Replace p205/70r15 with p205/75r15

[foureyedbuzzard]

Hi all,
I have a '98 Subaru Outback, recently purchased, with snow tires only, size p205/70r15. As spring approaches I'll need to replace these and have come across a deal from a coworker: Set of 4 p205/75r15 Goodyear Eagle all season radials used for about 2000 miles. Price $130

I think they should be okay, but figured I'd check with you all for comments on going up to the 75 series tires on this vehicle.

Thanks

[drdisque]

they'll make the speedmeter read 3.1% slower than the speed you're actually travelling.

[curtis73]

With the current tires on there, turn the wheel all the way to one side and get under the car. Look around for interference that might crop up with the larger diameter. Make sure to look at the lip of the fender, too. Often times cars today don't have the huge wheel wells of the past and it doesn't take much to cause interference.

Otherwise, you'll be decreasing highway RPMs by 3.1% and (effectively) increasing your final drive ratio by 3.1%. You might notice a slight change in accleration for the worse, but not much.

You might also have some mild issues with automatic transmission shift points. I went from 255/50 on my car to 275/40; a difference of 5% smaller and the computer read the reduced load as a cause for early shifting. It doesn't sound like 5% is much, but it made some huge changes in shift points.

The problem got slightly better when I reprogrammed the computer for the new tire size, but not quite right. It still won't downshift until much more load is placed on it with my right foot. Not sure, but your subaru can probably be electronically altered to compensate all of the above; shift points, speedo, odo, etc... which is a wise legal move, too. Your odometer will be off by 3.1% from now on if you don't and it may run into legal/title issues down the road if someone notices. Its rare, but I knew a guy who put bigger tires on his truck and just left the speedo/odometer alone. When he sold it, it had to be retitled TMU (true mileage unknown) and in effect made it about as desirable as a salvage title.

One last thing that might be affected but highly unlikely: ABS. Most cars use a reluctor ring in a three-channel configuration which wouldn't be affected by tire size, but a few out there still use the older systems and (depending on how its sensed) might make the ABS come on every time the brakes are applied, or it might cause them to be inoperative at all. Maybe ask over at the Subaru forums. I'll bet someone over there has done this.

[foureyedbuzzard]

Thanks Curtis. I checked the clearance under the fenders and that shouldn't be a problem. The actual radius difference is .4 " and there's nothing that appears to be a clearance problem. The car('98) has 113k on it and TMU isn't really a concern as the plan is to keep the car for at least a few years anyway. My biggest concern is any possible effect on the ABS system as you pointed out, and any possible effect on the auto trans and AWD system. I'll try the other forum(s) and perhaps the dealer as suggested. Thanks for the reply.