Impact of Model Five Low Profile Tires on MPG?

Actually on my car they could be considered as understated because my OEM tires are 855 revs/mile. Because the OEM tires create a +2mph overstated speed when the speedometer is compared to GPS, I believe they overstate the actual MPH. Therefore, even though my 17s have less revs/mile than the OEM tires, the mpg is closer to actual since the speedometer now agrees with GPS measurements.

I do not attempt to make corrections to my Fuelly.com entries based on revs/mile or anything. I simply record the Trip A readings and how many gallons I pumped.

 

I just take the numbers as they come, but that is a consideration.

I would think if the car comes with the 17" low profiles (like our Canadian "Touring"), with the slightly smaller Outside Diameter, the odometer and spedometer were corrected at the factory, to accomodate that difference. But I'm not sure. One way to verify would be to have the two Prius follow each other along the same route, one 15" OEM, the other 17" OEM, and see what shakes out.

When we're driving with our 15" snows on, I tend to push the speed up slightly, keeping in mind the larger OD factor. And FWIW, spedometers (but not odometers, at least I hope not), tend to underreport mileage, by about 2%. If it reads 50, you're likely doing 48. This is born out by hooking up a ScanGuage, at least it typically displays speed as being lower by about 2/50 factor. I think this is mandated, by the United Nations or some body associated with it.

 

Do slightly more or fewer rotations per mile matter in our cars?

In an HSD hybrid the engine RPM varies all over the place at highway speed, even on what looks to be a fairly level stretch of road. The engine RPM is dictated by the amount of power required to maintain what the driver or the cruise control is asking for at the time and to compensate for + and - demands of the battery.

I don't know of any tests which have determined the effect of slightly large or smaller diameter tires on the Prius. Maybe it's safe to assume we really don't know for sure what difference in fuel economy a few percent in rotations per mile will make. Perhaps the HSD system compensates so there won't be much difference.

Of course we do keep fairly close account of our fuel usage and I think, based on what I saw with my Gen2, wider slightly heavier tires do account for a 2 to 3 mpg loss as reported by others.

 

Mendel, the Prius GenII and GenIII with 15" tires overstates mph by 1-2 mph. This has been verified by GPS and scangauge. I do not have data for the V model. Most cars overstate rather than understate.

Tumbleweed, the size of the tire should not affect rpm but the extra rotational mass may. It will definitely affect the speedometer indicated mph. I've tested this a number of times this year. In every case the 15s overstated mph by 2mph when traveling over approx. 50Mph. When traveling at lower speeds the speedometer starts to read more accurately.

With the 17s the speedometer reads accurately when compared to scangauge and GPS readings (accuracy of 15ft. or less).

 

Oops, I should state that I have 185/65/15 tires which have a higher revs/mile than the 195/65/15 that come on the GenIII. However, other GenIII owners have reported a 1-2 mph overstated speed. If your experience is different please let me know so I don't speak inaccurately.

 

Cwerdna has a good point here. I'm self employed, so my Prius has equal amounts of local city vs highway driving. I don't put that many miles per day on my car...but then I go for extended drives to NC. Depending on what kind of driving I do...my average ranges from anywhere between 47mpg to 54mpg. Over my whole odometer range, I would say I'm averaging 50mpg total. I am keeping my plastic hubcaps on. I actually like the looks of them better then just having the alloy wheels...and the threads here say it does improve some on mpg. It might add just a few tenths compared to everything else...but really the main difference is your driving style. The more you're trying to "hypermile" the more you'll get even better mpgs (and it's more relevant then "oh, do I save a few more points with whatever hubcap").

 

I don't disagree with Andrew but when you drive well over 26,000 miles/yr like I do then 6mpg can make enough of a difference to buy a new set of tires each year (nearly $300). If you don't drive much then the difference is pretty small.

 

Summer tires are 45 series 17" and for the 1-2 mpg that I lose the safety of more rubber on the ground is all that matters to me, winters on the 15" wheels with thinner snow tires also make me as safe as possible

 

I find it very difficult to believe you only lose 1-2 mpg with the 17s. Have you run back to back tests with 17" and 15" summer tires?

 

Three summers on 15's average 53.7, three summers on 17's average 52.3, reset spring and fall, averages are for 15+ tankfuls...
Anyway, it's not the mileage, it's the safety factor IMHO,ymmv

 

I wonderful how much your driving skill played a part. My first couple years with Prius I had a hard time topping 50mpg even with 15s. Now it's a cakewalk and 60mpg is doable if I avoid my newer uphill commute.

Because I regularly swap the wheels back and forth during all seasons I can rule out most variables. Maybe my 17s very non- aerodynamic compared to yours?

 

I bought my prius v about a month ago. After 3 tanks, I have gotten 47.2 mpg (calculated) and 49.1 (car cpu). My car has the Bridgestone Turanza's. I understand those are not LRR tires. Has anyone replaced those tires with a LRR tire? Was there a MPG increase and which tire did you select?