How can I be sure my low mileage, isn't car related?

Can you give us some more detail? How long you have had the car, how you drive it, what the weather has been like, any and all things that you might have done/have happened to the car in recent memory, etc. would all be useful.

Having just bought a Prius and getting low mileage is one thing, usually attributable to either the driver or conditions, or occasionally measurement error. That's pretty easy to sort out. A car that suddenly loses mileage is another matter.

 

A car swap would be a straightforward way to answer this question. Do you have any friends with Prius?

 

Only 70-75? Why is your mileage so low? I routinely get 85-110mpg driving 70-80mph, uphill, with the ac blasting at full.

 

After four years it's not a lemon. The problem is the heat; your MPGs will improve as the temperature drops. The same thing happens with mine in Dallas.

Note that you *should* use the A/C in hot weather because it keeps the battery system cool.

 

I figured either that or you were driving with the brake fully depressed! I drive mine with my 2 tons of gold bars in the back.

 

Nimrodgrl: You in the middle of Prius country USA. Between the members here on PC and your contacts out there, you should be able to find a really expert Prius service. If it's that big of a difference and you can't pin point any changes that would affect mileage, since it's a 2004, it may be time for a good once over by a reputable Prius service shop. One with a dynamometer. It may be worth the ounce of prevention. Any good Prius certified service shop should be able to evaluate your car, especially since it does not seem to be an intermittant problem. And if they exist anywhere you should have one around your area. Sometimes I think CA must have half of the Prius' in the US. If they give it a clean bill of health, then you can focus on driving habits. We could speculate til the cows come home and not help you. That's my suggestion.
That plus 2 cents will get you, well! gee! nothing anymore.

 

I could be mistaken, but my understanding is that if you are letting it get too warm in the car, the traction battery might be running too warm as well. This could make the control system reluctant to charge the battery and/or discharge the battery when it should.

If what I just said is a load of hooey then I am sure (or hope) that someone more knowledgable will correct my statements.

I don't think it sounds like you are purposefully avoidng using AC and "driving like a Spartan", but if you are it might be counterproductive.

Another ventilation related issue is to make sure you don't have anything obstructing the battery's vent into the cabin (beside the right side passenger seat.) Doing so will make the battery run warm and prevent it from being used efficiently.

Is the car garaged or is it sitting in the sun? (And most importantly did this change with regard to previous years.) I expect a hot interior (and therefore a hot battery) to get worse mileage on short trips in the summer.

If you can think of anything that changed at about the same time you noticed the mileage drop, it might help...even if there appears to be no reasonable link at first.

 

Have you been getting all of the scheduled maintenance?

 

When was the last time the 12 V battery was checked or changed? I've seen some anecdotes about marginal 12 V batteries giving poor mileage. On other vehicles my batteries always show their age either when the weather first gets hot, or when it first gets cold--never during mild weather, it is the extremes that produce a stress test that fails batteries.