The eco tyres that most car makers fit to their cars today are woeful and have very low rolling resistance and by design hardly any grip, by fitting decent tyres you might lose 1mpg but I would sacrifice a bit of fuel economy for grip any day!
The thing that bothers me is the fact that Motability know this but refuse to let you have different tyres fitted, even when offering to pay any difference in cost! Just goes to show they’re not bothered about our safety at all.