Let's brake this down:

"mpg engine efficiency has come up to at least 55%"

• There's MPG efficiency (how far your car goes of a single gallon)
• There's Engine Efficiency (how much energy is actually harvested from the fuel)

The Toyota Miari has a measure Fuel Cell stack peak efficiency of 66.0% - That's much better than the most efficient gasoline engines out there reaching at beast 41.0%. 

As far as MPG - for a $50k car based on the Lexus LS, the Mirai is more efficient.

On the Toyota Mirai: ( 0.8 [kg / 100km] (worst) + 0.55 [kg / 100km] (best) ) / 2) * 1.6 -> 1.08 [kg / 100 mile] * 16.51 [$ / kg] = $17.83 to go 100 miles -> 18 cents per mile.

On the Lexus LS it's based on: 100 [miles] / 22 [miles / gallon] -> 4.54 [gallons premium gas] * $4.178 [ $/gallon (current avg.)] = $18.96 to go 100 miles -> 19 cents per mile.

The performance is different, but the Mirai and LS are about average for big $50k luxury cars.

Is there a more viable way to produce the Hydrogen without using a lot of energy?

This is a car forum, but here:

Either from fossil or the current way. Look with the world going to all of that EV crap - it's not like there's enough energy. According to studies, more people will die of lung cancer per 1,000 EV than 1,000 regular conventional cars.

And as Scotty keeps saying - no one is building the grid to support all of this craziness. And those that do, like Germany ask them how they're doing. In the US electricity is $0.10 for a kWh, compared to $0.36 in Germany - Imagine your powerbill being almost x4 - nothing as great as good old conventional cars have ever been created, gasoline engines are here to stay for a very long time (maybe in the form of Hybrids but those, exactly like EVs, tend to be a nightmare to fix and age like milk)

Hydrogen is still behind battery EV's in terms of development and infrastructure. Give it time.

The idea of using hydrogen to power cars is nothing new but as others have said more development is needed and the infrastructure is not there.

I'm trying to recall exactly how it was done in 7th grade science class but it was a mixture of lead (small fishing weights) & acid, probably hydrochloric. That probably isn't very efficient but from what I remember a balloon filled quickly when put over the test tube.

There's also the heat given off, that could be used for any number of things.

However I'm not a chemist & it has been about 30 years so feel free to disregard that.

Coming back to reality, replacing tried & true with something for which no infrastructure exists, that's decades off if at all.

Last month DOE Ames Lab announced ambient temperature-atmosphere dehydrogenation Liquid Organic Hydrogen Carrier (LOHC) breakthrough. ( see https://www.greencarcongress.com/2022/02/20220228-lohc.html&source=gmail&ust=1647099407830000&usg=AOvVaw0ETUol2scBF00kMNMxENW N"> https://www.greencarcongress.com/2022/02/20220228- lohc.html ). This is a game changer that promises to catalyze "The Hydrogen Economy" promoted by Jeremy Rifkin. It's important not to conflate fuel type hydrogen with gasoline/diesel. LOHC enables hydrogen as an energy carrier for our renewable electric power complex similar the ATP unit of chemical energy carrier essential to biological systems (pull out your high school biology textbook).  Both ambient LOHC and ATP use catalytic energy boosts which facilitate storage and means of distribution (LOHC can use petroleum transport infrastructure).  Our crises of energy independence and global warming suggests a Manhattan Project-like effort is needed to accelerate neo-biotech research to take advantage of this breakthrough. There are a small number LOHC technology EU and Southeast Asia companies and national initiatives (google search LOHC) which have a head start. Write letters(email) to Congress and the President who to direct their energy focus to a hydrogen economy.