Fuel Economy Measurements
Prius fuel economy depends on different variables.
First one needs to understand different factors involved in it
LIFETIME is the most useful measurement. Total kilometer driven, divided by total liter consumed, informs you how efficiently the car has performed overall.
TANK is the measurement between each fill up. You press the RESET button when the tank is full. The results are informative, but not perfectly accurate. In cold weather, the bladder inside the gas tank shrinks. This reduces the overall capacity making the "full" level variable. Also, "full" can be misrepresented if the pump doesn't shut off at the proper time. These factors make calculations based on fill-up less accurate.
TRIP is mostly for fun, since a multitude of variables can affect the measurement to a single destination. Watch the 5-minute summary segments shown on the multi-display. Remember though, if you were to start a drive downhill, with a tail wind, a warm engine, a fully charged battery-pack, and a warm outside temperature, the KM/L would be very impressive for that particular trip. But then if the return trip back was uphill, against the wind, with a cold engine, a drained battery-pack, and a cold outside temperature, the KM/L would appear very disappointing. In summary, trip results can vary greatly. The overall average is what really matters.
Tire pressure may have gone down. For every 10 F degrees colder, pressure will naturally decrease by 1 PSI. Verify you still have as much air in the tires as you think they do.
Increased use of the Heater or A/C (which includes the defroster) will force the engine to run more often. Try a less demanding setting.
When the temperature drops very low or below freezing, the engine has to run quite a bit longer to create heat for the catalytic-converter. This is to keep the Prius emissions extremely low. Avoid driving short trips; instead, take advantage of the time after warm up is complete by running several short trips at once.
A dirty engine air-filter will also cause the KM/L to drop. Check it routinely, especially as the seasons change. Once flow becomes visibly restricted (dark colored rather than white), the time has come replace it.
Don’t forget that new tires require a break-in period. Before that the tire surface and tread edges will be rough, causing KM/L to be lower than you expect. It takes about 1,600 km before enough wear (barely visible to a trained eye) occurs to allow less abrasive contact with the road. And since front tires wear more than those in the rear, expect another break-in period the first time the rear tires are rotated to the front. Fortunately, that reduced KM/L will only last a few hundred miles.
Increasing Fuel Efficiency of Prius
A/C & Heater:
Minimal use is the key. Using the Heater or the A/C (which includes the defroster) on anything but a low setting may prevent the engine from shutting off. That will reduce efficiency. So, try to avoid high demand use. Fortunately, on the highway using the A/C is still more efficient than opening the windows.
On the Highway:
Just like with traditional vehicles, efficiency drops the faster you drive on the highway. 96 km/h is more efficient than 113 km/h. Speeding up to 121 km/h, you'll observe MPG drop even more. It pays to drive slower (obey the speed-limit). Think of it this way, pedaling a bicycle rapidly takes much more energy than pedaling at a moderate rate.
Brisk Acceleration is an often misunderstood benefit. There's no need to hold back. A gasoline engine works more efficiently when running at higher RPM, about 70 percent of maximum. Take advantage of that by getting to cruising speed quickly (but not aggressively, please drive safely). And remember, while the engine running it is also generating electricity for later use.
Coast whenever you have the opportunity. Using the feather technique helps. By lifting your foot lightly from the accelerator-pedal, you can invoke an efficient computer-controlled glide without decelerating much at all (less than 1 km/h). With good road conditions and a bit of practice, you'll find yourself doing this instinctively.
Look Ahead. If you see a light turning red or a need to slow down in the distance, there's no reason to continue holding the accelerator-pedal. Remove your foot and allow the generator to decelerate the Prius. That will increase your MPG, charge the battery-pack, and prolong the life of your brakes
42/40 PSI (2.9/2.8 bar) is what many Prius owners strongly recommend. The original tires for the Classic Prius support a maximum cold pressure of 50 PSI (3.4 bar) and for the Iconic Prius 44 PSI (3.0 bar). So that pressure increase is well within the design specifications. Many of the alternate tires available support a maximum cold pressure of 44 PSI (3.0 bar) too. Whatever you decide, just remember that low pressure results in a fuel efficiency drop and the tires wear out faster. Tires will not bulge like in decades past; manufacturers provide much better quality now which maintains a flat contact surface all the way up to the maximum pressure.
Every 8,000 km the tires should be rotated, for best lifetime performance. Rotation is preferred in a roll-back, roll-forward pattern.
Measuring the PSI should be done only when the tires are cold, since driving heats up the air inside the tires making the results inaccurate... giving you the impression more pressure is higher than it really is.
Check Often since temperature causing pressure to drop, 1 PSI for every 10 F degrees. Air will naturally leak out from normal use too.