Hi guys. I have recently purchased 2015 smax. Its a 210bhp version with 2.0 diesel engine. I am getting around 40 mpg on the motorway with all meters setting being reset. I am not a fast driver and always stick to 70mph.

Is there something wrong with my car or is this mpg normal?

I'd say that was pretty normal, I've got the 2.2 diesel and I only manage on average about 33mpg

I was a bit shocked when I bought mine, it’s the 180bhp diesel. Then I realised the car weighs 2ton. I would say your mpg is about the norm fir the smax. 

My one 2.0 207bhp does in average 41-42mpg. 

On motorway 46mpg, tyre grade will also play a significant role ~5%

Not sure you will see any difference with tyres, unless you go down the cheap route. Mine come with continentals from new, then changed to Michelin’s, 245/45/19. And run 17 inch steel wheels with continental snow tyres for the winter 235/55/17.  To be honest, I can’t say I notice any difference in mpg between all 3 tyres, it’s more a handling and noise difference.