Chevrolet Cruze Forums banner
1 - 10 of 10 Posts

·
Premium Member
Joined
·
4,646 Posts
Discussion Starter · #1 · (Edited)
LRR A/S to Snow Tires, Mileage Impact (Michelin X-Ice Xi3)

On Friday Nov. 22nd I had a set of Michelin X-Ice Xi3 tires installed on my '12 Eco MT, size 215/55-17. This is the OEM size tire and is mounted on the OEM 17 x 7 Eco Alcoa forged wheels.

I was curious to see what impact these tires had on my fuel economy since I'm generally a hypermiler when it comes to my Cruze and I do whatever I can (within reason) to keep my mileage up. Michelin rates these tires as "GreenX", which is their own rating for Low Rolling Resistance (LRR) and is one of the main reasons I was interested in this tire, even though manufacturer ratings for rolling resistance are not regulated yet and mean almost nothing meaningful to the consumer. I will post a full review in the Wheels and Tires forum when I have time.

Sorry to make this so long and drawn out, but unless the background and conditions are understood the test is basically meaningless.

To evaluate the tire's impact on mileage I planned to do a before and after test, with the tires being the only change to the car. My appointment for installation was at 2pm, so before heading to the shop I did a few two-way average runs on the Rockliffe Parkway, a 60 km/h (37 MPH) road that follows the Ottawa river between Rockliffe and Orleans. I chose 70 km/h (43 MPH) as a test speed since I would be doing this test during the busiest time of the day/week and I didn't want to tick too many people off (it's also the speed I most commonly travel on this road). Weather was calm 2C (36F) with light rain and fog, during both before (~1:30 pm) and after (~3:30 pm) testing. The road surface was wet but with no significant amount of standing water. Runs are East and West, totalled and averaged, measured with the DIC in US MPG, reset at the same points along the route. The car was carrying the OE tires while testing the Xi3, so it weighed about 75-80 lbs more during the After test which should not be significant to rolling resistance. Both sets of tires were inflated between 45-50 psi.

Before (OE GoodYear FuelMax, 215/55-17):
East - 64.5, 64.6, 65.6 (Avg. = 64.9)
West - 60.9, 60.3 (Avg. = 60.6)
Overall Avg. = 62.75 MPG

After (Michelin X-Ice Xi3, 215/55-17):
West - 54.8, 56.7, 54.9 (Avg. = 55.47)
East - 64.3, 63.1, 64.3 (Avg. = 63.9)
Overall Avg. = 59.69 MPG

It's important to remember when discussing the mileage impact of tire rolling resistance, the tire's impact on the car's forward motion stays relatively constant as speed changes while the effect of wind resistance increases exponentially with speed.

Ideally I would have been able to test at an even lower speed to rule out more of the wind's effect, but this was not possible at that time of day. In a nutshell, the faster you drive the smaller the impact rolling resistance has on your mileage, and the slower you drive the greater the impact of the tire's rolling resistance has on your mileage, when calculating in %.Based on the total score alone the tires negatively impacted mileage by ~3.1 MPG, which in a situation like this where the car is getting over 60 MPG works out to 5.1%. Since the test was done at a fairly low speed (43 MPH), this percentage is much higher than it would be if the test was done at more typical highway speeds of 65-70 MPH.

I have an issue with the data, however. Conditions were pretty much identical for both sets of runs except for one variable; on-coming traffic. The runs I made travelling West with the Xi3 tires were going against an almost steady stream of oncoming traffic moving out of the city late in the afternoon (the same traffic I was holding up while travelling East :) ). On-coming traffic creates a headwind for cars coming the other way on two-lane roads, and is known as the "Corridor Effect" where it can actually improve the mileage of groups of cars travelling the same direction (See #6):

6 Common Hypermiling Techniques - CarsDirect

The air moving against me from the oncoming traffic contributed to the lower scores on that one set of runs, but I have absolutely no way of knowing how much of an effect it had. The optimist in me would like to base my conclusion on just the Eastbound data, but the truth seeker can't ignore what was recorded both ways. To satisfy both sides of my argument I will do the following…

Since the Eastbound data seems pretty solid and showed a 1.0 MPG change, I will assume the same 1 MPG change Westbound and then add half the delta and re-calculate.

Eastbound delta = 64.9 - 63.9 = 1.0 MPG
Westbound Before = 60.6 - 1.0 MPG = 59.6 MPG
New Westbound Delta = 59.6 - 55.47 = 4.13 MPG Divided by 2 = 2.07 MPG
Westbound After = 55.47 + 2.07 = 57.54 MPG
New After Overall Avg. = (63.9 + 57.54) / 2 = 60.72
Before Avg. = 62.75 MPG, New After Avg. = 60.72Change = -2 MPG, or -3.3% @ 43 MPH

I'm fairly comfortable with that number based on my observations, but I still wish the Westbound data hadn't shown such a drastic change vs. the Eastbound data. Oh well, that's about as good as I could do given the conditions. Once again, as a % change I would expect this number to fall as speeds increase beyond 43 MPH and the number to increase as speeds go lower.I'm pretty pleased to see the numbers as low as they are, even if the more pessimistic 5% is true. I was expecting a big hit in consumption and it looks like it may be smaller than expected.

I've gotten into the habit of resetting my Trip2 display before leaving for work each morning to keep tabs on how well each day's commute is going. Through last week I was struggling to keep my DIC showing above 50 MPG by the time I had parked at work since the temperature had fallen so much in the last couple weeks. Depending on the day and how smoothly I was able to get through a couple lights and slower sections of the commute my DIC was reading anywhere from 46 - 51 MPG with temps between -12C (10F) and freezing. This morning, with temps hovering around the freezing mark and a light headwind blowing from the West my DIC read 50.0 MPG when I shut the car off. Not bad at all for snow tires, especially considering the LRR GY FuelMax tires they are being compared to.
 

·
Premium Member
Joined
·
2,704 Posts
Thanks for taking the time to do this test.

Your procedure was as accurate of a comparison as we can get, the only issue is sample size. Manufacturers test these things for millions of miles and still don't get it completely right. Still, I would agree with your assessment that the difference probably isn't any more than 5% at the most. I'm even skeptical of it being that significant (the snow tire contact patch is the same). Literally the only thing you changed is the tire material.
 

·
Administrator
Joined
·
15,765 Posts
About 2-3mpg loss is what I've noticed as well, at a previous 40mpg.

Erik, the contact patch is actually a lot larger due to how soft the rubber compound is. In addition, the sidewalls are also a softer, and for many of us, bigger as well since we're going from 18" or 17" wheels. Lastly, the tread depth is higher at 12/32".
 

·
Premium Member
Joined
·
4,646 Posts
Discussion Starter · #6 · (Edited)
Literally the only thing you changed is the tire material.
The tread design is vastly different though, with all of the blocks cut up and heavily siped. This leads to more deflection in the tread blocks under load, more heat generated and energy lost. Check out the tread:

http://www.tirerack.com/tires/BigPic.jsp?sidewall=Blackwall&tireMake=Michelin&tireModel=X-Ice+Xi3

Intelligent tread design can minimise the losses, and I think that's what we're seeing here. I was expecting the impact on mileage to be much higher since I was starting off with a well regarded LRR all season to begin with. I'm also running my tires at 45+psi, so I'm not sure if the difference would be greater between the two tires at door-placard pressures.
 

·
Administrator
Joined
·
15,765 Posts
1 - 10 of 10 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top