Has anyone else noticed the effect of weight on MPG?

Added weight always affects mileage but what you experienced is a bit more than I would expect. Of course there were probably more variables a work here then the weight. Air temp, wind direction and velocity, wet roads, amount of traffic on the route and many more things can all have their effect on mileage.

 

I routinely get 47 MPG in my '09 when empty, that I get 42 MPG on the same roads with my Prius filled with computer gear. A 17 foot ladder and 4 18 gallon tubs of wire, parts, routers, etc. say 400 lbs. It all fits insde, so it is not increased wind resistance.

 

I don't feel like having or not having passengers has any measurable differences.

If you pack in 4 adults with luggage, then you have a different story.

The things that seem to affect the most is whether your running the air or not and how cold or hot you run it.

If the weather changes, it can toss several miles per gallon...
Cooler temps cause the engine to come on more to keep its temp up.

I've also found that all gas is not created equal....I avoid the cheapy places to buy gas if possible.. the car runs great on cheap gas and 10% ethanol, but if I can get it at the same price at a standard station I do.

Paying too much more for gas starts causing diminishing returns and is not worth it either.... High octane gas "does not" get better mileage in a prius.

Paying a premium for non-ethanol gas has diminishing returns too.

 

I realize that there are many variables, but I was frustrated when I only got 50.2 mpg on a 400-mile round trip to Kansas City this last weekend when I had two other people in the car. A couple of weeks prior, I got 54.9 with the same temperature and similar wind conditions...

Bryan

 

I had the same experience, two different trips of 150+ miles with 4 of us. I got several less mpg compared to the same trips when it was just my wife and I.

I believe that, under ideal conditions, the Prius is so efficeint that when conditions are less than ideal there is more of a difference than with other cars. That goes for effects of weight, weather, speed, terrain, etc. Of course, on a lower mpg car, the same percentage change wouldn't seem like as much of a difference, even though it is.

 

I got 62.7mpg one day going to work on my hour drive in and close to it a couple of other times... here lately, I've only managed about 53 -54.
I think its because I'm not trying as hard and just driving and enjoying the car.. but its been colder in the mornings too.

 

Exactly right... if you do something that knocks away 5% from your mileage, the higher you are when you started, the farther you fall when that 5% hits you.

 

Weight definitely makes a difference.

Last weekend, I made two round trips from Austin to Temple, a distance of about 60 miles, mostly on I-35.

On the trips up and back with the car loaded with model railroad equipment, I got around 46 MPG. On the trips up and back with the car empty I got around 52 MPG.

The weather was the same and I used the Dynamic Radar Cruise Control all the way on the highway. The only difference was some heavy traffic at the start of the first trip up to Temple (with the car loaded).

 

Adjusting your tire pressure will help offset some of the effects of the increased weight. When you're going to have a heavier load, add a couple of PSI to all 4 tires. It will help.

 

And of course, recall that the goal is NOT high MPG numbers, but to save gasoline. Four of you in a Prius is saving a lot of gas compared to two each in two cars, even if both are Prius.

High MPG may be fun, but reducing gasoline usage is the goal (If you goal is to save money, all the more reason to take one car) High MPG is a way of keeping score, but it does not effect the costs, or effects of Prius ownership. (I am doing 'better' getting 42 MPG on a 2 miles route, than I am getting 47 MPG on a 5 mile route to the same place)

 

This is exactly right and should be repeated all the time. I often have to remind myself that the goal should not be the best MPG, but the best *efficiency* and lowest overall consumption. It is hard to resist to take a longer route that yields a slightly better MPG when you try to hypermile, but you have to remember that you actually consume *more* gasoline this way, despite the fact that the number on the screen looks better at glance. And it is the same story with passengers. The hypermiler in me cries when I see my MPG falling, but it is *much* more efficient this way than taking two cars… Seems simple and obvious, but it is hard to resist the “MPG urge†sometimes .

 

This is why commercial airplanes post fairly impressive mileage numbers. They burn a lot of fuel, but they also carry a lot of passengers.

Tom

 

Yay, that's very true - The fully loaded Boeing 747-400 gets astonishing *91MPG* (yes, 91 miles per U.S. gallon) per passenger, if I remember correctly.

 

EVen that is subject to error. Which direction was the wind blowing from?

If you were headed North and the wind was out of the North and then you headed back with the wind still out of the North, you would get improved mileage..... or any variable thereof.

 

MPG per passenger is a standard metrics used to measure the consumption of airliners and it is a value of merit, used to express the consumption of a vehicle to carry one passenger for 100 miles at given passenger load. It is the MPG of the plane multiplied by the number of passengers aboard. Fully loaded Boeing 747-400 eats about 440 gallons of kerosene per 100 miles, so that gives you approx. 0.227MPG. Because 400 people are aboard, it is 400x0.227 = 90.8MPG per passenger.

If your fully loaded Prius with 5 people aboard still gets say 45MPG, this would yield 225MPG per passenger. When you are driving alone at say 55MPG, you only get 55MPG per passenger.

Average MPG per passenger of a given line is a very significant value watched closely by every airline, because it is one of the main factors determining the profitability of the line.

 

Mass (weight) affects MPG through energy changes. Energy changes in this case are accelerating (decelarating is energy change too but does not demands energy stored in fuel) and hill climbing. In other words: more stop and go's or many hill climbs - grater effect of mass on MPG. On straight route with constant speed this effect should be negligible. Additional mass on rolling resistance should be negligible too.

 

Weight is a factor that is why all my four tires are inflated to 44 psi, max sidewall pressure on my new Michelin Energy Tires, 195x15x65.

Also, wind has more of an effect that weight. I would rather carry 2 extra passengers than drive alone against a 20 MPH headwind.

When we went on vacation a few weeks ago we were fully loaded, I believe 3850 lbs. My wife, son, and plenty of camping gear.

We still averaged over 48 MPG, calculated by miles driven and gas used.

This was a lot of driving at 65 -70 MPH and against a 25+ MPH headwind, westbound, on the way back from Joseph Oregon via interstate 84.

Without that headwind we would have averaged over a true 50 MPG even with max weight.

alfon