroke, pay 15 bucks a month. Sure, it's not 100% accurate, but it would at least be something and is better than alot of people running arround paying no tax.