Why are Z4M tires taller than 3.0si's

[mhughett]

I'm going to order some non-run flat tires tonight to take advantage of DTD's special discount that goes through Monday. Probably getting the Hancook V12's as they have a rebate that expires soon too and have received very good reviews. I have the sport package on my coupe and was wondering why the M coupes have the same width and rim diameter as the 3.0si coupe but have slightly taller sidewalls on the tires. Does this have something to do with the fact they are non run-flats and need the extra width? Should I get the taller size for my coupe? I know it will add about 3.5% to my speed relative to the spedometer reading but I'm not concerned about that.

All comments and opinions are appreciated. Thanks.

[GP20]

Left-over tires from the E46 M3 production run.

[aerobod]

Originally Posted by mhughett

I'm going to order some non-run flat tires tonight to take advantage of DTD's special discount that goes through Monday. Probably getting the Hancook V12's as they have a rebate that expires soon too and have received very good reviews. I have the sport package on my coupe and was wondering why the M coupes have the same width and rim diameter as the 3.0si coupe but have slightly taller sidewalls on the tires. Does this have something to do with the fact they are non run-flats and need the extra width? Should I get the taller size for my coupe? I know it will add about 3.5% to my speed relative to the spedometer reading but I'm not concerned about that.

All comments and opinions are appreciated. Thanks.

The slightly higher profile tyres used on the ///M cars compared with the regular models with sport package is typical across the range (3-series to M3, Z4 to Z4M 5-series to M5). There are a number of possible reasons:

- Very low profile tyres are more for looks than performance
- Effectively higher gearing for higher speeds in each gear
- Standard ///M tyres aren't sized to give the wheel arch clearance to fit snow chains (alternative OEM winter tyre size for Z4M and M3 is the same as for Z4 or 3-series, to allow chains to fit).

[vachss]

Originally Posted by aerobod

- Effectively higher gearing for higher speeds in each gear

I would think that since the rear axle ratio of the Z4M Coupe is 3.62 vs. 3.46 for the non-M that any effective gearing difference due to taller tires is largely canceled out.

[mhughett]

I agree with the comment about the gearing ratio offsetting the change in tire diameter. What I was wondering was whether buying the slightly taller size would have a noticeable effect on the car's acceleration. I've also heard about this "3%" rule meaning that you should never change your size by more than 3%. What's up with that?

[aerobod]

Originally Posted by mhughett

I agree with the comment about the gearing ratio offsetting the change in tire diameter. What I was wondering was whether buying the slightly taller size would have a noticeable effect on the car's acceleration. I've also heard about this "3%" rule meaning that you should never change your size by more than 3%. What's up with that?

The Z4m is spec'd for 225/45-18 front and 255/40-18 rear, or 225/40-18 all around for winter use.

The 225/40-18 is the same size as used on the front of the Z4 with sport package.

If you can use both a 225/45-18 or 225/40-18 on the Z4M without any modification, then you can probably do the same on the Z4.

You probably won't notice any significant acceleration difference.

[aerobod]

Originally Posted by vachss

I would think that since the rear axle ratio of the Z4M Coupe is 3.62 vs. 3.46 for the non-M that any effective gearing difference due to taller tires is largely canceled out.

The gearing is 4.6% higher in the non-///M, but the ///M has a 14.3% redline, so with the taller tyres, the maximum speed in any gear is 13.6% higher in the ///M vs a 3.0si.

[mhughett]

Originally Posted by aerobod

The gearing is 4.6% higher in the non-///M, but the ///M has a 14.3% redline, so with the taller tyres, the maximum speed in any gear is 13.6% higher in the ///M vs a 3.0si.

I don't mean for this to be any kind of a competition between the M and the 3.0si so I'm really not sure how the comment above helps me in my decision. My 7k RPM redline is just fine thank you very much.

By the way, I did some direct comparisions this weekend since there was a 2006 M roadster for sale not too far from my house. I measured the diameter of the wheel/tires of both cars and there was about 1/2-3/4 of an inch difference. Not a lot of difference. I really like the fatter steering wheel and this might be on my short list for Santa this year. Pretty pricey from Tischer at about $1k. Ouch. The main thing I wanted to check out is how much different the tires made in the ride over small bumps and pavement irregularities. This convinced me to dump the run-flats, NOW.

[aerobod]

Originally Posted by mhughett

I don't mean for this to be any kind of a competition between the M and the 3.0si so I'm really not sure how the comment above helps me in my decision. My 7k RPM redline is just fine thank you very much.

By the way, I did some direct comparisions this weekend since there was a 2006 M roadster for sale not too far from my house. I measured the diameter of the wheel/tires of both cars and there was about 1/2-3/4 of an inch difference. Not a lot of difference. I really like the fatter steering wheel and this might be on my short list for Santa this year. Pretty pricey from Tischer at about $1k. Ouch. The main thing I wanted to check out is how much different the tires made in the ride over small bumps and pavement irregularities. This convinced me to dump the run-flats, NOW.

Doesn't help you with your decision, but illustrates why there is a difference.

I would buy tyres from a vendor that promises an exchange or refund on the tyres if you don't like them or if they cause problems. All indications are that the ///M sized tyres will work without a problem on your non-///M Z4.

[mhughett]

I looked in the Z4M owner's manual (available online at BMWUSA.com) and sure enough they recommend the smaller size as well for M+S tires. I guess that is to allow for chains if necessary but I can't imagine using them on such a low-profile tire. In any event I went ahead and ordered the M sized tires and bought the Hancook V12 Evo's due to the favorable reviews they have received as well as the great incentives they have (that end today). Thanks to everyone for your help.

