The gas tax should be abolished and a mileage & weight class tax enacted for all cars. That would be the equitable way to do it.
It is also incredibly simple to do and would save money.
To a degree, there's already a weight class tax, although not on fuel/road usage. In Maryland, for instance, if a passenger car has a shipping weight below 3700 lb, the registration is less than if it's over 3700. There used to be a loophole for anything classified as a truck, but these days I think most SUVs are classified as "multi-purpose vehicles." Actual "true" trucks, like a pickup truck, still get a bit of a break on the registration, though.
However, I'd argue that when the roads are built for fully-loaded tractor trailers and such, whether a 2000 lb Chevette or a 5000 lb Imperial goes over that stretch of road, it's irrelevant.
And in some cases, some smaller cars might do more damage than you think. It's not just the weight of a vehicle, but how much it's spread out. And weight distribution. For example if a 3000 lb Prius had a 61/39% weight distribution, and my 4000 lb '79 New Yorker has a 55/45%, that would mean the Prius is putting around 1830 lb of pressure on its front tires. My New Yorker is putting around 2200 lb, around 370 lb more. Or 185 lb per tire up front.
But wait, there's more. A Prius is probably riding on hard, skinny tires, like a 195 series, in the interests of less drag and better economy. My New Yorker is sitting on 225 series tires that aren't brick-hard. that extra 185 lb per front tire is spread out a bit more. And, because it's a softer tire, and most likely not inflated as high, it doesn't hit the road as hard.
The vast majority of vehicles used for personal transportation (cars, minivans, crossovers, SUVs, and half-ton pickups) probably weigh between 2500-6000 lb. To TRULY make things equitable, you'd have to come up with a formula that takes into account tire size, pressure, overall weight, how much of a load you typically carry (put four good sized people in a Prius vs just me in my New Yorker, and they might weight about the same), and how that weight is distributed across the road surface, and so on.
But again, in the overall scheme of things, with lighter vehicles these weight differences are irrelevant. Even in neighborhood streets. At some point during the week, a school bus, trash truck, UPS truck, etc probably goes past your house, and any of them are going to far outweigh the typical passenger vehicle.
