New Wheels/Tires- Bye bye OEM

So, after shoddy cornering and an uncooperative curb took a sidewall chunk out of a rear tire, I used the excuse and inspiration from this post, to buy four Pirelli Zero All Season 215-55-17- they can be had for just over $ 90.- a piece right now, and have decent ratings, except on snow (!). I just had them mounted and can't really comment on handling or gas mileage yet.

 

Looks good!

 

Out of curiosity: shoddy cornering by you, the car, or both?

 

Haha! Definitely me. The Prius was blameless.

 

That's a great idea & price! There are dozens of tire choices comparing to the stock size. Please let us know how it works out on ride, handling & MPG on your next fill up.

 

215/65R17 Michelin CrossClimate2 by Jim Stoll posted Sep 19, 2024 at 11:02 PM

Went with a taller tire to gain some extra ground clearance (in addition to 1.5" lift from PriusOffroad), as I live on an unmaintained road, and was constantly bottoming out with the stock tires and clearance.

Definitely an MPG hit - seems around 2-3 mpg 'around town' (not really a town, but driving at speeds from 0-40mph), and more like 5-6 mpg at highway speeds. For me though, it was either this compromise, or get rid of the Prius altogether. Thus far, I believe it was a reasonable compromise, as I need to drive on my bad road every day, and do mostly 'around town' speeds, with only the occasional highway trip. (That having been said, I recently took a 5.5 hr highway trip, and I'd ordinarily not had to have stopped for gas at all, but had to stop right before the end of the trip.)

BTW, went with the CrossClimate2's due to them being very highly rated for anti-hydroplane, as I live in an area that also regularly has water pooled on the roads. Have to say, these are the most amazing tires I've ever had, in respect to handling driving through water on the road - super confidence-inspiring.

 

Did you compensate for the 7% bigger diameter in tire size? Due to the bigger tires, your odometer is understating 7% and because of that, you should adjust it by 1.07 X MPG = actual MPG.

 

Yeah, still working through that. TBH, I think it was *overestimating* my original MPG, given that with the lager tires, the speedometer is almost spot-on with actual speed (tested with my phone GPS (granted, probably not 100% accurate) and had a little roadside chat with a (thankfully very friendly!) police officer the other day. GPS says the displayed speed is exactly right. The Officer said that I was going 1mph faster than what I had cruise set to (which was set significantly over the posted speed limit...). So, given this, it seems likely that the speedometer was originally overestimating my speed (and thus presumably distance), and I suspect that the MPG was being overestimated originally, as well.

The other thing that I've read is that the new tires tend to have a break-in period, where your MPG will improve once the new tires wear in a bit. That having been said, these are larger, heavier, wider, more-aggressively-treaded tires, so I'm sure there is a meaningful MPG hit to be suffered as a result.

 

Don't conflate speedometer with odometer. The speedo reads high on purpose, it's mandated, in a (mostly vain) attempt to get drivers to slow down. The odometer should be roughly accurate with stock tire sizes.

 

Driving without the hubcaps and the center caps may reduce your MPG especially at higher speed.

 

Interesting theory. (Have you seen that factually documented anywhere? Very curious!) I'd definitely assumed that the same rotation frequency/count signal would be used for both odometer and speedometer, but in today's day and age of electronics/software, it certainly would not be hard for them to scale those 2 metrics independently of a common-sourced data signal.

I need to get out with my bicycle GPS and measure off a mile-long segment of road, then see how the Prius odometer compares.

From a day-to-day functional perspective, I'm putting my money on the police officer's radar reading (and also using that to hedge my bet against getting pulled over again any time soon! , and assuming that I'm going 1mph faster than what the speedometer is reading. I really am surprised it is not further off, though. By the pure math of it, it should be reading around 4.5mph under now.

 

I've ordered center caps, but I suspect the very aero plastic stock LE wheel covers make a significant difference. (I had taken them off prior to having the new tires mounted, then was lazy putting them back on, but have now come to sort of like the look of the raw wheels w/o the covers.) I should put the covers back on for my next highway trip, however, and see what type of difference they make. At the least, I suspect they help keep gunk off of the brake rotors, particularly since I live in an area with lots of gunk.