[Starscream]

I'm sure it just boils down to what GP20 already said,
Leftover M3 stock,

Shame they didn't fit the Michelin Pilot Sports of the M3 run

But I do remember back in 06 when the E46 M3's were still in showrooms there was a 06 Vert with Conti's, and a 06 Coupe with Pilot Sports, (both with the 18" wheel),

Then as we all know all Z4M's came with the Conti's (with M3 on the sidewall)

[O-cha]

Originally Posted by Starscream

I'm sure it just boils down to what GP20 already said,
Leftover M3 stock,

Shame they didn't fit the Michelin Pilot Sports of the M3 run

But I do remember back in 06 when the E46 M3's were still in showrooms there was a 06 Vert with Conti's, and a 06 Coupe with Pilot Sports, (both with the 18" wheel),

Then as we all know all Z4M's came with the Conti's (with M3 on the sidewall)

Which is annoying, because what most people don't know is magazine performance testing and many others are doe on stock tires. And when it went up against all the newer models with way new tire technology the z4m got destroyed (actually was even with most competitors, but should have killed them) because of it (tires really will make or break the car).

[Starscream]

Originally Posted by O-cha

Which is annoying, because what most people don't know is magazine performance testing and many others are doe on stock tires. And when it went up against all the newer models with way new tire technology the z4m got destroyed (actually was even with most competitors, but should have killed them) because of it (tires really will make or break the car).

Indeed,

I actually was a big fan of the first gen Pilot Sports,
If you look at this pre-production Z4M you can see it has the PS's fitted to it,

and a nice exhaust can setup

Shame it didn't come like this

[AWM3]

BMW speedometers are notorious for displaying a speed that is higher than your actual speed. The odometer measures an accurate distance, it's just the speedometer that is intentionally made to read high. I read once that there is some German law that the speedometer must give the correct speed for the largest diameter tires than can possibly be fitted on the car. Since you ordered the M sized tires for your car, you will lose a slight bit of acceleration but you will gain a bit of fuel economy, your speedometer will more accurately indicate your true speed, and you will have a little more sidewall between your wheels and whatever road hazards they may encounter.

[jpl73]

Originally Posted by mhughett

I looked in the Z4M owner's manual (available online at BMWUSA.com) and sure enough they recommend the smaller size as well for M+S tires. I guess that is to allow for chains if necessary but I can't imagine using them on such a low-profile tire. In any event I went ahead and ordered the M sized tires and bought the Hancook V12 Evo's due to the favorable reviews they have received as well as the great incentives they have (that end today). Thanks to everyone for your help.

Hi,
are the new tires you bought (Hancook V12) of the Run-flat type? If not, what happens in case of a puncture?
I just bought a used Z4C and it has Non RunFlats fitted. the tires are a year and a half old so i'm not considering replacing them for the next 2 years.
As i see it i have two options:
1. buy a can of Tire-fix (don't know what its correct name is... ) to keep in the boot or
2. buy an emergency tire + jack

any suggestions?
(sorry if i'm off topic...... )

[mhughett]

Originally Posted by jpl73

Hi,
are the new tires you bought (Hancook V12) of the Run-flat type? If not, what happens in case of a puncture?
I just bought a used Z4C and it has Non RunFlats fitted. the tires are a year and a half old so i'm not considering replacing them for the next 2 years.
As i see it i have two options:
1. buy a can of Tire-fix (don't know what its correct name is... ) to keep in the boot or
2. buy an emergency tire + jack

any suggestions?
(sorry if i'm off topic...... )

They are in fact non run flat tires. That was the point of the purchase as my current run flat tires have quite a bit of tread life left. As for what provisions I'm going to make to compensate for this, I have a Dyna-plug system (that I've used successfully with my other BMW) along with a portable compressor in the trunk. I'm confident this will keep me on the road in case of a problem. And, there is aways AAA as a back-up.

[jpl73]

Originally Posted by mhughett

They are in fact non run flat tires. That was the point of the purchase as my current run flat tires have quite a bit of tread life left. As for what provisions I'm going to make to compensate for this, I have a Dyna-plug system (that I've used successfully with my other BMW) along with a portable compressor in the trunk. I'm confident this will keep me on the road in case of a problem. And, there is aways AAA as a back-up.

Thanks for the reply!! I'll have to buy such a kit . I guess it is easier to apply the repair plug on the front tires (the puncture is more accessible with the steering wheel is turned all the way) than the rear tires....
Any comments on the cans of foam spray which seal the puncture from the inside?
is it true that this is a temporary solution until you reach a tire shop? and the tire must be replaced after using this foam?
thanks!!

[mhughett]

It's my understanding that the tire goop is only temporary. The tire shops hate this stuff as they have to clean up the mess once they take the tire apart and put a patch in place.

[jpl73]

Yeah it make sense.
Happy miles with the new tires.....

[O-cha]

Originally Posted by Starscream

Shame it didn't come like this

Um, NEED those exhaust cans... stock ones stick out so much.... I'm probably going to paint them eventually.

[Starscream]

Originally Posted by O-cha

Um, NEED those exhaust cans... stock ones stick out so much.... I'm probably going to paint them eventually.

Yeah I hate the stock look too,

I actually just fitted the E46 M3 stock exhaust tips to my M,
They are longer than the Z4M tips and look really nice, I will have to do some photo's

and the cans I have seen painted flat black and it looks much better,

[O-cha]

Originally Posted by Starscream

Yeah I hate the stock look too,

I actually just fitted the E46 M3 stock exhaust tips to my M,
They are longer than the Z4M tips and look really nice, I will have to do some photo's

and the cans I have seen painted flat black and it looks much better,

Any more pics of the preproduction? It looks like the side are different too, I'm glad that trunk key hole didn't make it to production though.