Effect of Wider Tires on Gas Mileage

[WFalicoff]

According to Nokian Tires: "The rolling resistance of tyres may differ greatly. A nearly 40% difference in rolling resistance transforms into an approximately 6% difference in fuel consumption." So if we assume that the OEM tires on the I2, which are 175mm in width, are replaced with tires that are say 185mm, then this will increase the rolling resistance (assuming the rolling resistance of the tire is the same) by 185/175. However the increase in fuel consumption will be less based on the information from Nokian as there is a relationship between rolling resistance and fuel consumption (40/6). So the net effect would be to increase fuel consumption in this case by ((185/175) -1) /6.67) or less than 1%. If the new tires had 10% lower rolling resistance than the OEM tires, this would more than offset the losses associated with a wider 185mm tire. This analysis (assuming it is correct) is only true for dry conditions.

Have I got this right? I look forward to comments and opinions from others.

2010 Silver EX I2
(Shortly to have 185/60R15 Nokian WRG2 tires on the car)

[attilla]

Assuming that rolling resistance increases linearly with tire width (seems a good assumption to me) then I believe the math is correct.

This is also the same conclusion I came to, and is why I will go with slightly wider tires. There is a much wider selection of slightly wider tires.

[jf_cole]

I went from 165 to 175 on an Insight I, I lost about 5-7mpg. Both were LRR tyres.

[attilla]

A few questions:

Were the tires replaced in kind? (All LRR tires are not equal, see tire rack LRR test).

If not replaced in kind, what was the difference in revs/mile?

If replaced in kind, a new tire will look like it is getting lower mileage because the radius is slightly larger than the old tire therefore the wheel dos not spin as fast.

Some new tires apparently have lower mileage initially until they are broken in.

The point is be careful not to compare apples to oranges.

[jf_cole]

Yes I am comparing apples with oranges! My new tyres are slightly taller than the older ones. They are also Michelin's compared to Bridgstones. They worked out quieter, more compliant and in the same size as the Bridgestone, they outperformed them in grip tests in dry and wet.

Car drives far better and feels safer (again, this is a Gen 1 so 850kg on 165's when braking hard was a bit scary).

[WFalicoff]

Well I can report back that so far this is no loss of mpg using the wider Nokian WRG2 tires (185mm wide) on the I2. My wife just drove back from Southern Oregon to Southern California (with the new tires installed up there) and the mpg was essentially unchanged. This is a drive of over 600 miles with a wide variety of conditions (mountains, flats, etc). She makes this trip often and typically gets 42 mpg at average speeds around 75mph on long streches of Interstate 5. In the latest trip with the new tires she got 43.5 mpg. This is with the tire pressure set at 36 psi front and rear (I checked this when she got home). Correcting for the difference radius of the tires takes this down to 43.3 mpg. At slightly lower speeds I have achieved nearly 49mpg over the same distance but my wife consistently gets around 42 mpg. I will check the mpg over a "course" I know very well this weekend. Her immediate reaction to the tires is that they make the ride much smoother. I will give my own opinion on ride and handling after this weekend. This is a promising result for the Nokian tires. As we might be the first ones in the US to have made the switch to this tire we will keep everyone informed on how it stacks up over time (I will try bumping up the tire pressure to see what benefits, if any, can be had). I hope others to chime in when they change to a different tire.

2010 Silver EX I2
With Nokian 185/60 R15 tires

[waterdog]

Did you mount these tires on the OEM wheels, or did you have to get wider wheels?

I use Nokian WRs on all my other cars, as winter tires on the Accord (WRG2) and V70 (original WR) and as year round tires on the Forester (WRG2). I would love to have them on my I2 (though want to get some more miles/value out of the OEM tires before making a switch). I look forward to your continuing reports.

[WFalicoff]

They are mounted on the OEM wheels. As I have posted before the revolutions per mile for the Nokians are almost an exact match for the Dunlops (957 vs 952). The weight of the two tires is the same. Our reason for switching tires early was the safety issue -- travelling in the snow in Northern California and Southern Oregon.

[Purplegate]

These are good information !

Correct me if I'm wrong, but Nokian are winter tires (studless) right ? Size for size, they should have a higher rolling resistance than the OEM Dunlop. So if you don't even see a mpg drop, it confirm my fears, the Dunlops are not providing the LRR capability...

I was wondering the effects of changing for a larger tire (185), but going for a Michelin Energy Saver, or Goodyear Assurance Fuel Max. But they seems to only come in 185, no 175 as of now.

Based on you maths, the size comparison should be pretty similar to your Nokian comparison, but the roling resistance would have a good impact on mpg compared to the Dunlops.

You might have just triggered my desire to change my tires earlier than plan...

[WFalicoff]

The Nokian WRg2 were tested in the November issue of Consumer Reports, and were found to have very low rolling resistance, among the best of any tires they tested. They also performed very well in almost every category. Strangely they performed not quite as good as one Michelin tire and one for Hansook one ice and snow. (They still performed very well on snow and ice) However, this tire has an excellent track record in cold climates including parts of Canada. As several people mentioned, it does have the snow flake certification, which the Michelin on CR report does not (The Hankook does).

Now I have personally driven the car today (around 90 miles)and can report the following:

1) Handling and ride is improved dramatically. The car is more neutral on turns (with OEM tires the car had considerable understeer). The ride is noticeably less bumpy over bad roads. On smooth roads at say 65 mph the car tracks straighter, less wandering.
2) At 36 psi tire pressure over an approximately 30 mile section of Interstate 5, the mpg was better than with the OEM tires. This was an average speed of around 65 mph. The mpg typically for this route is around 62. The car got 67 mpg (including the correction for the tires). Now I have gone over 70 mpg in a previous run once with the OEM tires with a major tailwind. But I have made this trip perhaps 10 times and the 60mpg is more representative. On the way back we traveled 60+ miles over a wide range of conditions (stop and go on Pacific Coast Hwy, went over the Santa Monica Mountains, city driving in Santa Monica, some freeway driving). The car got 51 mpg over this route. This is several mpg higher than before. I think the car gets considerably higher mpg at lower speed (25 to 35 mpg) than with the Dunlops.

So far these tires have exceeded my expectations.