Prius Owner Advice #2

Increasing MPG

Last Updated: Sat. 8/20/2005

Here's a few suggestions from Prius owners:



Driving     back to top

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 MPH).  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.


Tire Care     back to top

The pressure recommended by Toyota is actually too low.  That soft ride sacrifices MPG and tire life. 

42/40 PSI (2.9/2.8 bar) is what many Prius owners strongly recommend.  The original tires for the Classic (2001-2003) Prius support a maximum cold pressure of 50 PSI (3.4 bar), for the HSD (2004-2005) 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 MPG 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 5,000 miles (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 10F degrees.  Air will naturally leak out from normal use too.


Engine Warm-Up     back to top

Short Trips are horribly inefficient for all vehicles.  Prius is no exception; however, itís far more noticeable since the Multi-Display provides immediate feedback to actually show you the lower MPG.  The efficiency benefits of the system are not utilized until after warm-up is complete... that's engine, emissions system, and tires.  So try to run several errands at once to take advantage of an already warmed up car.

SULEV (Super Ultra Low Emission Vehicle for the Classic) and AT-PZEV (Advanced Technology - Partial Zero Emission Vehicle for the HSD) is what Prius strives to remain whenever active, even during warm-up.  That means the catalytic-converter must be kept hot even if that requires using some gas to do it.  Fortunately, you still get better than average mileage, even if the engine doesn't shut off right away.


Highway Speed     back to top

Just like with traditional vehicles, efficiency drops the faster you drive on a highway.  60 MPH (96 km/h) is more efficient than 70 MPH (113 km/h).  Speeding up to 75 MPH (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.


Measurement     back to top

The Multi-Display rounds digits optimistically, so it will usually read about 2 MPG for the Classic and 1.4 MPG for the HSD too high for the typical owner.  The "bladder effect" caused by the flexible plastic bladder in the gas tank (used to significantly reduce evaporative type emissions) shrinking due to temperatures below freezing, is very noticeable in the Spring & Fall.  That will cause both the value displayed and what you manually calculate to vary greatly.  So the best way to get an actual MPG is to average several fill up amounts written down at the each time stop to pump gas.  For an example documenting MPG, refer to this webpage...


External Loads     back to top

Hitch Racks & Roof Carriers cause a lot of aerodynamic drag.  Expect a MPG drop when you use one.


Air-Conditioner:  Heater & A/C     back to top

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 MPG.  So, try to avoid high demand use.  Fortunately, on the highway using the A/C is still more efficient than opening the windows.


5W-30 Oil     back to top

5W-30 oil is strongly recommended.  Real or Synthetic may be used. 

Change the oil & filter every 5,000 miles (8,000 km) for the HSD Prius.

Change the oil & filter every 5,000 to 7,500 miles (8,000 to 12,000 km) for the Classic Prius.

If a service person puts 10W-30 in by mistake, you may complain since it will negatively impact your MPG slightly and may affect performance in below freezing temperatures.  The text on the engine oil cap clearly states 5W-30 should be used.


Synthetic Oil     back to top

Owners have observed minor MPG improvements by switching to synthetic oil.  Also, since it is also known to protect gas engines better than real oil (the natural kind), makes extremely cold startups even easier, and only costs an extra $10 to $15 each oil change, switching to synthetic is a wise choice anyway.


Oil Level     back to top

Too much oil will decrease MPG.  Insure that the level is never above the "full" mark on the dipstick.  The ideal is about 1/4 inch below that.   

Unfortunately, this is a problem often not even realized.  Oil change services routinely pump oil from large barrels, rather than using quart-size bottles.  That makes overfilling extremely easy to do.  Taking a moment afterward to check afterward is truly beneficial, especially since damage can occur if the level is significantly above the "full" mark... which is suppose to be a maximum indicator.  Why waste oil when you are better off with less anyway?


"B" Mode     back to top

Avoid using this unless absolutely necessary (like mountain driving).  It will cause MPG to drop, since it reduces electricity generation opportunities.


