Mileage dropped after oil, filters, & tire change. Or did it?

One piece of advice is that replacing worn tires with new tires is almost sure to give an actual, temporary, but fairly substantial mpg reduction.

I suspect the actual amount depends very much on the particular tires in question, but this general effect is widely reported, and one for which by odd circumstance I have pretty direct evidence in my own experience.

Warning--non Prius data here.

I own and Audi A4, which when new had Pirelli P6 tires. I live in a town (Albuquerque) for which much of the Northest Heights is built on a gradual slope, so that several roads I travel have stretches for which the slope in moderate weather means the car holds nearly constant speed when coast near the speed limit.

So, when I first replaced the original P6 tires with the exact came model of P6 tire, I had the chance to observe that on those places where just before the change my car had been slightly speeding up in coast it was now slowing down in coast (no, the air temperature had not changed).

I don't know how much of your observation is explained by this alone, and I also don't know whether your new tires just have considerably higher rolling resistance inherently than the ones you discarded, but suspect both are true, and may explain at least part of your loss observation.

By the way--is it colder now? All by itself that is a powerful mpg loss factor.

 

Hi 7Lowe. The first thing that you have to understand is that the Prius engine does frequently continue to run, even when no power is being delivered to the wheels. And during these times there is absolutely no indication of this shown on the energy monitor display (and all this happens even when the engine is fully warmed up). This unfortunate mode of operation is here (unofficially) referred to as "hybrid stage 3a". Read about the various stages of Gen2 Prius hybrid operation here: http://www.techno-fandom.org/~hobbit/cars/five-stages.txt

It's the tires. Almost for certain it's the tires. Trade them in for something with known LRR qualifications (eg Bridgestone Ecopia EP100), and your fuel consumption will go under 5.0 L/100km. Sure you may lose some money in the trade over, but it will save you a lot of money in the long run.

BTW. Also make sure that you don't overfill the oil, and use a good quality 5w30 synthetic.

 

Is it possible that you used a higher viscosity oil that it had before?

Other than that I believe it's your tires. Perhaps you can try higher tire pressures. If the ride's not too uncomfortable you should be able to reduce the consumption a bit.

 

The type of tyre makes a BIG difference to the fuel economy of a car. People just don't really notice in a traditional car where a drop of 1 or 2 mpg happens. On a Prius that can work out about 8 or 10 mpg which is significant.

Want good fuel economy? Buy a quality low rolling resistance tyre. I have the A/A rated Bridgestone EP001S's on mine and they're amazing. They grip in the wet really really well and the car just floats along. Being from Europe you are obviously familiar with the EU tyre rating system that's been available for nearly 2 years, and perhaps in future you could consider refering to it next time you're consider a new tyre purchase. It would save you time and money.

Tyre labelling | AA

 

As GC stated above the tyres must be LRR tyres to get the best fuel consumption figures. You state you have fitted "run flat tyres" these are definitely not low rolling resistance, in fact they are about as bad as you can get. They are also very heavy compared with ordinary tyres and this also makes a difference.
As others have stated fitting a set of LRR tyres will pay for themselves in fuel saved. You may be able to get your tyre supplier to give you a partial refund on the tyres you have or sell them on to say a BMW owner. If you need information on European tyres read the thread linked below.
Changing tyre size to 205/55/16 will save you money on the price of the tyres and will also improve the cars handling and feel.
Low Rolling Resistance replacement tyres (Europe)

John (Britprius)

 

I'm wondering if he has a bad 12v that has been killed off by leaving doors open when the car was serviced.

 

GC it is a possibility, but he does state he bought run flat tyres. If you look up the RR of such tyres they are terrible. They also weigh so much more and this weight is concentrated at the outermost diameter "the tread" where it makes the most difference with inertia.
Unfortunately for the OP they are also very expensive.

John (Britprius)

 

I was refering to the issues the OP was suffering with the engine continuing to run. Though perhaps the OP would car to elaborate. He does seem to have a marvelous grasp of the English language too.

 

But he cannot spell tyres.

John (Britprius)

 

I was going to suggest that he has an amazing grasp of American English and its spelling, but though better of it.

 

The EU tyre rating system gives information "based on a standard test" (1) for the fuel economy of tyres "rolling resistance".
(2) The wet grip of the tyre. (3) The noise level of the tyre.
The first two are graded "A" to "G", "A" being the best. The noise level is given in db's the lower the number the quieter the tyre.
This information must be displayed at the point of sale usually by a label n the tyre. This allows customers to be able to choose tyres on real results, and not the manufactures hype.

John (Britprius)

 

There's usually a big sticker stuck on the tread of the tyre, which I understood is a requirement of the new ratings. Obviously the sticker is removed before use - or should be.

 

Runflats are quite heavy, and have up to 20% more rolling resistance than non-RFT

 

I left mine stuck on, just to show off my green credentials.

Just joking, we don't even have those stickers here. I wish we did have those EP001s's though, the best I could find locally were the EP100's. They still give really good LRR performance though, and the tread pattern looks almost identical to those EP001s's.