Hi all. I've been hypermiling since 2006 in a variety of cars - Honda Insight, Nissan Maxima, Honda Civic, and even my wife's Toyota Rav4... all of them with stick shifts ( including my Yaris )
What i've noticed over the years is that each vehicle likes certain things, and it's on you to figure it out and develop a special way of driving the car. Lemme tell you a bit about the other cars' designs and then i'll talk about the Yaris..
My '98 Nissan Maxima had a 3.0L motor. It was fairly aerodynamic, but the drivetrain drag was the biggest fuel suck. The strategy for optimal fuel economy was to stand on the throttle and then coast in neutral as much as possible. Engine braking was very helpful too, as the engine uses a significant amount of fuel while idling. My fuel economy record in that car was insanely impressive - i would regularly get 32-35 mpg highway doing 60 and conserving momentum as much as possible in the city. One day in perfect conditions, i got 40.5mpg over 6 miles! this is on a car rated for 27 highway!
The 2000 Civic EX seemed to be well optimized across the board and the best technique for driving it was just to go slow and be in the highest gear possible at all times. My record run was 47mpg. There is not a lot of room to exceed stock fuel economy..
The 2001 Insight had a dying hybrid battery and i regularly saw MPG in the 50's to 60's. Because of it's diminutive size, it LOVED being behind semi trucks. I could see 80mpg-90mpg for the sections when i was a car length behind a semi. Drivetrain loss with the 1.0L motor was ULTRA low, so there was rarely a reason to pop into neutral - also, you'd reduce battery charging, which you absolutely needed.
2001 Toyota RAV4 2.0L AWD was non-optimal in a lot of ways. AWD gives you increased drivetrain drag, so the Maxima trick needs to be applied. The car is also an aerodynamic house of horrors. One has to combine the trick of going slow and disengaging the drivetrain as much as possible to get a maximum of... drum roll... 35mpg.
Now, what about the Yaris?
I'd say that the Yaris is much like the Honda Insight, in that it is well optimized across the board. The engine is exceptionally efficient for it's size ( major hats off to Mazda ). The aerodynamic footprint of the car is similar to a Prius. Just the other day, i parked next to a Gen 2 Prius.. the Yaris is roughly the same height, but about a foot narrower! The front end of the Yaris is less aerodynamically ideal than the Prius, but overall i'd guess that the cars are close in terms of their CDA.
This car doesn't seem to mind whether you keep it in gear or not. It also likes being lugged around 1,200-1,500rpm. Once i hit 40mph, i'm in 6th gear whilst in the city. If i need to pass someone, i might just wait for a perfect opportunity rather than downshifting. Engine braking does not yield much improvement in the fuel economy versus idling your way to a stop light.
This car is extremely two sided. On the highway, you will see fuel economy numbers that rival modern hybrids if you simply drive at 60.. 65mph.. thanks to the car's excellent aerodynamics and drivetrain.
In the city, the curb weight of the car doles out cruel punishment. It is critical that you maintain momentum, as this car is just as heavy as other vehicles in it's class. That means anticipating stoplights and rolling to stops and gradient braking as much as possible. Without these driving tricks, it's easy to get 20-30mpg city.
I actually picked this car over a brand new Honda Insight when i compared the highway fuel economy numbers in real world driving. In my test loop, i handily beat the Insight by +10mpg. Recently, MPGOMatic hypermiled a new Insight and only achieved 68mpg at an average speed of 40-50mph. My record at a steady 60mph is 69.5. I believe that this car could exceed 80mpg in the right conditions.
Mazda, err, Toyota left a few optimizations on the table when they designed this car that could be used to increase the fuel economy:
1) The car has a very large oil volume relative to the size of the engine ( hence the ~7500 mile oil change interval ). Unfortunately, this leads to longer warmup times and poor fuel economy in short winter trips. A variable grill flap could have been installed to counteract this and would shorten warmup time significantly. Luckily, this is fixable with seasonal application of duct tape, or preferably a foam insert.
2) The underbody wasn't given much attention as the Mazda 3 or other larger cars. There is a lot of room to improve here.
3) Weight reduction would be extremely helpful. I have not analyzed this yet because of worries of getting caught in a warranty trap with Toyota.
4) The rev hang programmed into the car is horrific and possibly a culprit in the substandard city fuel economy. This car wastes quite a bit of gas going nowhere in neutral while shifting.
So yeah. I believe that this is a fantastic hypermiling machine with a few flaws. An 'eco' version of this car would butt heads with hybrids.