[beemarman]

I am currently running 295-25-21 in the rear. Can I go to 285-30-21 with the same setup?

I want to improve the ride because the current setup isn't the nicest to drive.

[snowbimmer]

Originally Posted by beemarman

I am currently running 295-25-21 in the rear. Can I go to 285-30-21 with the same setup?

I want to improve the ride because the current setup isn't the nicest to drive.

You have an M850? Your profile says F10 M5.

[beemarman]

Originally Posted by snowbimmer

You have an M850? Your profile says F10 M5.

Yes, my old car was an M5. Currently got the M850i

[snowbimmer]

Originally Posted by beemarman

Yes, my old car was an M5. Currently got the M850i

Ok. So the M850 OEM tire profile is also a ridiculously low profile to begin with (IMO) and you made it even worse by going to 21" wheels. You are actually talking about adding around an inch in tire height, which would bring you to about the same cushion that an OEM height sidewall would have. You'd also have to upsize your fronts because the Xdrive requires that front and rear diameters remain with very close tolerances.

There are several forum members running the taller tires on the M850 (roughly the same as the Alpina runs) with no ill effects. I was going to do it, but chickened out when my dealer said don't do it and my performance shop said don't do it. Aside from changing the speedometer a bit, I don't see where it would be a problem, as long as your fronts and rears are very close in diameter (within 1% or so). It's not a problem with a rear wheel drive 840.

Maybe they'll chime in.

[beemarman]

Originally Posted by snowbimmer

Ok. So the M850 OEM tire profile is also a ridiculously low profile to begin with (IMO) and you made it even worse by going to 21" wheels. You are actually talking about adding around an inch in tire height, which would bring you to about the same cushion that an OEM height sidewall would have. You'd also have to upsize your fronts because the Xdrive requires that front and rear diameters remain with very close tolerances.

There are several forum members running the taller tires on the M850 (roughly the same as the Alpina runs) with no ill effects. I was going to do it, but chickened out when my dealer said don't do it and my performance shop said don't do it. Aside from changing the speedometer a bit, I don't see where it would be a problem, as long as your fronts and rears are very close in diameter (within 1% or so). It's not a problem with a rear wheel drive 840.

Maybe they'll chime in.

Thanks you for the quick and detailed reply.

So changing the 21 inch tires to match the current 20 inch profiles will improve the ride, or is the ride issue with me switching to the 21 inch alloys?

[Seaford]

Originally Posted by beemarman

Thanks you for the quick and detailed reply.

So changing the 21 inch tires to match the current 20 inch profiles will improve the ride, or is the ride issue with me switching to the 21 inch alloys?

As above if you are 4wd be very, very cautious about changing tyre sizes…..same applies to all 4wd.

[snowbimmer]

Originally Posted by beemarman

Thanks you for the quick and detailed reply.

So changing the 21 inch tires to match the current 20 inch profiles will improve the ride, or is the ride issue with me switching to the 21 inch alloys?

I think you're main problem was going to the 21" and dropping the profile down to the 25. Your sidewall went from approx 3.25" OEM to 2.9". Your overall tire height went up a bit, however, from 26.5" to 26.8"

When you talk about going to the 285-30-21 tire, you will gain about 1.25" in tire height and pick up nearly 1/2 inch in sidewall to 3.37", which is now bigger than OEM, but not by much.

If you do this, you must also address the front tire height. The 21" Alpina B8 tires run right at 27.75" in tire height, with a sidewall similar to the M850.

In a perfect world, I would run a 20" wheel with a 35 profile - like the M8s have. Then you end up with sidewalls between 3.8" and 3.9". This has gotta be way less harsh.

[ElloM8]

Originally Posted by beemarman

I am currently running 295-25-21 in the rear. Can I go to 285-30-21 with the same setup?

I want to improve the ride because the current setup isn't the nicest to drive.

Not sure what your offset looks like to answer this perfectly but you should absolutely be able to move up. I have 295/25 on my 22" (rear) wheels & will likely move up to a 305/30 R22 this Summer. I already have oversized tires on my 20" wheels (aftermarket) & I love the ride. It does throw the speed off by about 1.9MPH at 70MPH but that never bothered me. I'm use to it with my truck.

[snowbimmer]

Originally Posted by ElloM8

Not sure what your offset looks like to answer this perfectly but you should absolutely be able to move up. I have 295/25 on my 22" (rear) wheels & will likely move up to a 305/30 R22 this Summer. I already have oversized tires on my 20" wheels (aftermarket) & I love the ride. It does throw the speed off by about 1.9MPH at 70MPH but that never bothered me. I'm use to it with my truck.

Wow.

What size tires are you running on your 20" wheels?


Also, if you are OEM everything, and your speedo says 70, we're actually going about 68, right? At least that's what I see when the radar flashes back at me on the highway. So, if you upsize the tires, your speedo under-reports the speed - in other words if your speedo says 70, you're really doing 72. Hence, if BMW calibrates the speedo to be off by 2 mph in the first place, wouldn't a 70 mph speedo on the upsized tires actually now be a true 70 mph? Upsized tires would negate the BMW calibration.

Math is hard. They said there wouldn't be any math.............

[ElloM8]

Originally Posted by snowbimmer

Wow.

What size tires are you running on your 20" wheels?


Also, if you are OEM everything, and your speedo says 70, we're actually going about 68, right? At least that's what I see when the radar flashes back at me on the highway. So, if you upsize the tires, your speedo under-reports the speed - in other words if your speedo says 70, you're really doing 72. Hence, if BMW calibrates the speedo to be off by 2 mph in the first place, wouldn't a 70 mph speedo on the upsized tires actually now be a true 70 mph? Upsized tires would negate the BMW calibration.

Math is hard. They said there wouldn't be any math.............

Winter set: 295/35 R20 & 255/40 R20. Waze shows me moving slower by about 1MPH and Draggy has me moving 1MPH faster then the speedometer.

Summer set: 295/25 R22 & 245/30 R22. This set has me my speedometer at exactly what Waze shows. Not sure as I have not calibrated via Draggy yet. Didn't get the draggy until after I tuned the car in December.

[eldyf93]

Originally Posted by ElloM8

Winter set: 295/35 R20 & 255/40 R20. Waze shows me moving slower by about 1MPH and Draggy has me moving 1MPH faster then the speedometer.

Summer set: 295/25 R22 & 245/30 R22. This set has me my speedometer at exactly what Waze shows. Not sure as I have not calibrated via Draggy yet. Didn't get the draggy until after I tuned the car in December.

What are your 0-60 and 60-130 times with the 22" wheels?