The news in today's Autocar scoop that the next-generation 2012 Range Rover will be a massive 450kgs lighter than the current car couldn't be more timely.

A few days ago, the US government declared that the average SUV sold from mid-2016 would have to average 34.5 UK mpg.

Read Range Rover lightens up - full story

Although the formula that is applied to calculate the mpg rating is very complex (it's based on the 'footprint' of a vehicle) and there are some let-outs for building a car that will run on bio-fuels, the result will put Land Rover engineers on the spot.

While meeting the 34.5mpg average will be easy with the combination of a lightweight aluminium body and the frugal TDV6 diesel, building a Range Rover that will meet US market conditions will be harder.

Clean diesel engines can be sold in California, but they are rather more expensive to manufacture than petrol units and are not very popular with buyers. So how will Land Rover manage?

Some kind of new V6 is probably needed - possibly with a couple of small turbochargers - as well as an eight-speed autobox and a stop-start set-up.

Jaguar Land Rover could, of course, build a new V6 engine based on the new-generation V8 that's fitted to the new XJ. But while most Range Rover sales in Europe are diesel, the all-important US market wants a frugal petrol motor.

A four cylinder, electrically assisted, Range Rover is probably 10 years away.

In the meantime, downsizing is going to apply to the new Range Rover's emissions and not just its body.