I was wondering how much 27 inch tires would affect my gas mileage on a 98ish outback?

If you can fit them, probably around 5-8 MPG

If you can fit them, probably around 5-8 MPG

 

in the city. but in my experience, the freeway miles were only 2-3 mpg worse (well....at ~80mph anyway)

Tire size isn't the only thing that will affect mileage.

 

Tread pattern and air pressure will change it the most. I notice a huge difference in resistance when the tires are deflated to low pressure for offroading when I get them back on the road. Fill them to 40 lbs, and then it's all good again.

 

My street tires for the impreza are 28 inchers (225/55/17), and I get almost 30 mpg on the highway.

My wife's impreza, same year, gets 27 mpg on the highway.

 

If you can fit them, probably around 5-8 MPG

I'd have to argue Noah. A stock 98 OB comes with 205/70/15s. Practically a 27. So, your tire size isn't going to change much or affect your gas mileage one way or another with a "27". It will all depend on the tire tread you choose. Also the width of the tread, having to do with surface resistance and all that.

 

 

But just for good measure.

 

 

This gets asked a lot.

What Zap said about tread pattern and psi... depends on what type of "27" you're referring to.

 

Here's my personal experience FWIW:

 

97 Outback, 4EAT, 125K miles on it. With stock tires: 20-22 in town, 26-28 hwy.

 

With 205/75/15 AT tires (Firestone Destination A/T): 20-21 in town, 24-26 hwy. So, almost no city loss, a couple mpg on highways. And the car is due for new plugs and wires, so who knows, it could improve above that.

 

These are more aggressive than the stock all seasons but by no means a mud tire or swamper type tire. IMO they are more than adequate for anything I'll ever take my car to. Plus they do quite well on the street which is where the car spends most of its time.

 

Steve

are you guys compensating for the larger diameter tires and speedo being off? speedo will record fewer miles than actually travelled with larger tires.

 

i routinely install larger tires and get better highway gas mileage (adjusting for the increased circumference/speedo difference). my car is covering more ground (larger circumference) with the same RPM's. but i'm talking completely different application and tire choice. i want better mileage and dont get wide, aggressive tires. but that should compensate for some of the loss on the larger tires as well.

are you guys compensating for the larger diameter tires and speedo being off? speedo will record fewer miles than actually travelled with larger tires.

 

Can't speak for others but yes I am... but in this example it's only around 3% larger, 27" is barely larger than stock. There's no room for bigger.

 

Steve

it's only around 3% larger,

good point. that's probably not even enough to make up for the inherent speedo error!

 

either the road signs or the odometer was off on mine by about 2-3%. that's over a few hundred miles of straight highway driving, so something was off. odometer always reads less than highway mile markers.

DOH, didn't notice the 98 outback part, I was just thinking EA82...

 

I always accounted for the difference, at 50 mph, my speedo was almost exactly 10 mph off (according to my GPS). that's 20%. so everytime I calculated mileage, I added 20% to the trip odo, and then calculated it.

that's 20%.

WOW! that's huge!

Also, when fitting what are essencially truck tires, you need to do contact patch tests for proper inflation pressure. Inflating to 35 psi like the sidewall sugests is not going to be correct for a light subaru. It's also helpful to know the optimum aired down pressure for off-road. In my case with 28" tires on my EA81 I have to air down to about 7 psi for best traction.

 

GD

I have to air down to about 7 psi for best traction.

crazy!! so you can tell a difference between 7 psi and 20psi? (i know nothing about off roading). i do drive through snow and off road alot as necessary for hunting, etc but i don't know anything about ofroading in those terms. would have never guessed 7 psi though. is that mud, rocks or anything?

Calculated for mileage difference, I got about ~25 mpg in my outback at ~75 mph with my SSR's.

 

At 75 mph with the tripple treads, I get about ~27 mpg.

crazy!! so you can tell a difference between 7 psi and 20psi? (i know nothing about off roading). i do drive through snow and off road alot as necessary for hunting, etc but i don't know anything about ofroading in those terms. would have never guessed 7 psi though. is that mud, rocks or anything?

 

you wouldn't beleive it!!

 

the extra traction, and flex (since the tires can flex some too, you can more easily keep some of all 4 tires on the ground). I usually go to 12-15, but I'm using cheap tires, and on the rocks, so I have to be real careful of the sidewalls.

 

also, when airing down, it helps spread out the weight, if you're on fully inflated tires, you have a much higher chance to get a flat on a sharp rock!

WHen I do my gas millage calculations, I multiply my odometer by 1.2, then divide by the gallons. I get around 23 wheeling lots and beating on it, 27 highway cruising. And that's at 75+mph.

 

My car is a loyale with an EJ22, 3.9:1 diffs.

If you can fit them, probably around 5-8 MPG

 

when i went from 195/65r15 to 215/75r15 there was no real noticeable difference in mpg. it's a 96 legacy

Zap, your OBS came with that size tire?

mine came with 215/60/R15's... and that's what tire rack tells me (it's about 24-25" or something)

but yeah i hear yah on the tire pressures, and tread pattern... on my 16" rims, which have some touring tires on them, at 35-40psi i get much better milage than my snows... and the touring tires are larger than the snows, with nasty tread on them.

but whatever... if you're worried about gas milage, a set of offroad tires might not be the way to go

Although, if you get a larger touring tire, you can cruise at a slightly higher speed, with a little less rpmage... just food for thought... mm... food...