Wheels, tyres, pressures, other added options, all make a difference to the mpg.
Don't forget the driver... the OP suggests his wife is a slower driver, but that doesn't mean she is an economical driver. I've sat with drivers over the years who drive slower than me, but with greater fuel consumption.
Remember it is documented that the driver, and how he/she drives, can make a difference of up to 30% on mpg consumption over the same trip. Hence the EU drive for 'driver awareness' on how to drive for better economy.