Topic: Poor fuel efficiency - 2007 Sonata diesel

This may seem a little strange but the fuel efficiency on my 2007 2.0 Hyundai Sonata Diesel sames to fluctuate massively. In the mornings, especially when it's on the cool side, my fuel efficiency is excellent and regularly reaches 48 mpg on the 20 mile commute to work. However, the return home is rubbish with only about 36-37 mpg. The roads are clear so I'm not battling against traffic and it is actually more downhill going home. I'm guessing it has something to do with the temperature. Stranger still, sometimes if I stop the car and then restart it after it has warmed up, the fuel efficiency improves again (only sometimes).

The fuel efficiency fluctuates regardless of the route. It just seems to happen more in the afternoon or when it is warm and dry. The rear offside wheel does seem to emit a smell like something very hot and it ticks as if it is cooling down. This does seem to coincide with the drop in MPG. I have had the brakes stripped and cleaned and there was no evidence of scorching. Also, I have installed a new caliper.  The car drives normal and there does not appear to be any drag.

I have had it into 3 garages now and spent hundreds trying to get to the bottom of it but with no luck. Any suggestions would be welcome.