New tires affecting gas mileage on WRX

[Paul]

A few months ago I changed tires on my WRX from the stock Bridgestone which
had lasted nearly 40K miles to Bridgestone 950's. The difference in both
ride, noise and handling was night and day compared with the stockers.
However, immediately following that my gas mileage dropped from 20.4 to
17.8 even though my commute trip of 60 miles remained identical. My
measurements are very accurate as I fill up at the same place at about the
same time of day, and have kept accurate records since I bought the car new.
I have had the dealer check it over and there was nothing wrong, so it has
to be the tires. The new ones definitely feel "stickier", and I even have
them at 2 psi over the recommended pressure so I would not have thought that
the stickier rolling resistance would have had that effect, but it sure
seems it does.

Has any other owner experienced this phenomena?

 

[NitroTrike]

From: "Paul" (e-mail address removed)

Date: 10/5/2004 8:32 PM Eastern Standard Time
Message-id: <(e-mail address removed)>

A few months ago I changed tires on my WRX from the stock Bridgestone which
had lasted nearly 40K miles to Bridgestone 950's. The difference in both
ride, noise and handling was night and day compared with the stockers.
However, immediately following that my gas mileage dropped from 20.4 to
17.8 even though my commute trip of 60 miles remained identical. My
measurements are very accurate as I fill up at the same place at about the
same time of day, and have kept accurate records since I bought the car new.
I have had the dealer check it over and there was nothing wrong, so it has
to be the tires. The new ones definitely feel "stickier", and I even have
them at 2 psi over the recommended pressure so I would not have thought that
the stickier rolling resistance would have had that effect, but it sure
seems it does.

Has any other owner experienced this phenomena?



I noticed that last year. It happened when they switched to "oxygenated" fuel

here in NJ. I think it was later, like the end of October though. It drove me
nuts, I thought my brakes were dragging etc. Just a thought...

 

[Larry Van Wormer]

A few months ago I changed tires on my WRX from the stock Bridgestone which
had lasted nearly 40K miles to Bridgestone 950's.
However, immediately following that my gas mileage dropped from 20.4 to
17.8 even though my commute trip of 60 miles remained identical. My
measurements are very accurate as I fill up at the same place at about the
same time of day, and have kept accurate records since I bought the car new.
I have had the dealer check it over and there was nothing wrong, so it has
to be the tires. The new ones definitely feel "stickier", and I even have
them at 2 psi over the recommended pressure so I would not have thought that
the stickier rolling resistance would have had that effect, but it sure
seems it does.

Has any other owner experienced this phenomena?

I have a MY02 TS, not a WRX, but I've got the 16" WRX wheels and tires
from new, and last year changed over to 950's, as you have. No change to
the gas mileage I've been able to detect, and I also keep pretty close
track of usage.


Larry Van Wormer

 

[Mac Townsend]

must have gotten a load of California gas<G>

if the diameter is the same and otehr things are the same, gas is to be
suspected.

 

[Spudster]

I noticed a mpg drop when I switched out my stock 16s for STi take offs
(17"BBS w/RE070s) with basically 0 miles on them. After a couple thousand
the tires seemed to get "broken-in" and my mileage went back to where it
was. This was also very noticable in rolling resistance in my driveway. I
normally push my car out of the garage to wash it and save starting it up,
but found I could barely push it after the wheel/tire upgrade. Now it is
back to the normal rolling resistance as before.

Go figure,

Steve.

 

[Byron]

A few months ago I changed tires on my WRX from the stock Bridgestone which
had lasted nearly 40K miles to Bridgestone 950's. The difference in both
ride, noise and handling was night and day compared with the stockers.
However, immediately following that my gas mileage dropped from 20.4 to
17.8 even though my commute trip of 60 miles remained identical. My
measurements are very accurate as I fill up at the same place at about the
same time of day, and have kept accurate records since I bought the car new.
I have had the dealer check it over and there was nothing wrong, so it has
to be the tires. The new ones definitely feel "stickier", and I even have
them at 2 psi over the recommended pressure so I would not have thought that
the stickier rolling resistance would have had that effect, but it sure
seems it does.

Has any other owner experienced this phenomena?

I made the exact same tire change you did. I've had my RE950's on for
10 months and 16000mi. It's amazing how much better they are over the
RE92's. Even in snow.

