bmw545

Good year F1 GS-D3 no longer making 245/40/18
Can I put size 255/40/18 in the front for 545i sport?

Good year has $40 rebate until Jan-21/06 btw.

OliverB

Technically, it's possible ...
But the circumference of the tire will be higher ! You'll also change slightly the front ride height & the balance of the car ! :thumbsdown:
You should rather choose another make of tire to respect the exact ratio width/height !

EBMCS03

Ya you can... thats such a minor difference you'll be ok.

a 18X8" wheel will fit a 255 with no problems.

Diameter for a 245/40/18 is 25.72"
and for 255/40/18 its 26.03"

This ALTERNATE tire size will change the actual vehicle speed to 65.8mph when the odometer reads exactly 65mph.

Difference of 0.8 mph at 65mph is well within specs.

As for the "exact ratio width/height" of the car there isnt... Going from a 16" 17" or 18" wheel all OEM wheels from BMW ALL have a slight difference in overall diameter.

On street cars, it should have an overall diameter that is within ?3% of the factory specification. The ?3% variation is recommended for street cars in order to maintain accurate speed data going into the computer, thus keeping odometer and speedometer changes negligible.

Now I dont know what BMW uses as the factory specification for overall diameter

But I'm sure that 255/40/18 is well within spec and might be less change in "ratio width/height" than BMWs different OEM wheels itself.

bmw545

Thanks

Actually I found out when my car is at 68 but odometer read is at 70. (according to GPS device)
So I believe using 255/40/18 will make it a little bit better.

Maybe I should put 285/35/18 for the rear to make all 4 tires' diameter to 26"

bmw545

530i standard: 225/50/17 diameter 25.9"
545i 245/40/18 diameter 25.7"

255/40/18 diameter 26"
285/35/18 diameter 25.9"

EBMCS03

Ya actually I knew that but I just didnt want to say. I think ALL BMWs are like that they over estimate 2mph... my E60 is like that and my MINI Cooper S is like that also. Even my moms old E34 525 is like that... EXACTTLY like what you said. GPS shows 68 but my speedo shows 70 in both cars... Eh... doesnt bother me.

bmw545

I might be too picky.
it is about 3% different.
But, think about this way, by the time car reach to 50K miles (out of warranty) but it is actually only 48.5K miles. We lose 1500 miles. Not many but not little too.

heezy545i

I was looking to put wider tires up front, but ended up with stock sizes. Putting 255s up front while leaving 275s in the rear should decrease understeer.

EBMCS03

Is it 3%?

I'm calculating a 1.2% difference. Am I doing somthing wrong?

Then again... you're changing the front right? and is the odometer reading off the front or rear wheels?

bmw545

Speed at 78 show 80 on odometer.
78*1.03 = 80.34 so 3% right?

I thought the odometer reading is usually the front. I might be wrong.