 

The level of accuracy is written into the vehicle code. It is a range to account for different tire models and wear. Manufacturers can set it to anywhere within that range. They mostly tend to err in a direction that reduces liability for them when setting the meters. Speedometers read high cause people have tried blaming the manufacturer for speeding. Odometers are set low cause at least one car company was accused of setting it high to run out warranty periods faster, and lost that lawsuit.

 

The stock 19" is half inch bigger in overall diameter than the stock 17". I am wondering if Toyota calibrate the speedometer and odometer differently in each model to compensate for the difference?

 

You already changed tires? How many miles have you driven before you changed the tires? Did you change the size too? You can put in 215/55/17, but the wider tires will hurt your mileage somewhat.

 

Yeah, it's all done - results are shown in the posted photo.

I had about 15k on the tires, and held onto them in case I ever want/need to abandon the lift. The lift allows the 215/65R17's, which nets me another .8'ish inch of ground clearance, in addition to the 1.5" from the lift. (The math says it should've been .9", but in reality, it ended up being about .8") I could've squeezed 225/65R17's in there, but it would've been really tight (and only netted me another .25" of clearance), and I might've ended up needing to take a heat gun to the wheel well liners in a few spots. I wanted to not completely destroy my mileage, though, so the 215's where the choice. (I'd've loved to put a 205 in there, but turns out, nobody makes a 205/65R17 or even a 205/70R17)

And yes, there is a very definite impact on the MPG, but as noted earlier, it really came down to a choice of increasing ground clearance on the Prius, or getting rid of the Prius, due to road(ish) conditions where I live.

 

Highly doubt it.

 

To Mendel
Yeah, think you're right. I did the research and the .5" difference resulted in only about 2% difference in overall diameter.

To Jim Stoll.
Changing from the stock 195/60/17 to 215/65/17 size however resulted in 6.9% difference in the overall diameter. That means your odometer which used to calculate the MPG is now understating it by 6.9%. So if you're now only getting 45MPG, you should multiply 1.069 = 48 MPG; about a 3 MPG difference. I'm sure with the wider, bigger and heavier tires, your MPG must have suffered.

But the reduced wheel gap makes it looks better. This is your car in the picture below, right? I think the stock wheels look great!

 

The MPG displays on previoous gens were invariably "optimistic"' I'd suspect intentionally.

With our 3rd gen, tracking it with a spreadsheet, the dash display was around 7% high. I would think that was the "credulity limit", any more exaggeration would be laughable.

Anyway yeah, even tracking with a spreadsheet, assuming the car's odometer is roughly accurate with either stock tires, tires with signif increase in diameter will have the car thinking it's travelled less distance than the reality.

And to be fair, I've read here that Toyota finally left the mpg display accurate, with 5th Gen.

 

Yes, that's sort of where I was going, but Mendel pointed out that the difference I'm seeing in the speed readout may not match the difference that the odometer is seeing. (And while the math says the diameter of the new tires should be 28", it seems it's less, given I ended up w/ .2" less clearance than I'd originally expected - I suspect just due to tire compression with the weight of the car on it). But, if the odometer reading is actually accurate, then for sure, the MPG is probably a few percent higher than it's reading.

Bottom line, there is a definite MPG decrease as a result of all of this. How much is the tires (added width, mass, rolling resistance due to more aggressive tread design), how much is any added drag due to the car sitting higher, I can't really quantify. But, it does bang home the reality that while the Prius is a pretty amazing car, it doesn't take too many variable changes to have a significant effect on the outcome. The hybrid system is industry-leading, but clearly, it's not the hybrid system alone that leads to the stellar mileage. I'm presently sort of straddling a middle-ground between Rav4 hybrid and Prius MPG territory, it seems.

While it hurts not to be getting the astounding MPG I was getting prior to the mods, I'm still very satisfied with the daily use results. I can now drive the car daily, w/o fear of bottoming out getting to/from my house. It also increases my confidence driving through the regular pools of water we get on the road where I live (both in terms of hydroplane resistance, as well as just navigating the depth), and while it's not a 4x4 truck, I have now taken it on a short test drive in soft/deep sand, and it had zero problems. The fact that it came out looking a bit cartoonish is just upside to me! (And yes, that's my Prius in the posted photo.)