Lets clarify some misconceptions, the grams per kilogram, the measurement for CO2 emissions for vehicles does not vary according to vehicle.
Manufacturering standards and UK requirements demand that capacity ( cubic centimentres) of engines will meet CO2 emission requiremts. Therefore if a 2500 cc engine is in a 4x4 the emission will compare with a 2500 cc engine in a car. In some cases, performance cars have higher g/kilo outputs than comparable 4x4. It is also evident that some luxury saloons have a higher putput than 4x4s. All of comparable engine size.
The continued argument that 4x4 are gas guzzling monster also falls down under scrutiny. The variance between saloons, 4x4 and sports vehicles, all of similar engine sizes, is only 2 to 3 miles per gallon. Now, when you consider the fact that 4x4 are usual of a lower gear ratio than performance saloons it is evident that the 4x4 engine is actually more efficient.
The arguments of size and height of bumper needs a physical tests and finding the variance. A long standing joke was that the comparisons of bumper height between a Mini and a Rolls Royce was virtually nil. Because of the Over riders on the Rolls covered a greater area and allowed a matching height with the mini.
It is easy to target a style of vehicle to use as social misfit but the whole mass of dis-information about the vehicle does not stand close scrutiny. The need to compare like with like and not just to use some of the infomation in an attempt to justify some misconcieved argument. The most surprising element is that professional bodies make unfounded claims without any statistical information to substaniate their claims.
Having someone say more 4x4 kill speople than other vehicles is, at close inspection a spurious and totally misleading statement. The therory that a side impact would overun a smaller saloon again is spurious and misleaading.
The smaller veehicle will, as has been proved, slide along a road before any possibility of overturning is ever reached. A Foden 32 ton truck pushed a rover 2200 saloon along the entrance road to the Hammersmith fly-over and both vehicle came to a standstill without any ' turning over' of the smaller vehicle. The discrepancy in bumper height to car is greater than the 4x4 and the average car.
The smaller sorts car, Mazda, Lotus, etx have a much lower profile and are at equal risk from vans, commercial vehicle and trucks as from 4x4 so it would be unrealistic to make the statement without qualification.