My mileage did not decrease when I switched tires. I've averaged 19.14
mpg since I started keeping track - but that includes 3000 miles of
pulling a trailer. My average tank is ~21mpg. It's actually about 1
mpg better then it was with the old tires. I also run ~2psi higher than
the doorjam label.

I'm guessing that somehow your alignment got messed up - either hitting
a bump or even at the tire shop. If there's too much toe, handling can
still be great, but there's a lot more resistance. Just something to
check maybe.

- Byron '02 WRX wagon

 

[Rick Courtright]

Has any other owner experienced this phenomena?

Not with my Subie, but an almost exactly 10% loss with a change of tires
on another vehicle (based on very good records.) Same as your
experience: going to "stickier" tires.

Rick

 

[ThaDriver]

I'm thinking the alignment thing too. If you have just a tad too much
toe-in, & you put on sticker tires, then the rolling resistance will
increase & cause the drop in mileage.
~ Paul
aka "Tha Driver"

Giggle Cream - it makes dessert *funny*!

 

[2 Stroke]

Many people, myself included, run tire pressures 6-10 pounds above the
Subaru recommended pressures. As long as you stay below the maximum
molded into the side of the tire, checked when cold, there won't be a
safety issue. I run my 205/55-16's at 38-40 psi front, 36-38 psi
rear. Ride is not as soft, but fuel efficiency and steering feel are
improved.

That said, the previous comments about an alignment check are good
advice.

===========================================================

 

[y_p_w]

Many people, myself included, run tire pressures 6-10 pounds above the
Subaru recommended pressures. As long as you stay below the maximum
molded into the side of the tire, checked when cold, there won't be a
safety issue. I run my 205/55-16's at 38-40 psi front, 36-38 psi
rear. Ride is not as soft, but fuel efficiency and steering feel are
improved.

I don't know about that. What about premature wear down the center
and a reduced contact patch?

 

[Danny Russell]

A few months ago I changed tires on my WRX from the stock Bridgestone which
had lasted nearly 40K miles to Bridgestone 950's. The difference in both
ride, noise and handling was night and day compared with the stockers.
However, immediately following that my gas mileage dropped from 20.4 to
17.8 even though my commute trip of 60 miles remained identical. My
measurements are very accurate as I fill up at the same place at about the
same time of day, and have kept accurate records since I bought the car new.
I have had the dealer check it over and there was nothing wrong, so it has
to be the tires. The new ones definitely feel "stickier", and I even have
them at 2 psi over the recommended pressure so I would not have thought that
the stickier rolling resistance would have had that effect, but it sure
seems it does.

Has any other owner experienced this phenomena?

Big time. After wearing out the RE-92's on my '02 OBS and swapping on some RE-950's, my city
mileage immediately dropped from 24mpg to 22mpg. Interestingly, as Wormer mentioned, now that I've
got over 30K miles on the RE-950's the mileage has come back up to 24.

The RE-92 is a LOW ROLLING RESISTANCE TIRE. It has flexible cord structure in the sidewalls,
whereas the RE-950 is a very staunch-feeling tire for an H speed rating.

My main beef is the the negative camber built into the rear suspension geometry, which causes the
inside tread-blocks to develop an extremely noisy feathered edge.

BTW, you might want to try a little higher pressure (38-40 psi) with this tire. That way you'll use
less gasoline to heat buildup in the sidewalls, and the steering feel will open up too.

 

[2 Stroke]

I don't know about that. What about premature wear down the center
and a reduced contact patch?

At the recommended pressures, my tires roll over onto their shoulders
easily during hard cornering, and this wears the ouside edges of the
tires at an alarming rate. I've put chalk on the sidewalls, then
increased pressure little by little until the chalk doesn't get rubbed
off while driving on twisty roads. That's how I found the pressures I
prefer.

In spite of that, on my WRX, while running -1 degrees camber in the
front, and 8 psi more pressure than recommended, I still get faster
wear on the outside edges than inside or center. This has been true
for 4 different sets of tires I've had on this car.

If I drove more conservatively, this would probably be less of an
issue. If I were to always drive as if there was (for instance) an
infant in the car, I would slow down more for curves and corners, and
my tires might well wear more in the center.

As far as contact patch size, it changes dynamically when you drive,
and I use the criteria above and don't worry about it. After all, the
biggest contact patch is on the flattest tire.

I think I'd better put on my asbestos underwear now.

