I replaced my OEM (Bridgestone) 225/60-16 wheel/tires with 235/50-18 (BF Goodrich G-force KDWS) tires (and 18" wheels) No clearance problems, although they are slightly oversize (+8mm dia). My speedo is not as far off as it used to be, (it's about 2% high now, better than the 5% error it used to have). But my MPG dropped from about 25mpg to about 20mpg. inflation is 32 at all corners. I used to go about 420mi per tank, and allowing for the bigger tires, I should see 400 on the odo. But I only get 340.

Can the tires be that big a difference? Can the 3% change in tire size throw off the car that much? The tires are 10mm wider, can that be a problem? (again, there is no interference)

Wide high performance tires will have a higher rolling resistance than the stock tires.

Heavy, 18" wheels take more power to accelerate up to speed as well (what wheels did you get?)

You can try inflating the tires even higher, try 36-40psi, that should help improve your mileage a bit.

i have the same tires and see even wear at 40-42 psi. and yes minde did go down also but i know my rims are heavier (P1=21.5lbs or 42lbs per corner which is up 7lbs per corner from stock) then add more friction from the wider tread and there is your culprit.

just a side note.. below 40MPH its all road resistance

All true (but a 20% drop in MPG?). My tires are 10mm wider, the wheels are light (SSR Comp, <15 lbs)--I think the tires are 20lbs, so I am about the same weight as stock.
I'll try inflating them higher- The handling is certainly nicer, but the money on gas is disconcerting. I was thinking to try Dunlop SP9000s, but I don't know if just tread pattern/tire construction could make such a HUGE difference in MPG. <sigh> Thanks for the hint though--

without question your gas mileage will be affected. I have seen this on numerous vehicles that added larger wheels/tires and even just wider (heavier) tires can have this effect. The only car I ever saw go down was my wife's '03 lancer that had strangely heavy stock wheels. The plus 1 size 15 inchers I replaced them with were lighter. I replaced the OEM Bridgestones with 225 width Dunlops on factory WRX wheels and noticed a drop in MPG. Another poster was correct in stating that raising the tire pressure will help. Keep the factory ratio (front 2 or 3psi higher than rear) but pump them up. That will make them easier to get rolling and give them what I like to think of as a little energy return. Think of how a basketball bounces higher with more air in it. The tire pushes back from the road and all of it keeps you moving....IMHO. 16 to 18 is a big jump and even if the weight of the combo were the same as stock (VERY expensive to do) the rotaional inertia of the combo would increase because the weight is pushed further out from the center of the wheel by virtue of it's larger diameter. Big looks great and in the proper application may handle great but it robs acceleration and MPG everytime in my experience.

It might be a combination of weight, circumfence and rolling resistance.
Did you watch the mileage over a longer distance or is it the first fill up after changing the wheels?

Do you feel a difference in acceleration? Maybe you just try to make it and hence the drop in mileage? I guess the "lead foot" influences the mileage more than the tires ...

By the way, 420 miles is a very good mileage for the 2.5 Turbo ...
340 miles sound more reasonable to me.

It might be a combination of weight, circumfence and rolling resistance.
Did you watch the mileage over a longer distance or is it the first fill up after changing the wheels?

Do you feel a difference in acceleration? Maybe you just try to make it and hence the drop in mileage? I guess the "lead foot" influences the mileage more than the tires ...

By the way, 420 miles is a very good mileage for the 2.5 Turbo ...
340 miles sound more reasonable to me.
The mileage per tank can't be directly compared with the other 2.5 Turbo offerings like the STi or the Forester XT because the Legacy comes with a larger gas tank.

The mileage per tank can't be directly compared with the other 2.5 Turbo offerings like the STi or the Forester XT because the Legacy comes with a larger gas tank.

I guess that the Baja has the same 64 l (appr. 16,.. gal.) as the Legacy.
But 420 miles with one tank gives a great mileage for the 2.5 Turbo. Hard to believe for me that this mileage could be achieved under "normal" driving.

So my guess is just that the pedal has just more influence on the mileage drop than the 18" wheels.

The answer is very interesting for me because I am going to put on 18" wheels also (225/40/18). I am now getting appr. 300 miles per tank! :eek: May depend on my driving style ... :devil:

My previous numbers were from many tank-fulls of gas before and after the 18" upgrade. (I'm a data junkie, with a CarChip on the OBD port, a GPS and a SmarTire on the dash)
It isn't 'normal' driving, it is sustained highway miles, travelling to a vacation house, 150+ miles each way, at a cruise-controlled average speed of 60+ The tires are now at 39F, 37R psi, and it looks much better for MPG. My previous tank got me just about 25MPG, and if that holds out for a couple more tanks, is actually the same as before. I'll post a follow-up when I'm more certain.

You did three things with your wheel/tire change that might negatively affect your mpg.
-You increased rolling resistance by increasing tire width.
-You increased the inertial resistance the wheel/tire has to starting to roll and increased the wheels resistance to stopping too. Its harder to get the car rolling and getting it to stop now.
-You increased wind resistance slightly with the wider tire.

