Compact Space Saver Tire/Wheel Solution

[JST]

The nominal diameter of a 245 45 19 is 27.7".

Nominal diameter of a 245 35 21 is 27.8".

Nominal diameter of a 155 80 18 is 27.8".

So the compact spare I have is exactly the same nominal diameter as a 21 OEM wheel, which is 1/10" bigger than a 19" wheel.


I don't see how running a wheel 1/10th of an inch larger in diameter could cause any issues with the driveline--like I said, the reason you have a differential is to allow one tire to turn faster than the other, while going around corners.

With a big enough size difference and limited slip diff you might have a problem. But this is not a big size difference and there is no LSD.

But if (and this is a big if) there were any negative effects from the small size difference, those effects should be the same whether you are running a 19" OEM wheel on a car with 21s, a 21" OEM wheel on a car with 19s, or a 155 80 18 space saver on a car with 19s.

Of course, on a car with 21s the 155 80 18 space saver is the same size.

 

[Larry Chanin]

They are similar in terms of diameter, but not exactly the same.

As for using a compact spare on the rear--I wouldn't worry about hurting the drivetrain. That's what differentials are for, and if the diameter is close enough that it doesn't freak out the electronics, I doubt it will be an issue for the mechanicals.

I haven't test fit the 18" compact wheel on the rear, though, so I am not sure it will clear the brake calipers back there. That's one minor issue.

The other is that since the MS is a powerful RWD car, I'd rather have the lower-traction wheel on the front than on the rear. Understeer is more predictable than oversteer, and why try and put that kind of torque through a 145 or 155 mm tire of you don't have to?

The nominal diameter of a 245 45 19 is 27.7".

Nominal diameter of a 245 35 21 is 27.8".

Nominal diameter of a 155 80 18 is 27.8".

So the compact spare I have is exactly the same nominal diameter as a 21 OEM wheel, which is 1/10" bigger than a 19" wheel.


I don't see how running a wheel 1/10th of an inch larger in diameter could cause any issues with the driveline--like I said, the reason you have a differential is to allow one tire to turn faster than the other, while going around corners.

With a big enough size difference and limited slip diff you might have a problem. But this is not a big size difference and there is no LSD.

But if (and this is a big if) there were any negative effects from the small size difference, those effects should be the same whether you are running a 19" OEM wheel on a car with 21s, a 21" OEM wheel on a car with 19s, or a 155 80 18 space saver on a car with 19s.

Of course, on a car with 21s the 155 80 18 space saver is the same size.

Even if tire diameters are not expected to effect driveability, you have made another case why using compact tires in the rear might not work well. Refer to highlighted text in your prior posting.

Larry

 

[jerry33]

I don't see how running a wheel 1/10th of an inch larger in diameter could cause any issues with the driveline--like I said, the reason you have a differential is to allow one tire to turn faster than the other, while going around corners.

1. You can't use the diameter for that, you have to use the RPMs, which should be listed on the spec page for the tire. Pi*D != RPM in tires.

2. Nominal sizes can easily be +- 5mm or more. So use the spec sheet when selecting.

2. The differential is for use while turning only (intermittent), not all the time as it would be if you drove with tires with two significantly different RPMs. There is more than one reason why temporary spares have speed limitations.

3. The 21" and 19" are very close to the same RPM (749 vs. 751) The 155/80R18 is (726*), which is 3.3% higher. That's a lot compared to the difference between the OE sizes. It might even mess up traction control. This is like a lot of things; it will probably work if you're careful and lucky. I'd put this size on the free rolling axle so I wouldn't have to worry about it.

* This is not the manufacturer's spec as I couldn't find one of those, it's based on a calculation.

 

[JST]

1. You can't use the diameter for that, you have to use the RPMs, which should be listed on the spec page for the tire. Pi*D != RPM in tires.

2. Nominal sizes can easily be +- 5mm or more. So use the spec sheet when selecting.

2. The differential is for use while turning only (intermittent), not all the time as it would be if you drove with tires with two significantly different RPMs. There is more than one reason why temporary spares have speed limitations.

