I started March 23 2009 with a 2009 Prius Touring, and read this site to get the best tips on driving with great mileage. Naturally, I wanted to eventually get it into the 50's as others have done. Tires were inflated to 42-40. Wasn't carrying boat anchors in the trunk for added weight. Didn't really use the AC much. Thanks to FireEngineer, I had a block heater for colder nights as winter approached. At last fill-up Dec. 19 2009, my lifetime mpg was 43.78 mpg. Disappointing. So, I decided to be happy with what I had ... 44 mpg still beats the Taurus I traded in by quite a bit. Stop monkeying with it. Winter hurts everyone's mileage anyway. Well, with winter showing itself somewhat in Michigan, I put on 4 Michelin X-Ice snow tires, using 15" rims instead of 16, and left the tire pressure for the time being at the 35-33 the installer set it at. So, to test them, I drove a 30-mile circuit I've used other times I just wanted a ride in the country, eg, on my (former) motorcycle. Roads not straight or level. Mid 30's temperature. Turned the heat up to be toasty worm, and ran the seat heater until that was comfy too. I turned the mpg screen off ... I'm testing new snow tires, and their extra rolling resistance should drop my mpg to even lower than I've had in the past. Why watch even lower mpg present itself. Used the cruise control for most of the drive. As in the past, I did let it coast down to some stop signs etc if no one was behind me. Got home and looked at the 5-minute bars. Except for start-up, all over 50mpg on the computer. Some at or approaching 75 mpg. Huh??? In the past, on that route, I never got out of the mid-40's for mpg in the Prius. Never seen numbers above 40's before at all ... even on flatter roads, with no traffic, with regular tires, warm temps, and trying to glide etc guided by the mpg screens. So, today I made the same run again ... roads were slick, so some of the cars in front of me were pretty uneven as to their speed. Should have hurt my mileage. Got home. Same computer results as yesterday ... 50 - 75 except for the first 5 minutes. I'll refill the tank tomorrow and start clocking it for real. But I do feel as if I've driven into ... the Twilight Zone of mpg.
Does your new tire/wheel combination have a different outside diameter than the old one? That could be the difference. You can check your revolutions per mile for each tire on a site like tirerack.com.
+1 here, your odometer and speedometer may now be off a bit. What tire sizes, exact numbers now, did you have before and what are they now? - D
Good question, and one I double-checked right after the first set of high readings, thinking that had to be the cause of the increased mpg. OEM tires: OEM Turanza P195/55R16 New Tires: Michelin X-Ice P185/65R15 I repeated the calculation I made before purchase, using the comparison tool quoted elsewhere on this site -- the one at miata.net/garage/tirecalc -- Tire size calculator Results: Specification-Sidewall--Radius--Diameter--Circumference--Revs/Mile 195/55-16----4.2in----12.2in----24.4in---76.8in----------825 185/65-15 -- -4.7in----12.2in---24.5in----76.9in----------824 -------------Difference 195/55-16---0.0% 185/65-15---0.1% And so, the difference is so small it's can only explain about two mpg in 30 miles. Have to figure out where to look next for an explanation. Maybe the car's computer figured this was a good time to go nuts. But I'm not complaining if real world calcs bear it out, and I admit I'm still a skeptic!
First, congratulations at being at the unveiling of the 2010 Generation 3 Prius! What an opportunity. I'm exactly with you on believing it after I run a few tanks to see what comes of it. Hence, my prior comment -- "I'll refill the tank tomorrow and start clocking it for real. But I do feel as if I've driven into ... the Twilight Zone of mpg." I've set up the usual spreadsheet to calculate mpg for individual fill-ups and lifetime mpg, and I'll continue on with it and see what comes of it. The only other things which I can think of as having changed are that: (1) I didn't put tire pressure sensors on the snows, so I get the little light on the dashboard and (2) the odometer just passed 5,000 miles [I drive a convertible in summer] -- and maybe it became "broken in" all at once. But I doubt that either of these explains the difference in the computer's calc of mpg; I take my cue on things like this from Sherlock Holmes speaking to Watson: "How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?" And, if the mpg really is up there .... who knows ... I may run snow tires all next summer regardless of the wear !!
