Two up with gear at those speeds, maybe a headwind too, 140 to the light isn't that unreasonable if a little dissappointing. You'll find that if the bike is arse heavy the light will come on sooner as the juice sloshes to the rear and the float drops more, you're not necessarily reading the same level of emptiness as you would do with the bike level, not that far off mind.
I used to find that two-up, and particularly fully loaded, The 'economy' falls like a lead brick above about 85-90, knock it back 10mph and you'll get much better milage, just depends how the equation stacks up in your head: get home 20min sooner but pay a couple of quid extra, your choice but most importantly, I don't think there's anything wrong with your bike.