Different tyres sizes?????

[omodos]

I had the same questions and had a staggered setup and was toying woth the idea of different tires , in the end i bought a set of used alloys 17" and tires are the same all the way round and the correct tires as you see them listed on the fuel cap flap

[Fensterb]

Hey guys, does anyone know if having a standard tire and a Z tire on the same car makes a difference/has an effect on this?

We bought our 04 X3 in August, and now when I drive it, I dont like how the transmission feels. It is a hard shift at low speeds sometimes and I get a weird RPM "flutter" when slowing down/coasting at approx 30mph.

Anyways a local mechanic pointed out to me that the tires in the front are different sizes than those in the rear. The measurements are all the same (235/35/17), however the rears are Z rated. I can really see a difference in size though. The fronts definitely look bigger.

Can tires have the same measurements on their side walls but really be different sizes?

Can this cause issues?

[Qsilver7]

Originally Posted by Fensterb

1. Hey guys, does anyone know if having a standard tire and a Z tire on the same car makes a difference/has an effect on this?
2. Can tires have the same measurements on their side walls but really be different sizes?

1. Having different tire brands with different tread patterns can create a different feel/response from each axle
2. Yes, tires with the same measurement on their side walls can actually have different overall cirucmference/diameter

TIP: spending approx 30 minutes on a site like Tire Rack - Your performance experts for tires and wheels educating yourself to various tire technology & info can go a long way in expanding your knowledge about the questions you asked. You can come away with a lot of information about the tire industry and what all the specs & stuff mean that is provided on tire sidewalls...and you can actually compare many tire brands & specs side by side and see exactly how different some tires may be even though they are the same size, same category, etc.

[oldskewel]

Originally Posted by JCL

Tire size differences can certainly cause damage with the pre x-drive transfer case. I would strive for 3-4 revolution per mile difference, but I don't think that is a hard cutoff...

Originally Posted by JCL

...
*The non x-drive (nxd) is an open diff. It can handle speed differences
...
*BMW has a towing limit, with one axle raised, of 150 km at 50 km/hr for the nxd. Towing the xd with one axle raised is not permitted due to the risk of transfer case damage.

I've got a 2001, non-xdrive, NV125 transfer case, 19" style 63 BMW wheels, 285/45R19 107V rear, 255/50R19 103V front. The spare is of course a 235/65R17 with zero wear.

My rear tires are worn. That is a fairly rare size, but I can get a pretty good deal on some lightly used, nice 275/45R19 tires, but of course I'm concerned with causing TC problems.

I know that buying a new set of 4 Michelins at the dealer, and replacing the never used, but 14 years old spare as well will work, and many here would do just that. Not me.

So first question on the TC tolerance on difference in rotations front to rear - JCL quotes 3-4 per mile should be good, and I think I saw that somewhere else too. But then the quote on towing (also confirmed in the owner's manual) - that seems EXTREMELY more stressful than the 3-4 revs/mile difference for driving. It would make me think that the real limit may be higher than 3-4. Comments?

Also, I ran some numbers for myself calculating the differences in tire diameter and revs per mile for different tires. Following is the output for my current situation.
1 - 285/45R19 on rear, with about 8/32" of wear gone (not remaining)
2 - 255/50R19 on front, 4/32" worn
3 - 235/65R17 spare, 0/32" worn
4 - 275/45R19 candidate replacement for rear, 2/32" worn

revs_per_mile =
705.2 700.5 694.8 704.7
diameter_mm =
726.4 731.3 737.3 726.9

Comments - the candidate tire fits better than the existing one due to lower wear. The spare is further out.

BTW, if I did not account for tire wear, here are the results (assuming all tires were brand new with no wear):

revs_per_mile =
693.1 694.5 694.8 701.6 (brand new, all specified tires are in a tight range)
diameter_mm =
739.1 737.6 737.3 730.1

I know this does not account for manufacturing tolerances, inexact tire design, inflation, load, etc.

Does this seem like a safe plan? BTW, the TC is pretty new, dealer installed by the PO a year before I bought it.

[JCL]

I don't have a single answer for you, but I can give you some data points and a suggestion.

The BMW workshop manual lists a 1% maximum allowable difference between tires to avoid transfer case damage, on the x drive transfer case. I don't have the published figure for the non x drive transfer case. 1% works out to about 7 revs/mile, so that is their hard limit. I have used 4 revs/mile as my own limit, based on previous discussions with dealer service personnel and my own experience with other awd vehicles.

I deal with a good tire shop. I recently had a tire receive a cut to a sidewall on a lightly worn Pirelli Scorpion on an x drive vehicle. They supplied one new tire to replace it. I had purchased 4 tires from them not too long ago. Turns out they covered most of the cost via a road hazard warranty I had, but that was a bonus. They checked the other tires, and told me I was OK with replacing only one. I asked, and the limit they used was 40% wear on a same size tire compared to the new one. Not sure how many revs/mile that is, but your calculator would tell you. I guessed it was around 5 revs/mile, but I didn't calculate it.

You can see from your calculator how tightly BMW aimed to have them grouped when new, less than 2 revs/mile.

It appears that with your worn front tires, the used tires you found would work.

It appears that if you do that, when you next replace your front tires you may need a plan B. Perhaps change the front size to match the rear if the tread wear is close.

I wouldn't worry as much about the spare.

Good luck

Jeff

[oldskewel]

Originally Posted by JCL

...
You can see from your calculator how tightly BMW aimed to have them grouped when new, less than 2 revs/mile.

It appears that with your worn front tires, the used tires you found would work.

It appears that if you do that, when you next replace your front tires you may need a plan B. Perhaps change the front size to match the rear if the tread wear is close.

I wouldn't worry as much about the spare.
...

Thanks for the confirmations - everything you say makes sense to me. BTW, I expect that if I get these ~new ones on the rear now, they will wear out when the fronts do and I'll get 4 new ones at that time.

[jdstrickland]

To the guy that asked about the 285/45 and 255/55 combination. These are identical in over all diameter. One of them makes 693 revolutions per mile, the other makes 694. The 255 makes the extra revolution.

The overall diameter is 29.098 and 29.039, respectively. These are identical for any practical purpose. You have no worries about the drive train using these two sizes.

[jdstrickland]

And, to dot the i and cross the t, The 235/65x17 is also in there at 695 revs. per mile. All of these sizes are suitable for use at the same time on your car. That is, the spare can be used anywhere and will play well with the other three.

As with any spare, you will want to get the damaged tire repaired or replaced as soon as is practical.