3. The 21" and 19" are very close to the same RPM (749 vs. 751) The 155/80R18 is (726*), which is 3.3% higher. That's a lot compared to the difference between the OE sizes. It might even mess up traction control. This is like a lot of things; it will probably work if you're careful and lucky. I'd put this size on the free rolling axle so I wouldn't have to worry about it.

* This is not the manufacturer's spec as I couldn't find one of those, it's based on a calculation.

I defer to your superior knowledge on tire diameter issues, though I note that if I plug in 245 45 19 I get 728.6 revolutions/mile, compared to 726.4 for 155 80 18. Not sure why the difference from the numbers you got. But that's a 0.3 percent difference.

And for the 245 45 21 size, I get 726.7 revs/mile.

On the differential issue, I am still struggling to understand why even a continuous and relatively small speed differential would harm a non-limited slip diff. I mean, there aren't clutch packs or anything to wear; it's just gears, and they are spinning in precisely the way they're designed to do.

 

[tezco]

I'm not a differential expert, but it seems like a small difference in rpm spread over a mile wouldn't put much strain on the diff. If you were doing 60 mph (1 mile per minute), and the tire rpm difference was 5, then the differential cage might be rotating at 2.5 rpm, which doesn't seen like much.

 

[jerry33]

I defer to your superior knowledge on tire diameter issues, though I note that if I plug in 245 45 19 I get 728.6 revolutions/mile, compared to 726.4 for 155 80 18. Not sure why the difference from the numbers you got. But that's a 0.3 percent difference.

And for the 245 45 21 size, I get 726.7 revs/mile.

On the differential issue, I am still struggling to understand why even a continuous and relatively small speed differential would harm a non-limited slip diff. I mean, there aren't clutch packs or anything to wear; it's just gears, and they are spinning in precisely the way they're designed to do.

The 19" and 21" sizes were taken from the Michelin spec page. The 18" was from that calculator page because i couldn't find a manufacturer's site with the real numbers. So that was the best I could come up with.

As to why, well, if you go slow it should be okay, but like any other mechanical device there are limits. Since I don't know what those limits are, I err on the side of caution. I've seen far to many things break because they were run out of spec--even though everyone said there should be no problem or they used them and there wasn't a problem.

- - - Updated - - -

I'm not a differential expert, but it seems like a small difference in rpm spread over a mile wouldn't put much strain on the diff. If you were doing 60 mph (1 mile per minute), and the tire rpm difference was 5, then the differential cage might be rotating at 2.5 rpm, which doesn't seen like much.

You're likely right that over one or two miles wouldn't be an issue, probably. However, sometimes I see temporary spares that are left on for months until they are showing the cords, or people drive them spiritedly. Now surely no Tesla owner would do this, but if the Tesla was driven by a family member who possibly didn't realize there was a temporary tire mounted...

 

[Cool-Model S]

Thanks for the information.

Would you mind elaborating on why you would only use this arrangement of the front wheels?

Larry

I did not want to take a chance with a less than perfect spare tire with all the torque from the motor and getting software messages. I was worried about ABS, traction control and other issues. I did not have time to test this out yet.

I found 145/80/R18 wheel & tire on E-bay sold by LKQ for $110. It was a deal. Just arrived and brand new. It's VW Tiguan spare set.

 

[JST]

I did not want to take a chance with a less than perfect spare tire with all the torque from the motor and getting software messages. I was worried about ABS, traction control and other issues. I did not have time to test this out yet.

I found 145/80/R18 wheel & tire on E-bay sold by LKQ for $110. It was a deal. Just arrived and brand new. It's VW Tiguan spare set.

The Tiguan spare wheel has a different bolt pattern, doesn't it? Thought those were 5x112, as opposed to the Tesla 5x120.

Or are you just planning to take the tire and put it on a BMW wheel?

 

[Larry Chanin]

The Tiguan spare wheel has a different bolt pattern, doesn't it? Thought those were 5x112, as opposed to the Tesla 5x120.

