Tire Question -- HELP!!!!!!!!

[James Bond]

I recently got some new rims for my car, and now I'm trying to find some tires.

The rims are staggered--18x8.5 up front, and 18x9.5 in back.

The "recommended" tire sizes, for those, are 225/40/18 up front, and 255/35/18 in back.

The tires that I want, and planned on getting, come in the 225/40/18, but not in 255/35/18. I could, instead, put 245/40/18 on the back... but it would be .5" higher.

My question IS...... What sort of impact would that have on my car? Would my speedometer be off? Would it affect my safety? A half inch seems like SO little... but then again, I don't know much about tires.

 

[IronWing]

The extra height shouldn't matter as long as they don't rub. Ask the tire shop folks about the width. The big no-no is using the incorrect radius for the wheel.

 

[mercanucaribe]

If it's an AWD car, that is bad. If it's RWD, it's fine. I doubt it's an AWD car if the tire sizes aren't the same front and rear, simply because even if they calculate the proper dimensions for identical diameter front to rear, the actual diameters will be different.

The speedometer will be slightly off. Are you sure that diameter will fit? It should since that's only .25" increase in radius.

 

[James Bond]

Originally posted by: ironwing
The extra height shouldn't matter as long as they don't rub. Ask the tire shop folks about the width. The big no-no is using the incorrect radius for the wheel.

That's good news, and exactly what I wanted to hear.

Is there anything else I should be worried about?

 

[spidey07]

Could change handling and traction. For the better or worse? Don't know. But there is an impact on the contact patch of the tire.

 

[James Bond]

Originally posted by: mercanucaribe
If it's an AWD car, that is bad. If it's RWD, it's fine. I doubt it's an AWD car if the tire sizes aren't the same front and rear, simply because even if they calculate the proper dimensions for identical diameter front to rear, the actual diameters will be different.

The speedometer will be off. Are you sure that diameter and width will fit?

Its RWD.

I'm 99% sure the diameter and width will fit... I will have to ask the guys at the tire shop I guess.

 

[overst33r]

I'm pretty sure those measurements are in millimeters.

 

[Howard]

Originally posted by: mariok2006
I'm pretty sure those measurements are in millimeters.

That's correct.

 

[IronWing]

On the safety issue: with an aspect ratio or 35 or 40 don't be hitting any potholes, speed bumps, curbs, or soda straws.

 

[spidey07]

Originally posted by: ironwing
On the safety issue: with an aspect ratio or 35 or 40 don't be hitting any potholes, speed bumps, curbs, or soda straws.

ummm, it's called an aspect ratio for a reason. The number alone is meaningless.

-edit-
nice 350 or GTO.

 

[natto fire]

Originally posted by: Howard

Originally posted by: mariok2006
I'm pretty sure those measurements are in millimeters.

That's correct.

Yeah, and the difference will be 17.5mm or .689 inches. Keep in mind those aspect ratios are for section measurements, therefore you have to multiply the difference by two.

Your car might use the traditional way of driving a speedometer, which will be thrown off by the change in tire circumference, but it might also calculate the speed by sensing the difference in wheel speed sensors compared to a FD sensor. Given that it is a BMW, and probably has Bosch ESC, which is a very advanced system, and could use the latter. Not 100% on this, though.