A bit of advice would be nice

I have returned to the joy (and sometimes frustration) of have a Prius as my primary driver.

in 2011 I bought my first Prius, a 2007 II. About a year later I leased a new 2012 II. I understand the reliability advantage of the 2nd gens, but age is now a factor as well and I liked the way my 3rd gen drove better.

I recently purchased a ‘12 III with 135k on it and the solar roof package. Shortly after doing so I came across a ‘12 five with the advanced tech package and picked that up. Driving 30k / year I’ll get more benefit from the adaptive cruise (which works amazingly well) than the roof.

I live in a mountainous area and 90% or more of my driving is highway. The III is knocking out 48-52 MPG and the five is getting only 42-46. I have already ditched the 17s in favor of a set of 2019 Corolla 16s with 205/55 eco tires. I haven’t weighed them, but based on design I’m certain they are lighter than the 15 or 17” Prius rims, and they look great and immensely improved ride quality and noise compared to the 17s.

Should I swap the batteries, or are their some other things to look at first? I’m figuring that an average 6mpg difference couldn’t be fixed by any one thing besides the traction battery?

I drive speed limits (mostly 75 here) but am otherwise pretty gentle. I’m up in the air if I see any benefit in Eco mode or not. I’ll likely do a multi tank test to see if there really is a benefit.

Air filters are fresh on both but I haven’t gone any farther yet. I’d like to plan an approach with some sort of logical order to it instead of just attacking at random without any real direction.

Both cars run well and have no dash lights on. Although I haven’t done any real testing, I’d say the III feels a little livelier, but wouldn’t notice an issue with either if the other wasn’t present.

I did put a CarPlay deck in the five and I’m certain that has nothing to do with it. Very cool though with steering wheel controls and factory backup camera working.

I’m mechanically adept and plan to do as much as possible on my own. I have ordered a OBD-II device and am planning to use dr prius when it arrives unless someone has a better suggestion?

I appreciate any recommendations.

Thanks in advance.

 

I had that thought in my head. I wasn't giving it much credence based on the fact that with 15s or 17s the rated MPG is the same.

Adding in your advice and the fact that it's relatively easy, I think I'll try it. I've got a 1300 mile road trip starting tomorrow. I'll do the swap today before I go and report my findings on return. To be honest, I'm kind of mixed about what I'm hoping for. I really like the look of the 16s, but I don't want to accept a 12% MPG penalty for it. On the other hand I guess I'd rather it was this than something more significant. I could always get 15s that I like and resell my 16s.

Thanks for the push in this direction.

Still very interested in other ideas. At this point, with 125k on the clock, I'm thinking plugs and an EGR cleaning are necessary anyway. Maintenance history shows that transmission service, engine and inverter cooling systems were done at 100k. It also shows PWR MGMT & MG ECU updates done at 105K. Brake fluid was flushed @ 66K. I wish the maintenance history on the car was more complete. There is a gap from 9/14 @ 23K to 10/17 @ 66K and then again from 2/18 @ 77K to 11/18 @ 100K. No info from 5/19 @ 110K. Are there any other critical maintenance items I should be looking at.

 

So I have some interesting information to add.

One thing I hadn't previously considered was air pressure. I found the 15s @37, the 16s @ 32, and the 17s @ 35. I adjusted all to 40 before proceeding.

My test route is probably not long enough to be considered scientific. It is 35.3 miles from my home and back. The route consists of 2 miles of 30 mph, 26 miles of 75 mph, 3 miles of 65 mph, and the remaining 4.3 at 55 mph. No traffic to interfere anywhere on my route. There are two stop signs and 1 traffic light. None of them long enough for a Prius ICE to start while waiting.

No jackrabbit starts. When accelerating to highway speeds I try to keep the power meter in the center of the PWR section. Cruise control at speed limit as much as possible, which is most of it. The first half of my drive is a net gain of about 800 ft of altitude, the second half balances it to a net zero. I do think the penalty climbing is more than the benefit when descending from what I can see.

All three test drives began with only two bars on the hybrid battery state of charge. I used the Prius power ports to run the compressor to charge up the tires before each drive so it was on without driving. Neither car typically wakes up at only two bars.

First drive was in the '12 III with 17" rims and 215/45r17 non-eco rated tires that are not quite down to the wear bars. They also feel like they are not quite perfectly balanced. Reported miles was 35.9, reported MPG was 48.2, adjusted MPG to actual length of drive is 47.4. Ride is very rough and tires are noisy. I only tested these as they are on the car that I perceive to be getting better mileage.

Second drive was in the '12 Five with 16" rims and 205/55r16 Eco rated Dunlops that have only 8K miles on them and look like new. Reported miles was 35.5, reported MPG was 45.6, adjusted MPG is 45.3. Comfortable and quiet, plus provides the look I like the best.

Last drive was in the '12 Five with 15" rims and 195/65r15 Eco rated Michelins that have about 20K on them but still look nearly new. Reported miles was 35.9, reported MPG was 47.8, adjusted MPG is 47.0. Also comfortable and quiet and I'm sure I could get a set that I like the look of.

The first test was earlier in the day with the temp about 80. The last test the temp was up to 86. Both cars have excellent AC and it was running during all tests.

So the III still got the best mileage, even with the most likely MPG robbing tires in place. The difference between the 16s and 17s on the Five was 1.7 MPG or about 3.5%. That's a price that I might be willing to pay?

I don't have time to move the 16s to the III and run the same route today, but I may check that in the future.

Both cars seem to charge from two bars to six in a reasonable period. If anything, the Five takes longer, which if I understand correctly could mean the Five's battery is in better condition. Once at six bars they both stay there with the exception of going to seven on an off ramp. The Five stays at seven significantly longer than the III.

The other thing that I think could be relevant here is that the cars when mounted with the 15s or 17s are traveling slower that with the 16s when set at the same cruise speed, which I did. With this in mind, with cruise control set at 75 and equipped with 15s or 17s the actual speed is between 72-73, when with the 16s the actual speed is between 74-75. Although a small difference, this would theoretically bring the gap between the 15s and 16s on the Five a little closer.

I still think I have some further investigating to do. The 17s on the Five were only getting about 44 reported MPG and on the III they're pulling off 48. I've heard the 12 volt battery can affect MPG on a Prius. I'll be looking into that, replacing plugs, and cleaning the EGR valve next.

 

1,150 mile trip in the Five with the 15" tires on it and averaged 46.5 at the pump and adjusted for speedo error. Much of the trip was 75 MPH interstate and even 80 in Utah. I set cruise at 5 over and with speedo error that becomes about 2-2.5 over at those speeds.

I guess that's good, but I'm still going to try to find more. I know the immediate response is going to be to slow down, and I get it. There's math for that too, and for the cost, in time, of driving slower. At those speeds 5 MPH difference would cost about an hour of travel time and would not have increased mileage enough to save the time of a fuel stop. An hour has a value that varies from person to person. For me, the hour is worth more than the fuel savings. Much of the reason that I am wanting to get the best mileage out of my car is that also indicates excellent running condition and hopefully a long life and not so much the minor $$$ savings of a couple miles per gallon. The time I put into searching for the improved mileage is therapeutic and results, hopefully, in success and achievement, which is good for the soul.

At the end of this project, I'll end up putting the 16s back on the car because I like them. As long as I've maximized the mileage from the car, having the wheels I want on it has value too.

 

On a side note, I really believe how impressed I am with the adaptive cruise control on this car. Much better than my wife's '17 CRV or my '20 Ridgeline.

The Lane Keep Assist on the CRV is far and away better than the other two.