Comparison: 2 prius's 1driver very differnent MPG??

Hi Joe. I would try two more tests before worrying about the main battery.

1. Check the odo on the 2007 against the 2008 to make sure they are both reading the same (as this can obviously make a difference to the apparent fuel consumption).

2. Do a few manual fuel consumption calculations for each Prius (distance travelled and gas used at the pump) to make sure that your MFD figures are reasonably accurate. I just found out that the MFD displayed MPG on my 2004 prius is quite a bit less than the actual MPG, though apparently this is pretty unusual.
BTW. When doing "at the pump" tests it helps if you average over a few tanks to improve accuracy.

If the consumption figures are accurate then perhaps it really is that your battery is not as strong as the other one. Someone mentioned in another thread that if the auxiliary 12 volt battery is in bad shape then the main traction can waste a lot of power trying to keep it charged (through the inverter). Honestly I don't know how relevant that is but it might be worthwhile getting the little battery checked out first.

 

One's a touring the other isn't.

The touring will always take a bit of a hit due to the bigger wheels/tires.

Try swapping wheels tires for a while and see what happens. Also, as uart mentions, calculate the fuel economy manually for at least 3 tanks, preferably 5+ and double check that the odometers are calibrated.

What tires do you have on each vehicle and how much tread is on them?

 

Hi Joey...,

I concur with Drees. The 2007 Touring car has the 16 inch wheels/tires, which are wider (as air volume in the tire sets the load rating, and the load rating has to be the same or better between wheel/tire combinations). The wider tires have more patch area, better handling, and more rolling resistance.

People who swap out their 15 inch tires for 17 inch see even lower fuel economy.

 

Hi Joey....,

The newness of the tires supports the greater difference. My opinion is that allot of the new-car improvement in mileage is due to tire break-in. When I got Hydroedges at 5000 miles, it was like starting with the car from scratch. Although at 3000 miles most of the break-in should be over. Do you know any rolling resistance specs for these tires? Are they summer performance tires (softer compounds of rubber - more rolling resistance) ?

The comments regarding having difficulty getting the car to glide (no-arrows coasting in transmission D mode) or stealth (electric cruise) also support the tires as the source of the mileage issue. What road-toque is permitted to be powered by the battery alone is dictacted by battery SOC and temperature. So, its not complete evidence the tires are the problem.

 

I'm not totally sure but I notice a similar thing when I'm coasting/gliding. Most of the time the engine cuts off and I get 99.9 (actually I get 0 L/100km but it's the same thing) but every now and then it keeps the engine running while I'm gliding. Not sure why but it's certainly not every time. Are you saying that yours keeps the motor running every time you try to glide?

If I were you I wouldn't try too hard to make it engage all electric mode in regular driving. I've found I'm mostly better off to keep the charge on the battery and just let it use it for assist up hills or slow start stop driving. Having said that I can usually coax mine into all electric mode pretty easily on terrain that's flat to very slight downhill by just backing the accelerator right off for a few seconds and then re-applying the gas very lightly. This will usually make it engage all electric mode, at least for a while, especially if the speed is below about 40 mph. Like I said before though, I've found it's better not to coax it into this mode and run down the battery as the motor just has to work harder later to make up the lost charge. I like to use that mode only when the down hill is steep enough to give some gliding and some regeneration too, so the motor can stay off for a while but overall the battery is not run down.


Try driving it for a while where you're not really trying to get it into all electric mode and see what happens. You might find that by keeping more charge on the battery that it will start going into all electric mode more readily when you really want it to (as in where it's really useful like real downhill glide + motor + regen combo).

 

Comparing stock vehicles (meaning stock tires), the touring Prius is down a couple mpg compared to the regular one.

Now add sticky, ultra high performance summer tires like the Toyos you've got and subtract another couple mpg.

Voila - there's your 4-5 mpg difference.

 

Yep just take a look at the tyre test posted here last week :
http://priuschat.com/forums/gen-ii-...ity-against-energy-saver-ecopia-fuel-max.html

Up to almost 8% difference in just the limited range of tyres they tested there.

BTW. Can someone please help me understand why the touring Prius is generally a couple of MPG down compared to the regular package. I always thought larger diameter tyres would give a slightly lower rolling resistance. Are there some other additional differences that account for this?

 

Touring edition tires are wider (185mm vs 195mm) so there are more contact area with the road; therefore more rolling resistance.

