[ProDigit]

MPG is a combination of engine size, engine and vehicle efficiency, and gearing.

You can gain good results by getting a more modern engine, with tighter tolerances, and aimed more for fuel efficiency. Modern cars usually are built for higher MPG, because of the pollution control.

You can also improve efficiency, by making sure the model is aerodynamic, and light.
Aerodynamic for freeway riders, and light for start-stop traffic.

Last is for city riding of up to 45 MPH, a 3 cylinder 1 liter engine is good enough.
The less cylinders, the better for MPG, and the smaller CC, the better MPG.
But it also depends on gearing.
If your car does 6k RPM to maintain 60 MPH (600 cc Kei cars in Japan, do this), your MPG will suffer, and be worse of a larger car with a larger engine doing the same speed at 2,5 k RPM.

So in order to get best MPG, you'd have to determine in which areas are you going to ride the car most, and how much fun (opposite of granny driving) you want it to be?

Like for instance,

While a 1 liter engine with 75-100 HP is okay for the city, of up to 45 MPH, these kind of cars won't be any fun to drive on the highways!
They're sluggish, and accelerating or surpassing someone doesn't happen over a cup of coffee, even with a gearshift.

Bump it up to a 150 HP motor, 1.4 liter turbo or 1.8 liter NA with on a 3300 LBS chassis, and it'll be kind of fun for speeds under 60-75 MPH, but get over it, and it'll be the same problem as the 1 liter engine.
You can surpass other cars on the highway, but, boy, it takes time..

So if you're going 75MPH, you have the choice to go with an eco car, or bump it up in the 200-275 HP range.
That'll be a 1,8-2.0 liter turbo, or a 2.4+ liter NA.

Once you increase the 1,8-2.0 liter turbo on a standard eco sedan style body, the fun factor on the highway increases, but the MPG will start to suffer.
3-4 liter cars make better MPG than 2.0 liter turbos at speeds well over the legal speed limit.
For that reason they're not an option if you care about fuel efficiency.

A Camaro, A Kia Stinger, they have 2 liter turbo engines that are on the threshold of great MPG and fun. Any larger, and your MPG will come down into the high 20s.

It all depends on how the manufacturer has geared the car, but generally speaking, that kind of engine gets avg MPGs in the low thirties, and highway MPGs in the high thirties.

Especially with Turbo engines, you should look at the speed you're driving the car most (eg: 75MPH), and the RPM range of the car in final gear at that speed (eg: 2250RPM in 6th); and where the engine starts making power. Like, if the engine starts making power at 2k RPM, but below is kind of sluggish, you'd want a car that's geared tall enough to get you to the speed you like, in final gear, above the RPM range where the car starts making power..


The smart for instance, has a 1 liter engine that's hopelessly undergeared.
It revs at like 3k RPM at 45MPH. This is the main reason for it to have low MPG.
But the manufacturer had the choice, to either make it sluggish in final gear, or have MPG suffer.

This all to say that there are many factors that come into play of MPG