Or are you just planning to take the tire and put it on a BMW wheel?

In posting #67 he shows the spare mounted on his Model S. In some of the ebay descriptions of the Tiguan spare they say it comes on Mercedes wheels.

Larry

 

[Cool-Model S]

The Tiguan spare wheel has a different bolt pattern, doesn't it? Thought those were 5x112, as opposed to the Tesla 5x120.

Or are you just planning to take the tire and put it on a BMW wheel?

I am planning on putting the tire in BMW wheel

 

[JST]

In posting #67 he shows the spare mounted on his Model S. In some of the ebay descriptions of the Tiguan spare they say it comes on Mercedes wheels.

Larry

Larry,

The Tiguan and (at least some) Mercedes use 112 mm bolt patterns, so they may be interchangeable.

From the text, I think the pic in post 67 is a borrowed wheel, not the Tiguan wheel. Just want to be clear on this so no one buys a Tiguan wheel and is disappointed--it won't fit, since the bolt holes are too close together.

 

[CHG-ON]

Great info JST. Thanks!

 

[gzerninplatz]

From a fellow in Portland who has done it:
18 inch BMW X5, BMW part no. 36-11-6-768-861. (about $100 from dealer) The tire is 145/80-18,Ebay. (about $85 shipped-better fit is 155/80-18.scissor jack and lug wrench.

 

[WhiteStar]

Am I correct in thinking that the frunk has shrunk in the latest vins rolling out? Think I recall reading it's 2" shorter to accommodate the new sensors. If so, does this mean forget about putting any full size spare in the frunk, say a 245 that will work front or back in a P85+?

A compact space saver might be the only solution unless one could fit in there with the frunk liner removed.

 

[jerry33]

Am I correct in thinking that the frunk has shrunk in the latest vins rolling out? Think I recall reading it's 2" shorter to accommodate the new sensors.

The two inches shorter is in the back of the cubby. It wouldn't affect the tire carrying space.

 

[CHG-ON]

Am I correct in thinking that the frunk has shrunk in the latest vins rolling out? Think I recall reading it's 2" shorter to accommodate the new sensors. If so, does this mean forget about putting any full size spare in the frunk, say a 245 that will work front or back in a P85+?

A compact space saver might be the only solution unless one could fit in there with the frunk liner removed.

From what I can tell, only the microwave shrunk. I'd say approx 3". The main area looks to be the same.

 

[4SUPER9]

I'm about to pull the trigger on getting all the parts for this. I simply cannot find the 155/80-18 anywhere. Yes, I could try the dismantlers, but seems too complicated with what I may get.
I can easily find the 145/80-18's, either new for $200+, or about $100 on eBay.

Does anyone think there is a big risk or difference going with the 145's?

 

[jerry33]

Does anyone think there is a big risk or difference going with the 145's?

The RPMs of the tire will be lower. That might set off traction control and affect other braking safety systems.
The carrying capacity is lower.
If you're going to use that size, 30 mph is the maximum speed I'd use. And be sure they are pumped up to the maximum psi. And then only drive on them from wherever you are to the nearest tire store. I'd avoid putting them on the rear (if a rear flattens, remove the front tire, install the doughnut, replace the rear).
Carry a plug kit and try that first.

 

[JST]

I'm about to pull the trigger on getting all the parts for this. I simply cannot find the 155/80-18 anywhere. Yes, I could try the dismantlers, but seems too complicated with what I may get.
I can easily find the 145/80-18's, either new for $200+, or about $100 on eBay.

Does anyone think there is a big risk or difference going with the 145's?

I don't have personal experience, but someone up thread reports using the 145 successfully without setting off error codes.

 

[Cottonwood]

I don't have personal experience, but someone up thread reports using the 145 successfully without setting off error codes.

It will be interesting to see if any of these will fit in the frunk of the new "D" cars. If it's close, the 145's have a better chance of fitting in front of that second motor. Wonder if the P85D larger, front motor takes up more of the frunk than the S85D front motor?