Mick787
Posts: 46
Joined: Wed Jul 11, 2018 2:43 pm

Spare tyre option

Fri Dec 06, 2019 10:31 am

Our 94AH REX has the wider tyres on the rear. 155/70 R19 on the front 175/60R19 on the rear. I have noticed uneven tyre wear and as a result, with the winter weather approaching here in the UK, it would be wise to fit new rubber. There has been a delay in obtaining a set of 4, the 175's taking the longer time to obtain. What is concerning me is the time taken to obtain tyres, it appears that these are not something you can drive in and have fitted while you wait. I was present when the dealer rang Bridgestone and could hear that this was not someone messing around, it does appear they are harder to get hold of than 'normal' tyres. Thankfully we have not had an incident such as a sudden tyre deflation, it is wear and uneven at that, but at the end of the day, the reality is that the car could be off the road for a while if tyres aren't available. I fully accept that all tyre sizes cant be stocked by everyone.
So, holding a spare rim seems to be the most logical solution, but the rims are different sizes. I very much doubt the rear rim will fit on the front. Can a front rim be safely fitted on the rear or is it the case that a front rim and a rear rim have to be kept in storage?

jadnashuanh
Posts: 4766
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Spare tyre option

Fri Dec 06, 2019 10:55 am

The front tires/wheels do fit on the rear of all i3's. If you decided to get winter tires, the only ones available are the smaller, front size in a square (same all around) setup. THere is a second supplier for winter tires - Nokian, but not for the summer or all-season tires.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

JohnKelly
Posts: 70
Joined: Sat Oct 07, 2017 7:02 am

Re: Spare tyre option

Sun Dec 08, 2019 9:34 am

The Nokian 155 snow tires will absolutely fit on the 5-1/2" wide rear wheels, even though they are not supposed to. I ordered mine from a large tire company and had a local tire shop mount them. I told them the wheels were staggered and that the tires would fit both front and rear. Over a thousand miles on them with some hard cornering and without any problems. Others have done the same.
2015 REX / Participating on plugshare w/level 2 Clipper Creek 32 amp, Moclips, Washington coast

jadnashuanh
Posts: 4766
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Spare tyre option

Sun Dec 08, 2019 9:04 pm

When you put the same tire on a wider wheel, assuming it is within the allowable range, it will fit, however, that does change the OD of the whole assembly. That will change your speedometer and odometer and maybe confuse the ABS logic. THe difference in this case won't be much, but it's recommended to keep all of the wheels' diameters within a max of 4%.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

JohnKelly
Posts: 70
Joined: Sat Oct 07, 2017 7:02 am

Re: Spare tyre option

Mon Dec 09, 2019 10:14 am

The difference in diameter is only .38 of an inch... a very small difference.
2015 REX / Participating on plugshare w/level 2 Clipper Creek 32 amp, Moclips, Washington coast

jadnashuanh
Posts: 4766
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Spare tyre option

Mon Dec 09, 2019 2:25 pm

JohnKelly wrote:
Mon Dec 09, 2019 10:14 am
The difference in diameter is only .38 of an inch... a very small difference.
That's about 1.4%...close to the maximum allowable with an all-wheel drive vehicle, but not as big a deal with one axle driving things, which would cut down your allowable 'real' mileage under warranty, give you a bit more torque (not great when the roads are slippery), but probably close enough, most people would never notice. That also means your speedometer will be more conservative...saying you're going that much faster than you really are (there's about a 2mph bias built in, fixed, but the added variance from this is linear with speed, not fixed).

There's a reason why the aspect ratio varies on the OEM tires when the wheel size changes.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

JohnKelly
Posts: 70
Joined: Sat Oct 07, 2017 7:02 am

Re: Spare tyre option

Tue Dec 10, 2019 9:10 am

Are you sure the speedometer reading is taken from the rear wheels? I drive by a speed reader regularly and I always&still read 2mph above what it reads.
2015 REX / Participating on plugshare w/level 2 Clipper Creek 32 amp, Moclips, Washington coast

jadnashuanh
Posts: 4766
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Spare tyre option

Tue Dec 10, 2019 2:46 pm

Not positive of anything, but, the difference in OD is about 1.4%, which, at say 30mph, may not be enough to show up. The 2mph buffer is a constant above the design speed, however that's detected. But, at 30mph, 2mph is 6.67%, so 1.4 isn't that big of a difference (less than 0.5mph). The vehicle is not using GPS for determining speed or odometer, though. That leaves the gear ratio and the tire size. I'm sure it's not a mechanically driven cable, but the detector must get its reference from some rotating device, and changing the OD of the tire/wheel assembly will throw off that actual correlation.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

Mick787
Posts: 46
Joined: Wed Jul 11, 2018 2:43 pm

Re: Spare tyre option

Wed Dec 11, 2019 8:24 am

Thanks for all the responses. I still don't know whether to buy just one front tyre and keep it as a spare for any position on the car or whether to buy a rear axle specific rim as well. They seem to be harder to get hold of.

jadnashuanh
Posts: 4766
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Spare tyre option

Wed Dec 11, 2019 8:08 pm

As a temporary, emergency spare, a single one sized for the front would seem to be more than sufficient.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

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