check for sticking brakes as previously suggested.
I assume the mechanic checked the CO setting which may have a small effect on fuel consumption.
You really need to check the consumption with fuel you put in to fill up and the odometer and not the light.
Nevertheless, for comparison, my light comes on between 130~ 160 MILES (depending on where and how I ride).
In addition to what you have checked and what has been suggested, I would do the following:
- change the spark plug
- what engine oil are you using (a good synth will give you and extra 10% range)
- tyres affect consumption. chunky ones are worse
- check your tyres are not under inflated
- fuel quality and ethanol can reduce your range
- change in riding: more traffic and city ride = reduced range
- change lube and slack adjustment

I don't know the climate and temperatures where you live but winter riding (0~10deg) will reduce your range by about 10%