Thanks experience counts for alot.
Because of the torque out of the 5e and 5efte depending on turbo there will be a good 10mpg difference on the 4e 4efte, also gearbox dependent on both the 4e and 5e.
I've run the c160, c161, c54, and starlet na box with very different weekly mpg figures so monthly mpg will be very different the c54 was the worst because of the large gear gaps needing more throttle to react to rpm drop and hit target speed, the c160 out of the bzr has a nice 6th overdrive gear droping the rpm down on the motorway, whilst the c161 you can Access 6th at 40mph droping the rpm down. using the corolla inlet and na cams, low rpm torque is amazing so power at this low 6th gear rpm is great and surprisingly you can accelerate still nice. I've calculated the torque out put is on par if not better at low to mid rpm than the 5e engine. also the smaller turbos like the ct9 will hit boost pressure (chatter) just in stop start traffic this will have an impact on fuel delivery at that rpm. Impacting worsening mpg efficiency.
Even tyre width increases or reduces rolling road resistance\friction.
Seasonal change in temperature also will affect mpg in warmer weather the mpg will increase by around as much as 5mpg a week due to the fact hot air is less dense, and better fuel vapourisation will be happening (indirect batch firing injection\carb setups). Most if not all rolling road tunes will not account for this at all. Just look at the difference in temperature between the coldest months and hottest months, upto 25degC.
Mpg tells a very different story about how the car is tuned\setup if it's very low then the engine or tune is not very efficient this could be for many reasons, fueling for safety\setup quick, setup for strip/track and used on the roads. Either way this will show with the mpg\engine efficiency.
Using excess fuel also impacts oil grade and oil degradation. The higher the excess\safer tune the quicker oil will degrade.
Engine with poor mpg will get hot because some of the excess fuel being used and/or ignition retard mainly to keep it safe will now produce heat energy instead of kenetic energy.
The modern engines use alot more sensors running faster ems allowing closer talerences.
Threw winter with the modified oil delivery system on this ep91 I'm running, I was able to use 5w30 oil grade I've recorded the mpg figures. I'm not quite at a temperature point yet to run 5w30 grade past the month of may yet, but by next year I should be able to.
I'm starting to get close to 40mpg figures on a basic ems system with not a massive amount spent applying normal everyday driving and the figures are on a increase coming into summer thow impacted by the thicker oil grade I'm using threw summer. Just my top level power\rpm to tune for 220+hp and very best efficiency.