[carseatsm5]

Hi I just picked up some 863M wheels and they came with 275/30 front and 285/30 rear PS4S tires. Stock is 275/35 front and 285/35 rear.

Is there any issue running these? I am obviously not a tire expert. Car is on M Performance HAS (so lowered slightly) and has 11mm spacers up front.

Thanks for info!

[pbonsalb]

Same difference front to rear so no possible harm. But gearing will be shorter so rpm will be higher and fuel economy lower and speedometer reading higher than actual. Also less sidewall so rougher ride. Probably sharper turn in.

[carseatsm5]

Originally Posted by pbonsalb

Same difference front to rear so no possible harm. But gearing will be shorter so rpm will be higher and fuel economy lower and speedometer reading higher than actual. Also less sidewall so rougher ride. Probably sharper turn in.

Thanks so much.

How much worse would you say ride quality is? Concerned that could get annoying. Car is already lowered and rides probably 10-15% rougher than stock. Guess I’ll find out for myself when I put them on.

Do you think I need an alignment moving from one to the other?

I also wonder if somewhere in iDrive I can update the specs so gearing/speedo aren’t thrown off.

[pbonsalb]

I would not align just because you swapped wheels and tires.

No computer can change your gearing — the circumference of your wheels/tires is smaller. It is probably like going from a 3.15 to a 3.23 diff but I am guessing at the math.

I do not know if you can input custom wheel/tire specs into the idrive or what the idrive would do with that info if you could, but maybe the speedometer could be corrected that way.

What I usually do is calculate out the difference and remember it. Maybe it is 5%. If you use Waze or another GPS map, you can see your real speed and compare it. Maybe the speedo is 5 mph off at 80 mph, for example.

[carseatsm5]

You’re the man thanks.

[pbonsalb]

I did this on my E90 M3 — I was thinking about going from the stock 3.85 to a 4.10 diff for performance but a diff is expensive so the next time I needed rear tires (I got only about 12k out of rears), I bought 285/30/18 instead of another set of 285/35/18 for my wider than stock Apex wheels. That gave me the equivalent of a shorter 4.10 diff. I liked it but decided I would go back to 285/35/18 next time. A little better ride, a little less buzzy on the highway, and a little better looking than the rubber band like 30 series. Sold the car before the next tire change, but recall that the new owner went back to 285/35/18.

On my turbocharged E36 M3, I went the other direction and ran wider tires but did not go down in aspect ratio to keep the same diameter. Stock is 245/40/17 and I run 275/40/17. I like that setup. The diff ratio is longer, which normally hurts performance but that car is RWD and has about 700 rwhp so it has traction problems and the effectively longer gearing works great. Speedo is a bit lower than actual.

On my F90, I might go to 295/35/20 and 305/35/20. I remember the math since I did that calculation not long ago. It effectively changes the gearing from 3.15 to 3.09. My car is decently modded and has more torque so I am not worried about a performance drop.

[carseatsm5]

Originally Posted by pbonsalb

I did this on my E90 M3 — I was thinking about going from the stock 3.85 to a 4.10 diff for performance but a diff is expensive so the next time I needed rear tires (I got only about 12k out of rears), I bought 285/30/18 instead of another set of 285/35/18 for my wider than stock Apex wheels. That gave me the equivalent of a shorter 4.10 diff. I liked it but decided I would go back to 285/35/18 next time. A little better ride, a little less buzzy on the highway, and a little better looking than the rubber band like 30 series. Sold the car before the next tire change, but recall that the new owner went back to 285/35/18.

On my turbocharged E36 M3, I went the other direction and ran wider tires but did not go down in aspect ratio to keep the same diameter. Stock is 245/40/17 and I run 275/40/17. I like that setup. The diff ratio is longer, which normally hurts performance but that car is RWD and has about 700 rwhp so it has traction problems and the effectively longer gearing works great. Speedo is a bit lower than actual.

On my F90, I might go to 295/35/20 and 305/35/20. I remember the math since I did that calculation not long ago. It effectively changes the gearing from 3.15 to 3.09. My car is decently modded and has more torque so I am not worried about a performance drop.

That makes sense.

What about my drop? Do I have to go adjust the HAS since the tires aren’t as tall?

[pbonsalb]

That is a cosmetics thing. You can look up the size difference on a tire size calculator online. Half the difference will be what you see as increased gap from tire to fender. I’d guess it might be 1/4 to 3/8 inch. You will have to decide what you like.