It would be so fast you won’t see it coming
This reminds me of something...
I had a Ford F-150 4x4, for towing heavy trailers out of muddy fields. It got about 12 miles per gallon on the freeway.
I decided I could do better. I started a design program to improve all the fuel-sucking weaknesses. I removed the fan assembly and replaced it with electric fans and an electric thermostat. I revised the air intake. I reprogrammed the shift points on the transmission. I changed the fuel mapping. I got it up to around 18 or 19 mpg.
One of my neighbor's, John, followed my exploits with interest and amusement. Why was I trying to make this old beater more efficient, when I could just buy a newer truck?
I realized the biggest weakness now was the aerodynamics. I designed a convoluted "Aerocap" to fit on the truck bed. This reduced the size of the back of the truck by 45%, and cut drag from a calculated Cd of around .45 to around .33. Fuel economy on freeway trips jumped to 22 mpg - *almost* DOUBLE the factory spec.
Finally, I decided I needed to completely revise the motor's timing system. The variable valve timing system was poorly designed and dependent on a common oil feed for both sides. This single oil feed tensioned the timing chain, operated the timing control valves and shifted the variable valve timing cams.
So, John comes over. I'm sat, covered in oil and grime, with half an engine in bits around me. He asks what broke, and I tell him my latest adventures in search of fuel sipping.
"You want to be careful," he said. "When you're finished, make sure you stop every few miles and drain off the excess fuel!"
That truck drove 80 miles further then the timing ate itself. It's sat in my driveway unused the last 3 years. I can't even bear to look at it. I went out and got a diesel Passat, and now I get 50 mpg on the freeway and no maintenance hassles. I just don't pull caravans out of fields any more