My 2018 Eco Diesel engine(engine replaced at 41K miles) has a noticeable drop in MPG when temp goes below 21. Just returned from two trips where I drove through areas where the temp ranged from 27 to 34 then down to 15 and then back up to 35. I averaged 29 MPG the first 92 miles. (manual calculation) when the temp dropped below 21 the MPG dropped to an average of 18 MPG. MPG went back up after temp increased again. My 2500 5.9 always ran better in colder temps. Is the ECO diesel different or is it just my truck?