[aerohead]

Not to be overly critical, but, in mechanical engineering, the car would be driven 50-miles, at 50-MPH just to achieve thermal equilibrium for all the dynamic parts, at whatever the ambient outdoor conditions were, before actual testing began.
This removes powertrain efficiency variability, after which, the actual 'source' of any observed 'change' to mpg can be properly credited with the 'attribution.'
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Coolant - 203 F
Engine oil- 214 F
Transmission / Transaxle lube - 188 F
Differential - 159 F
Tires only require about 2-miles of driving to achieve equilibrium rolling-resistance conditions.
'New tires' require 4,000-miles of treadwear on them before they'll reach their advertised R-R coefficient, And as they wear out, they'll achieve their lowest R-R.