Gas Mileage Bad on Long Trips.... WHY?

How fast? Hills?

 

Wind direction and elevations will affect MPG's. I take it the return trip was better ?

 

Have you had anything done to the car prior to noticing lower MPG's. Say like an oil change ?

What's the oil level at and what are the tire pressures. Did you add luggage/weight to the vehicle or have passengers ?

 

First off, you have a North American generation 2 Prius. This means you have the wonderfully hated gas bladder. YOU DO NOT HAVE A GAS TANK. This means you cannot do the calculations based off of mileage driven per tank, gallons pumped into the tank, miles until half a tank, or any of that. Your "tank" is variable. So when you fill it up all the way, you might only have 70% the capacity of the full bladder on 1 day, 90% the other day. If you have a fold in the bladder, might be half capacity. If you are driving up/down large elevation swings, the pressure can effect it. If you are filling up in really cold or really hot weather, it can effect it. You just cannot guesstimate at all how much fuel is in the bladder.

What does the MFD say? That is the only reliable place for average fuel economy. Also you took 1 road trip. Very small variations in weather, road surfaces, ambient temperature and all that will make a big difference. Also very slight topographical changes make huge differences on average fuel economy. Going from here to Denver is a 8000ft to 5000ft change over 60-70miles. I can get 60mpg easily going 80mph or more. Going from Denver back to here is now uphill even though it looks mostly flat with a couple of actual up and down hills. 40mpg is more normal at traffic speeds of 80mph. There is nothing wrong with the car, it just is a change that is almost unnoticeable but it is there.

Wikipedia the city names of where you started vs. where you ended. Usually the elevation at city hall is in the page somewhere. Even a few hundred foot elevation change can make a big change in one direction versus the other, and higher speeds make the problem almost exponentially worse.

 

Lots of things affect MPG differently, but high speed in the Prius does not equal high MPG. The difference between 60 MPH and 72 MPH can be quite a lot at times. There are other factors too that affect MPG such as wind, temperature, air pressure in tires, road type, road grades, wet or dry pavement, climate control settings, altitude changes and so on. But speed is always the major factor. What you reported does not surprise me in the least. I have seen similar numbers if I drove our Prius fast with the cruise especially if there are some hills.

Also, remember that your car has the bladder. Some tanks just last longer than others due to the way the bladder works. YMMV.

Ron

 

it's the 72 mph. 40mpg is about right, but it could be a little higher. there's no way to really know.

 

The gen 2 oil wants to be 2/3rd's up the dipstick for maximum mileage, but on a road trip I wouldn't take the chance of burning oil, the tire pressure can effect the overall mpg's, if it's 32 vs 42 pounds, is per chance your emergency brake dragging, how dirty is your interior cabin filter, and how dirty is your ice filter.
37-42 mpg's at 70+mph is good, better then any other car on the road, a TDI might give it a run.

 

current mpg indicator is not really accurate due to varying road and weather conditions. there's no way you'll get same mpg's at 72 as at 55 under the same conditions.

 

I don't think so. Even at 90mph, I don't see 40mpg...

 

really? what do you see at 72? i know bob w. has a chart here somewhere.

edit: just found it, looks like low 50's at 72. dang, these cars are efficient!

 

How many miles do you have on your car? Have you kept up on maintenance? Spark plugs, belt, Throttle Body Cleaning, clean air and cabin filter? You are using the ICE more at those speeds and will see a greater affect on MPG if those things need to be addressed...

 

It would be helpful if the OP filled out the questionnaire that cwerdna posted here on Prius Chat. That could give us a lot of information whereas maybe we could pin down the area that could be causing the problem. I for one am very curious about the air pressure's on the tires as well as the tires themselves. It is well documented that tires can affect the MPG drastically on the Prius. There are many strings to attest to this.

Fuel economy complaints/queries? Please copy, paste & answer these questions, esp. if you're new | PriusChat

 

I usually get less mpg every time I use cruise control. At 72 mph, at that speed it will also hurt your overall mpg. And as mentioned before, road terrain and traveling weight are factors to consider.

 

What I find is a variance in MPG based on the gasoline used, so when traveling it can vary from tank to tank. I've become a big believer in using Ethanol Free gasoline, we get around 5-mpg more using it. (approx 51 vs 47 summer)

Part of the difference may be lax gasoline standards in Louisiana where I live. We were in California once and couldn't find EF, but found the gasoline containing ethanol got mileage as good the EF at home.

 

Check the fuel economy ratings for your car, it's likely to have better numbers for lower speed city driving.

 

We all have different experiences. I drove 250 miles to Little Rock from TN in the night, 80 degrees out, cruising 77 most the way with A/C on, and got 50.1 MPG there. On the way back I got about 48 MPG because of the wind, heat (95 degrees), and extra traffic.