87 Octane Gas     back to top

Prius was designed to run with 87 Octane gasoline (85 in high altitudes).  Owners have experimented with higher octanes, but found there wasn't any MPG improvement.  Also, keep in mind that higher octane gasoline may trigger an emission sensor alert.  Save money and continue using the inexpensive 87 octane gas.


Break-In     back to top

For the first 200 Miles (322 km):

  - avoid rapid deceleration (hard stops)

For the first 600 Miles (966 km):

  - avoid rapid acceleration
  - avoid racing (high RPM) the engine

After roughly 10,000 Miles (16,100 km):

  - enjoy a MPG increase, from the moving parts having loosened

Even at 30,000 Miles (48,300 km):

  - you may continue observe minor MPG increases as the car ages


Your Mileage May Vary     back to top

That simple statement about the EPA ratings shown on the new vehicle window sticker is often overlooked, yet it makes a significant difference depending on the type of driving you do.  Reading this quote provided by the EPA about HSD Prius reveals why: "Actual Mileage will vary with options, driving conditions, driving habits and vehicle's condition.  Results reported to EPA indicate that the majority of vehicles with these estimates will achieve between 51 and 69 mpg in the city and between 43 and 59 on the highway."

EPA tests are generalizations (performed under ideal conditions) intended to make vehicle comparisons easier, not to specify what MPG you will actually get.  In fact, they rarely reflect the MPG in real-world driving experiences.  The following explains how those tests are performed... notice how results can be quite a bit lower if you live in the north or if you drive fast: 

The fuel economy estimates are based on results of tests required by the U.S. Environmental Protection Agency (EPA). These tests are used to certify that vehicles meet the Federal emissions and fuel economy standards. Manufacturers test pre-production prototypes of the new vehicle models and submit the test results to EPA. EPA re-tests about 10% of the tested vehicles to confirm manufacturer's results in EPA's lab. The vehicles are driven by a professional driver under controlled laboratory conditions, on an instrument similar to a treadmill. These procedures ensure that each vehicle is tested under identical conditions; therefore, the results can be compared with confidence.

There are two different fuel economy estimates for each vehicle in the Fuel Economy Guide, one for city driving and one for highway driving. To generate these two estimates, separate tests are used to represent typical everyday driving in a city and in a rural setting. Two kinds of engine starts are used: the cold start, which is similar to starting a car in the morning after it has been parked all night; and the hot start, similar to restarting a vehicle after it has been warmed up, driven, and stopped for a short time.

The test used to determine the city fuel economy estimate simulates an 11-mile, stop-and-go trip with an average speed of 20 miles per hour (mph). The trip takes 31 minutes and has 23 stops. About 18 percent of the time is spent idling, as in waiting at traffic lights or in rush hour traffic. The maximum speed is 56 mph. The engine is initially started after being parked overnight. Vehicles are tested at 68 F to 86 F ambient temperature.

The test to determine the highway fuel economy estimate represents a mixture of "non-city" driving. Segments corresponding to different kinds of rural roads and interstate highways are included. The test simulates a 10-mile trip and averages 48 mph. The maximum speed is 60 mph. The test is run with the engine warmed up and has little idling time and no stops (except at the end of the test).

NOTE: To make the numbers in the Fuel Economy Guide more useful for consumers, EPA adjusts these laboratory test results to account for the difference between controlled laboratory conditions and actual driving on the road. The laboratory fuel economy results are adjusted downward to arrive at the estimates in the Fuel Economy Guide and on the labels seen on new cars, light trucks, and vans. The city estimate is lowered by 10% and the highway estimate by 22% from the laboratory test results. Experience has proven that these adjustments make the mileage estimates in the Fuel Economy Guide correspond more closely to the actual fuel economy realized by the average driver.

For more information, please refer to...

ALSO NOTE:  The EPA tests are all performed with 100% gasoline, summer-formula.  Owners using E10 (that's 10% ethanol, 90% gasoline) will observe efficiency about 3.4% lower than with 100% gasoline.  And during the winter months in the United States, the refining formula for gasoline is altered to help reduce emissions.  As a result, winter-formula is less efficient, making MPG lower than with summer-formula.


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