[RedDevil]

My sister has a GoldWing 1100 with sidecar. It is getting a new engine and Interstate guts and fairings right about now (it was an Aspencade, but she does not care for the air suspension as it was not strong enough for sidecar use anyway).

There are 2 basic problems with her (and probably most) sidecars: speed and range.
We are allowed to drive 130 km/h on half of our highway system. But she can't if she does not have a strong rear wind or so. This is an 1100 CC motorbike with more power than some small cars, but the air resistance is much more than any small car has.

The GoldWing has no small tank for a bike, but as it easily uses 1 liter of fuel every 10 km or so (about 23 mpg) even at a more moderate 100 km/h; you need to tank within 200 km or 120 miles.

Driving it more slowly makes it somewhat less thirsty but at over 600 kilograms for the combination the sheer mass will always work against it.

On the plus side, it can carry 3 adults and a small kid (or 2 and 2 not too large kids) and quite some luggage. It can even tow a small trailer. But so can a small car.

If bike and sidecar are (much) more aerodynamical then you can drive that on the motorway, even with a slightly smaller engine, but you'd have little to spare. It takes the fun out of motoring.

The small engined motors with sidecars are fit only for city and rural use, And that's the only environment where any sidecar combination can get 'eco'-like FE.