 

Ok that's interesting. The tires on mine are 195/60R15, this is also what it says they should be in the specifications section of the owners manual. What else should I look for to see if mine is a "touring edition". I don't think it says "touring" or anything like that anywhere on the car. Maybe they specified the standard model to take 195 tires in some countries (I'm in Australia BTW.)

The manual says my car is : Prius 5 Door Hatch, 4A Base / Super White II. It's date of manuafture is Dec 2004 and it was first registered (first sold) July 2005.

When these tires wear out I think perhaps I should repalce them with 185/65R15 tires. According to my calculations 185/65 should by 6.5mm larger diameter too (65% of 185 is bigger than 60% of 195).

Can anyone help explain why my prius model was specified to have the 195 tires, I'm sure it's not the touring edition.

 

Yep, and the rims are bigger, too, which means that both the wheels and tires will be heavier in general. The overall diameter of the stock Prius tires for the Touring/non-touring is very, very, close to the same.

 

Sounds like they just speced slightly wider tires in Australia. The Touring models come with 16" wheels.

195/60R15 are only about 1% smaller [1] in diameter than 185/65R15 tires - not larger. But the difference is basically negligible. Some owners in the US use the slightly wider tire as they prefer the additional lateral grip those tires provide.

[1] Try one of these two links to calculate tires sizes:
Tire size calculator
Tire size calculator

 

Joey,

Since you are in Florida, I'm wondering if your Toyota tech isn't correct. If your traction battery starts the drive warmer, that would cause the car to use battery less until the AC had cooled the cabin, and the fan to the traction battery has cooled down the traction battery.

 

Check to see if your in stage 4 of prius operation. If your engine isn't warmed up AND you haven't stopped for 7 seconds while the engine is running and if the coolant is over 70c, then your not in stage 4.

 

A wider tire does not necessarily mean more of a contact patch with the road. Given two tires (same manufacturer and model) with similar aspect ratio and tire pressures the wider tire will present a wider but shorter patch compared to a narrower tire which will make a narrower but longer contact patch. In terms of area they'll be about the same but the shape of said patch will be different.

Think of the tire as a 'flotation device' - for tires of similar type it's contact patch will be largely determined by vehicle weight and tire pressure.

When Gordon Murray was designing the McLaren F1, that was one of the reasons he went for the 315's on the back rather than the 'standard' 335 or 345's that everyone else was going for - the longer contact patch gave better traction under very hard acceleration.

The biggest differentiators are tire weight (~17lb for a 185/65/15 and ~21lb for a 215/60/16 - using the Michelin Engery Saver A/S as a guide) and frontal area presented to the wind. The larger/wider wheels would probably weigh more too. Effective gearing would also be changed with the 15's having a 1.6" smaller diameter.

 

Joey,

I concur with your observations and mpg readings. My 2007 Touring model behaves just about as you describe and my mileages are right in line with yours:
(As a retired engineer I keep careful mileage logs.)

My lifetime average mpg @ 31,600 miles) is : 42.1 mpg

Where as my Nav Unit Consumption readout is: 42.6 mpg*

( * Note the Consumption unit was reset accidentally so this value reflects only the consumption for about the last 5,000 miles) .

BTW: I am pretty feed up with the "if you don't get 55 mpg it's your fault" criticisms.

Particularly annoying to me is the "feather footing". On my vehicle the difference in "touch" between "stealth" (electric) and normal mode is so slight I cannot actually feel any difference....most of the time. However, occasionally, rarely...weather??, gas??, how I hold my mouth??, it will feel like a different vehicle and I can actually engage the "stealth" configuration.

Also: I have similar hesitation/low acceleration issues starting from a stop and it is somewhat unpredictable (even at sea level; see next comment).

This caused some near death, hair raising start-from-a-stop experiences on a recent vacation to the high plains where the elevation was 6000 feet. It seemed, logically, even more pronounced.

I also experience this hesitation occasionally when trying to scoot through a yellow light (when I misjudge the timing.)


If you figure it out please let me know. I still like my Prius and plan to buy a later model.

Best Regards,

Harry

 

As a bit of random waffle, the wonderful State of California is considering banning the sale of black (and maybe all dark colored cars) due to the added burden of running the A/C more which decreased fuel efficiency... The goal is to have all colors available be capable of reflecting at least 20% of the solar engery.