2001 Honda Insight gets 52 mpg combined (48 mpg city, 58 mpg highway).Antony Ingram/High Gear Media

The first-generation Honda Insight hybrid, sold from 2000 through 2006, is renowned even today for its ultra-high gas mileage: 52 mpg combined (48 mpg city, 58 mpg highway).

Those ratings remain unmatched today by any other all-gasoline-powered vehicle.

Considering all fo the engineering advances in auto making that have occurred since turn of the century, why don't we have regular cars without plugs that can top those numbers? MUST READ: Chevy Volt: How It Really Works Vs Common Myths & Conceptions

As the owner of a used 2001 Honda Insight hybrid -- currently averaging more than 60 mpg, including a few long highway journeys at 70 mph or more -- this lack of progress seems a frustrating state of affairs. Shouldn't the modern equivalent of the Insight, 15 years later, get more like 70 or even 80 miles per gallon?

After all, it would have a cleaner-burning and more efficient engine, a more powerful electric motor, better aerodynamics, perhaps even lower weight with more advanced design of its aluminum construction material. And given the passage of time, it would also likely be quieter, better-equipped, safer and offer better performance.

Well, it's not quite that simple. Here are four reasons that we haven't (yet) seen a true successor to ultra-high-mileage cars like the original Honda Insight.

The 4-cylinder Honda CR V gets the best gas mileage for an all-gas-powered car. Honda

Making cars is never cheap. Producing a more advanced vehicle, like our hypothetical 80 mile-per-gallon gas-sipper, even less so.

It's widely known that Honda lost money on every first-generation Insight it made. That's not really surprising. Not many were built, it used brand-new technology at the time, was made on a specialist production line and was constructed from expensive aluminum rather than pile-it-high steel.

Modern car companies are less willing to experiment at a loss, so any car combining the latest fuel-saving technologies would need to make money. Building a modern remake and incorporating the latest technologies simply drives up the cost.

To date, the best rating for a gasoline-powered car is Honda's CR-Z coupe, which gets 37 mpg. It costs less than $20,000, but Honda has struggled to maintain 5,000 sales per year.

One factor in the CR-Z's high price for shipping it from Japan. If it were manufactured stateside, could bring costs down, but there's little incentive to build a truly niche vehicle like that on U.S. shores.

Corvettes do not get high gas mileage, but they sure are pretty.DSC

The Insight, and its spiritual successor, Honda's CR-Z coupe, are both two-seater cars. While this gives the cars a sporty demeanor and a surprising amount of luggage space, it also limits their appeal to the vast majority of buyers who want a larger vehicle.

Even two-seater sports cars like Miatas and Corvettes sell in relatively small numbers.

Any true Insight successor would have to have four seats, at the very least. That would make the car grow, increasing weight and frontal area, working against two attributes that make the car so efficient in the first place.

Going from an all-gasoline-powered car to a hybrid like a Prius could save a driver hundreds of dollars a year on fuel. Toyota

The sort of person interested in getting high miles per gallon from a car is likely to own one already.

This means they're already using relatively little gas, and even fairly large increases in economy would save proportionally very little additional fuel, compared to someone moving into a moderately economical car from an inefficient one.

An example: A driver doing 12,000 miles per year in a 30-mpg car will use 400 gallons, costing just under $1,500 at current fuel prices. Should they trade up to a 50-mpg car -- a Prius, for example -- fuel use will fall by 160 gallons, and their gas bill will drop to less than $900, a $600 saving.

But a second driver who trades a 50-mpg Prius for our hypothetical 70-mpg fuel-sipper makes seemingly the same 20-mpg jump. This time, though, the overall saving is only 70 gallons, cutting the gas bill only $250.

It's why we've said that miles per gallon is a stupid way of rating a car's efficiency. It isn't linear, so doesn't illustrate the cost reductions of moving from one car to another.

Granted, a driver trading from say, a 35-mpg car to a 70-mpg car will realize some hefty savings: Fuel use is halved, as are the gasoline bills.

But for those already driving an efficient vehicle, savings could be minimal. Particularly if they already own that 60-mpg first-gen Insight.

The Chevy Volt is all-electric.Chevrolet

This is the real "elephant in the room" as far as a hyper-economical small car is concerned.

If car manufacturers could make a fuel-efficient small car for a reasonable price, they might not anyway.

That because the sort of buyers they'd be aiming at -- the kinds of buyers really intent on saving fuel and reducing oil use -- may already have moved on to electric vehicles.

Electric cars don't suit everyone, naturally. But if you're making that much effort to buy a car that gets 70 or 80 mpg, you may want to just skip the gasoline-powered middle-man and get a pure EV. Why spend extra to get an ultra-efficient fuel-sipper, when you could buy a car that doesn't sip fuel in the first place?

Unless you regularly do longer journeys, it would be a no-brainer for eco-conscious buyers.

The BMW 328 328d X Drive is a larger, gas-powered vehicle that uses diesel fuel to get 35 mpg combined.BMW

There are still some buyers for whom our hypothetical 80-mpg car would be ideal.

I, for example, tends to drive mainly long distances -- making electric cars impractical.

That's why my Insight is ideal. But I'd like a 70- or 80-mpg modern equivalent even better, one with better sound insulation, the extra surge and response of a more powerful, yet more efficient, hybrid system. Throw in a modern infotainment system and it'd be perfect.

Unfortunately, drivers like that are few and far between -- not really numerous enough to present a solid business case to a major automaker.

So while we can expect regular cars to become more economical as the years pass, don't hold your breath for a true, affordable successor to the highest-mpg non-plugin the EPA has ever tested.

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