Car and Driver: Effects of Upsized Wheels and Tires Tested

Interesting but this "Car and Driver" report doesn't match my "Prius Driver" experience:

• P175/65R14 -> P185/70R14
• 919 rev/mile -> 867 :: 6% slower revs
• 41 mph indicated -> 43 mph true :: hybrid mode keeps up with traffic
• highway steering is more stable :: probably gyroscopic effect
• adjusted for tire size, MPG remains at 52 MPG of smaller tires
• acceleration :: don't care, I accelerate at 2,600 rpm anyway

By using a larger diameter tire, I'm in effect always running with a 6% 'over-drive'. This means when keeping up with traffic on a 40 mph posted street, the car still treats it as under the hybrid threshold, stopping the engine as needed to maximize fuel economy.

Normal Prius driving, I'm not trying for maximum acceleration or any form of 'street racing.' I haven't even bothered to measure 0-60 times but don't really care. I still climb 8% grade hills at 55 mph, efficiently. The center of gravity has not changed so I still take turns at 'Prius speed.'

The car still gets me from point A-to-B with the expected 52 MPG. However, I can keep up with traffic on 40 mph streets while the car continues to cycle the engine ON/OFF to minimize fuel burn.

Bob Wilson

 

Doesn't speak to grip advantages of wider wheels and contact patch, as opposed to "the look" of plus-sizing, though wider is inevitably heavier, and suspension geometries can't always use -- or in some cases even tolerate -- much wider.

 

The 18 was .04-.06g (significantly) better than the 17 and smaller, while the 19 was .01g (marginally) worse than the 18.

In any case, with a 235/35 section tire on an 8.5" rim, they were lucky to be able to maintain a seal and not to roll the tire off a rim way too wide for the rubber.

 

Bob, in traditional upsizing or Plus Sizing, the original tire diameter is kept the same as original so no appreciable change to revs/mile should occur. That is what they were attempting to do in this test.

In such cases, the Prius does indeed lose fuel economy. Quite a bit in my experience. 17s equal a 3-6mpg drop depending on the tires being compared.

 

In this season and depending on where you live, the wider the tire the more snow it has to displace. Same with a heavy rain. There are always tradeoffs.

 

If they are not increasing the diameter or improving steering stability, it is a waste. But we're Prius people, not 'Car and Driver' art appreciators of wasting gas going 0-60 mph. <grins>

I have speculated about getting four 'donuts' for our NHW11 . . . for a little 'street racing.' Gut the interior, remove the spare, get quality gasoline, and tune it. Then go 'trollin' for the motorcycle clubs in the early summer. . . . <wink>

Bob Wilson

 

Great article Jimbo... Thanks for sharing... It makes me curious about downsizing?

For example I love the look and size of Prius V but the SUV like design takes an MPG hit. So what if the car was lowered and the underbody profile raised, side view mirrors replace with smaller ones, stripping the interior of excess weight and some of those super-light CE28 racing wheels made for prius that are 15x5.5 and the best LRR tires put on it? Maybe even get some body work done, perhaps some rear wheel covers and de-bulking some of the trim?

Do you think that'd improve the V's MPG at freeways speeds? Or would it just make the speedometer and engine settings out of sync and cost MPG?

 

The small weight savings you are talking about really won't make much of a difference at freeway speeds. The big difference between the prius v and prius liftaback is drag cdA, not weight.

Lowering the car reduces frontal area -A, but its a tall vehicle. Lowering it 2" will only reduce A by about 3%. Smaller mirrors and faired rear wheels might drop cd 3%-4%.

There is not much savings going from 16" to 15" low rolling resistance tires. I'm sure the car was designed for the 16" ones. If you change to 15" you may increase drag, unless you are very careful in selection. This will hurt highway mileage. Going smaller than 16" may help city mileage because of a smaller contact patch.

 

Interesting, I was wondering this myself with the fusions 17 vs 18 option. Looks like braking improved though, distances grew shorter as the wheel size increased.

 

The article said going from 15-19 inch wheels dropped fuel economy by 10%.
How's an effect that big happen? It doesn't add anywhere near that to weight of car.

 

20% wider tire section width (~44% larger contact patch, a proxy for tire rolling friction/drag), 35% greater tire/wheel mass, with much of it at the periphery, hence a greater polar moment which varies as (r^4)/2