Help!

My 2015 Fiesta Ecoboost started using more petrol than normal a couple of months ago, around the same time that I started losing pressure in my front tyres quite often (I was informed they were cracked). I replaced both front tires, and paid for wheel alignment etc. Still no joy and my petrol was still decreasing more than normal. I put it in for a service last Friday, they worked their magic and confirmed engine was in good shape.........petrol STILL going down rapidly compared to miles travelled!! Back to the garage on Tuesday - they rang diagnostics, checked for leaks, holes, etc and couldn't find anything wrong! They advised to fill my tank up, take a note of the miles showing, and reset my mileage round trip. SSSOOOO, I filled to 422 miles and reset the 'computer', and the latest reading was '379 miles to go' according to the Petrol, but only '15.4 mi TRIP' - big difference! My driving hasn't changed, I don't 'rag' my engine or use the wrong gears.

Please help me get to the bottom of this. If I wanted to get through petrol like this I would have brought a gas guzzler :-(

2 hours ago, TA86 said:

SSSOOOO, I filled to 422 miles and reset the 'computer', and the latest reading was '379 miles to go' according to the Petrol, but only '15.4 mi TRIP'

I don't understand this. Procedure I use is,put 25 litres in when its near empty,set trip clock to zero and cover about 230miles with 25 litres 

I am using over double the amount of petrol I have always done. Despite not extra driving. It told me i had used approx 3 miles in petrol when i had only travelled approx 1 mile