If it is a 30 mile route that you use often then does the odometer measure the same distance as it did previously. That's a better idea of tyre outside diameter than calculating it.
Interesting situation; MPGs in warmer months ~45, MPGs in colder months ~50, contrary to all expectations. (I take it that speed on your test route is 30-40 MPH, with overall average speed reduced due stops signs, etc.) I take it you are not complaining. On my normal commute in 23 degF weather, I'd be overjoyed to get 51 MPGs. It looks like the new tires aren't the cause. What then? It may be something as simple, but easily overlooked, as a change in the conditions or your route. Say the addition or removal of a stop sign or other cause to come to a full stop. Strange. To get max MPGs the car needs to be in what is called Stage 4, S4, which reflects the engine and cat converter being fully warmed up. Also to get to S4, you to have gone 35 MPH, and the engine needs to come to a stop on its own while the vehicle itself is stopped. If neither of these last to conditions is met, the car stays in S3, a less efficient mode of operation. It is possible to drive around all day and not get into S4. It may be your driving style. As you get more familiar with a route, you can anticipate small variations inroad conditions and "set up" for best MPGs. Finally, would you accept magic or divine grace as an explanation?
And Rae Vynn wasn't the only, or the most fortunate person to make it to the unveiling!! :cheer2: Some people went to Detroit for the unveiling from well outside North America. Anyone seen the Flying White Dutchman lately? :focus:
Good point. unfortunately, I never before worried about the exact distance of my test route ... it's very roughly 30 miles but not nearly exactly so ... I just drove it for pleasure when I didn't want to have to concentrate quite as much on where the road turned next etc. For example, lots of trips on my (former) Triumph motorcycle when I wanted a short and pleasant ride at night. Also, I'm not sure I had even driven that route with the Prius, much less tracked the mileage. What got me was that when I drove that route to see what my new snow tires were like, I found I had sudden mpg readings above anything I ever saw before (previous lifetime average below 45 mpg). I'm left looking at the diameters of the two tires, which vary by a tenth of one percent, and the revs per mile, which are off by 1 rev mile (824 vs 825). I should also note that the test route was just that, and I'll be driving to accommodate my normal driving needs so that I can see what that brings.
Well, the touring tires/wheels are well known for taking off at least a couple mpg and maybe the Michelin X-Ice tires are better than expected in rolling resistance. Michelin tends to make some nice low rolling resistance tires, and usually are fairly light, too. I know people with Nokians often report similar fuel economy or sometimes even better than with the stock 15" tires. Would be interesting to do the same test back to back with both sets of wheels/tires.
"I'm not sure I had even driven that route with the Prius, much less tracked the mileage" So maybe it's just that other parts of your total driving pull down the average from your commute? Give it some time.
We already know form other members with the 17" wheels that a hit of 3 to 5 MPG is not uncommon. So if the Michelin's actually improve the rolling resistance over the OP's OEM 16" tires and the difference in tire weight and rim weight are significant then why couldn't he see a 6 or 7 MPG improvement when all things are considered. I bought the 2010 IV rather than the V to get the 15" wheels even though I like the look of the bigger wheels. I wanted the better fuel efficiency rather than the look of the 17", I did however replace my OEM wheels with aftermarket wheels but stuck with 15".
I have been pondering the same issue when it comes time to change tires. I consider the 16" touring tires too wide but I do not want to spend big bucks on new aluminum wheels. A narrower tire that is a shade taller should improve MPG!
I may have missed it, but did you post what speed you are going on this route? In my experience, the Prius does better when under 50mph. Also, if you've never tracked mpg on this route before, hard to look at something like the tires as the reason why mpg increased. But hey, I'd take your results!
The previous lifetime MPG wasn't horrible but not great either. So I'm starting to think there must have been something slightly wrong with the previous setup that got fixed with the new tire install. Interesting stuff anyway! - D
I know that since I put the 16" touring rims and tires on, my car is not doing very well mileage wise at all when compared to how it was doing in similar temps last year. Looks and drives a lot better though.