Its possible those changes had that much impact on your MPG. My mpg dropped about 15% on my old Legacy going from 14" to 16" wheels, 185 to 205 section width.

ss

According to the specs on tirerack.com, your new tires rotate 759 times per mile. Your old tires (RE-92's?) were smaller at 837 revs per mile. So you're going farther per tire revolution, by over 10%. When you calculated the 3% change, did you multiply by pi?

Edit: Removed links to Tirerack.com, which needed cookies not to choke.

Next three lines copied from tirerack.com w/o permission.

235/50-18 BF Goodrich G-F KDWS = 759 revs per mile

225/60-16 RE-92 = 837 revs per mile

Recommended 18" tire for 2004 Baja: 235/45-18

*** end of tirerack data ***

837 ÷ 759 = 1.102, or about a 10% difference.

Doh. I did not use the revs per mile, I just used the tire nominal size and calculated it. (Pi is the same for both tire sizes, so it wasn't a factor)

10% probably explains it, thanks for pointing that out!

I'll let everyone know where I end up.

According to the specs on tirerack.com, your new tires rotate 759 times per mile. Your old tires (RE-92's?) were smaller at 837 revs per mile. So you're going farther per tire revolution, by over 10%. When you calculated the 3% change, did you multiply by pi?He's got a Baja, his new tire size is actually within a few percent of the stock size. Have a look at his first post.

He's got a Baja, his new tire size is actually within a few percent of the stock size. Have a look at his first post.

Looked at it, 3% didn't seem right. Checked tirerack & found out it's actually ~10%. Sorry the links didn't work, my bad. I've edited my previous post to include relevant data instead of broken links.

Doh. Well, it looks like the RPM (revs per mile) on Tirerack.com for the RE92 is wrong, Bridgestone's site lists 790 RPM (26.6in diameter) for the 225/60R16 which should be used on the Baja. The KDWS is correct at 759 RPM and 27.2in diameter.

Using these numbers show about a 4% difference in rolling diameter.

<chuckle>
I'm a newbie on NASIOC, but not to other forums, and this is a good site. I started here getting some info on how to turn off my DRL (since I don't want my cibie HIDs running all the time :) (boy, that was a B&*^% reversing the right-hand projector pattern)), but this is a well-attended forum, with a strong group. Good news actually, since my GPS speedo agreed with my original math, at about 3% difference (as best I could read the analog speedo).

p.s. i bumped my pressure up to 44 psi each and the car feels like it is not dragging as much. remeber you can play more with the pressure on these tires due to the "contact patch technology" that is incorporated in this tire. hope that helps get you even better MPG

why in the **** are you running 44 psi.. you are supposed to go by the door plate... the tire rating is a full load for the tire... damn man you are going to **** your tire.. max pressure will cause blowouts, quickly.. on a 2002 wrx you are supposed to run 32 in the front and 29 in the rear.... the other thing to mention is that running the wrong pressure puts stress on the drivetrain...

i have 18's so i have to run a higher pressure just to make sure i do not bend a rim. also you should never go over what the tire recomend. the manufacturer is set for comfort.. not MPG or performance. also please note that most people who autocross run 45+ psi.

LOL, if you keep it below the max on the sidewall you'll be safe for sure, but usually you can exceed that by quite a bit without any danger.

It's low pressure that causes most blowouts!

Actually, big *differences* in the pressure stress the differentials (because it changes the rolling diameter of the tire). If they are all at 44, there won't be extra drivetrain stress.

Actually, big *differences* in the pressure stress the differentials (because it changes the rolling diameter of the tire). If they are all at 44, there won't be extra drivetrain stress.

you are correct. howeve subaru does recoemnd a 2-4psi difference between fron and rear again due to comfort. this is to get rid of the clunk casued by the 1.1 to 1 ratio in the center diff. i also believe they recomend this for oversteer


:devil:

Good point on the "clunk"!

After a bunch more dirving, here's a recap:
On OEM tires/wheels/inflation pressure, I got 25mpg on long trips.
I upgraded to 18" wheels & tires which are about 4% larger in Miles/rev, and after 2100 miles, I was seeing 20mpg, a HUGE drop in fuel economy, which I could not explain. The fourm's wisdom was to inflate my tires more. I did, going from 32 all corners to 39F, 37R. After 830.2 miles on the same route, I have used 33.114 gallons, so I am right back up where I was. Those 830.2 odometer miles actually compare with 863.4 miles, (since my tires turn 4% more slowly than before the upgrade) so I am getting BETTER (26mpg) gas mileage than before. (not really surprising that a taller ratio would do better on highway driving)

So, this FORUM has saved me, over the 24K miles I drive every year, 6MPG = 280 gallons per year, = $650/yr. Time for the next upgrade, And thanks to everyone!