 

[Rick Courtright]

In spite of that, on my WRX, while running -1 degrees camber in the
front, and 8 psi more pressure than recommended, I still get faster
wear on the outside edges than inside or center. This has been true

Hi,

Have you tried playing with the toe to offset some of that camber wear?
I'm thinking back to mid-70s when I got my first front wheel drive car,
and out of habit, even though it was brand new, took it right in to my
then favorite alignment shop just to be checked.

The fellow reset it with a tiny bit of toe out, despite the book spec
for some toe in. His reasoning had to do with whatever oddities go on
with front wheel drive, particularly in cornering, though I'm not sure I
understood exactly what he was getting at. I traded that car about 32k
miles later, with the tires still in great shape. I got another FWD,
repeated the drill, and that car didn't see the inside of a tire shop
until it had 107k miles on the clock (five tire X rotation pattern, so
each tire had ~85k miles. Not bad for 155-13s!) I wore out two sets
(four tires only) of the same tires in the next ~80k, with the alignment
set back to the book cuz I'd moved and didn't know exactly what to tell
the new alignment guy. So my original guy musta known something...

I don't know if the AWD shares any of the "characteristics" of FWD, but
it might be worth asking around.

Rick

 

[Danny Russell]

Have you tried playing with the toe to offset some of that camber wear?
I'm thinking back to mid-70s when I got my first front wheel drive car,
and out of habit, even though it was brand new, took it right in to my
then favorite alignment shop just to be checked.

The fellow reset it with a tiny bit of toe out, despite the book spec
for some toe in. His reasoning had to do with whatever oddities go on
with front wheel drive, particularly in cornering, though I'm not sure I
understood exactly what he was getting at. I traded that car about 32k
miles later, with the tires still in great shape. I got another FWD,
repeated the drill, and that car didn't see the inside of a tire shop
until it had 107k miles on the clock (five tire X rotation pattern, so
each tire had ~85k miles. Not bad for 155-13s!) I wore out two sets
(four tires only) of the same tires in the next ~80k, with the alignment
set back to the book cuz I'd moved and didn't know exactly what to tell
the new alignment guy. So my original guy musta known something...

I don't know if the AWD shares any of the "characteristics" of FWD, but
it might be worth asking around.

Rick, similar to deviating from the manufacturer-recommended tire pressures, the alignment specs can
be deviated from for the purpose of trading one characteristic for another.

The slight toe-in setting is usually desireable for reducing "lane wander", and to provide a on more
on-center feel.

Also, as the bushings and tie-rod ends begin to develop wear on a FWD car, the actual toe and camber
settings will move around with varying throttle input.

 

[John Rethorst]

The fellow reset it with a tiny bit of toe out, despite the book spec
for some toe in.

The slight toe-in setting is usually desireable for reducing "lane wander",
and to provide a on more
on-center feel.[/QUOTE]

Outside of the impressive tire mileage you got, what's the advantage to toe-out,
if not recommended by the mfr?

 

[2 Stroke]

Yes, I have played with the toe. At 1/8" toe-out, I was getting some
slight feathering across the whole tire, more pronounced at the
outside edges. Right now the toe is set to zero. It's been there for
about 6 months, and tire wear looks good, but the steering was more
lively and responsive when I had the toe-out. Oh well.

Part of the reason for the negative camber is that I always seem to
wear the outside edges of the tires out first. I figure that the
higher tire pressures tend to wear out the center and preserve the
edges, and the negative camber tends to wear out the inside, so that
compensates for me scrubbing all that rubber off the outside edges in
hard corners.

Many of the more hard-core WRX guys run camber at -2 or -3 degrees. I
chose my alignment settings primarily for tire wear, and I think they
are as good as can be for that. It's my driving that's wearing the
outside edges. Something to reduce body roll might help, but I've not
decided how far I want go with that.

===============================================

 

[Rick Courtright]

Outside of the impressive tire mileage you got, what's the advantage to toe-out,
if not recommended by the mfr?

I can't ~really~ say there was any advantage, other than tire mileage.
The points Danny made about lane "wander" and on-center feel are well
taken, but I didn't really notice any negative change in either. In
fact, while the car was aligned with the "toe out" situation, it
~seemed~ to be more stable than later when it was returned to the book
specs. My unofficial "stability" test at the time was driving with one
knee while pouring coffee from a Thermos (don't try this at home, kids!)
so don't take what I'm saying as anything but anecdotal. You'd want to
talk with a knowledgeable alignment guy for better input than mine.

Rick