How do I design car for MPGs in thrme teens and have some towing.)

[Unavailableartist]

How fo you know how much torque/rpms is enough for a somewhat fuel efficient car. I ve been trying to design somewhat fuel efficient cars. And I figured that torque iand Rev count are both important. I try for 1500s ~ for rpm and at least 100 NM to get 14 MPGs. However I also try to harmlessly decrease high/low end gearing to get better tow weights (Ive trying roleplay a truck. I found thay that all three 100 NM, 125, 150 all end up same amount 16MPGs = 8 or 7. I suspect its probably my gearing but I lowered the gearing and notice no change from the estmates on how many mpgs the car would have. I could try for more eco... but I doubt I could hit 28-32 mpgs at 2-3l at least not @ 1902. Its really strange considering on most occassion I find the estimate to be fairly accurate.... my 50 mpg engine at first becomes 10 mpgs (due to low power). If make the most powerful I-6 I can and leave low gearing/high gearing on high 7mpgs = 10mpgs... based on estismates and that's what if turns out to be.
Could it be my gearbox isnt "powerful" enough, I get that message even if I set the gear torque limit = to the motor's.