"Space-Saver" spares

Steverino

Steverino

black sheep
Donuts, whatever they're called, they're useless to me. If I blow out a tire 45 miles from home, I doubt I could count on it.

I looked under the bed of my truck, and I don't think it's ever been used, and it looks like hell, dry rotted. I've seen motorcycle tires wider than that thing.

I had to order a replacement for a RR tire that's starting to separate (ran it under-inflated for 40+ miles :embarrassed:) and didn't want to have just one new tire and 3 others at 75% so I ordered 2. I figured I'd put the other new tire on the LR. That leaves a tire that could be a decent, full-sized spare. So I found a 16x8 wheel on Ebay. When it gets here (and the 2 tires) I'll take them all down to the tire place and get 'em swapped around.

I think the Porsche Cayman carries no spare, just a plug kit and compressor. I'd rather have that, plug a tire than put a stupid space-saver on.
 
I've had to use the donut to drive about 30 miles before. I was worried the entire time.

Then again, I've seen people on the road that have had donuts on their car for weeks at a time.

What I really don't get, on my 2000 Ranger, the option package I have came from the factory with 245/75R16 tires, yet the spare is a 235/75R15. Not only that, but even though it's a full size tire, it still said right on the sidewall "Temporary Spare - Do not exceed 55 mph" or something like that.

Luckily, our Blazer has 235/75R15 tires on it, so the last time we got new tires for it, I kept the best one and had it mounted on the spare rim for the Ranger. Still, I'm not sure how far I'd want to drive with 1 tire being an inch and a half shorter than the other 3.
 
Many new cars are coming with tire slime, patch kit and compressor to save weight and $. My DD has one and included roadside assistance. I picked up a better plug kit for peace of mind.
 
I wonder if that "Temporary Spare" uses some kind of rubber compound that's not like a regular tire :shrug:

I didn't realize how difficult locating a 16X8 wheel with the right bolt pattern and backspacing was going to be.
 
My Jaguar (2003) came with a full sized spare wheel AND a spare mag rim (nice touch).
My Toyota Tundra (2014) came with a full sized spare with a real tire on it, but it is on a cheap, black steel rim.

I remember we were talking about this a while back.
I know my old neighbors had a brand new Cadillac SUV that came with the inflator kit and Roadside service instead of a spare. They had a flat in a parking garage in Boston and were stranded for HOURS because the tow truck couldn't get the SUV out due to the height.

I've driven on donuts and I'm not a fan.....but I am even less of a fan of the inflator kit.
 
Steverino said:
I wonder if that "Temporary Spare" uses some kind of rubber compound that's not like a regular tire :shrug:

I didn't realize how difficult locating a 16X8 wheel with the right bolt pattern and backspacing was going to be.
Click to expand...
The space saver spares use a very different compound and construction. The rubber Is exceptionally soft compared to a regular tire. This helps maintain traction despite the smaller footprint. They have a very limited lifespan in terms of miles before they are completely bald. People that run them for an extended period usually find this out the hard way, on the side of the road, again, with a flat spare that had the inner belts poking through what's left of the tread. Then they learn that their car has no room for a full size spare, and freak out when they discover the cost of a new space saver.
Prages said:
I've had to use the donut to drive about 30 miles before. I was worried the entire time.

Then again, I've seen people on the road that have had donuts on their car for weeks at a time.

What I really don't get, on my 2000 Ranger, the option package I have came from the factory with 245/75R16 tires, yet the spare is a 235/75R15. Not only that, but even though it's a full size tire, it still said right on the sidewall "Temporary Spare - Do not exceed 55 mph" or something like that.

Luckily, our Blazer has 235/75R15 tires on it, so the last time we got new tires for it, I kept the best one and had it mounted on the spare rim for the Ranger. Still, I'm not sure how far I'd want to drive with 1 tire being an inch and a half shorter than the other 3.
Click to expand...
I believe your Ranger actually came with 245/70/16. 245/75/16 is very odd.

The difference in absolute rolling radius between a 245/70/16 and a 245/75/15 is nearly nil.

The wheel/rim diameter is the least relevant thing in this equation :

245= tread width in millimeters
75 (or 70) is the aspect ratio. That ratio is measured as a percentage of sidewall height from bead to tread in relation to tread width. (An aspect ratio of 75, means the the sidewall height is equal to 75% of the tread width). Therefore, a 255/70/16 would be "shorter" than a 265/70/16 because even though they are both fit a 16" wheel, and one is 10 mm wider, the 265 is also thereby 7mm taller.

So, if the standard tires are 265/70/16 and the spare is 245/75/15, the final rolling radius is very close despite the smaller wheel diameter because 75% of 245 is about the same as 70% of 265... make sense?

Now, a 235/75/15 has a noticeably shorter rolling radius than a 265/70/16 and would be a bad choice for a spare.

Even on a 2 wheel drive truck, you would never want to put the spare on the back. The shorter rolling radius means that the smaller tire has to complete more revolutions to cover the same distance as the larger tire. As such, it will destroy a differential in no time. Limited slip or not. Even on the front, where the wheels roll independently from each other, it can cause big trouble on a modern vehicle with the now mandated traction control. Speed sensors at the corners count the number of revolutions each tire makes. When the computer senses that one wheel is turning faster than the other 3, it activates the traction control and tries to compensate.

It's ALL about rolling radius.
 
My car has one of those space saver spares. It worked fine to get me off the highway and to my house (maybe 30 miles) and then to Costco for new tires (10 more miles). I was nervous as hell driving on that little tire, especially with all the max speed warnings printed all over it (45 mph, I think). My car rode very funny with the spare on it, and the suspension geometry was clearly not ideal, resulting in very odd steering and balance. Odd that I see people blasting down the highway going 80 mph on those things all the time. Ignorance is bliss, I suppose.
 
My Porsche 944 blew out a tire on the freeway (I was switching lanes and a pothole in the space between lanes sliced the sidewall) and I was shocked to see one of those "inflate it yourself" spares with no compressor in the spot earmarked for one. :mad:

So.... I jacked up the car, had someone pick me up so I could take the rim to a shop and have a new tire put on. While I was there, I checked to see if they had a suitable lugged rim in their used inventory. Sho nuff, I was able to grab a second rim, slap another tire on it, and I carried a full sized spare from that point forward. The old space saver was stored vertically in the end of the boot... the new tire layed flat in the hatch, taking up a good chunk of space but I didn't care. Better than getting stranded somewhere. :tongue:
 
  • Like
Reactions: Tig
My current vehicle has a subwoofer mounted inside the space saver spare tire. I have yet to use the spare on this car, but it looks kind of complicated to do so.

Here is a pic from the web of the arrangement of subwoofer and spare.

IMG_1099.jpg
 
OGG said:
The space saver spares use a very different compound and construction. The rubber Is exceptionally soft compared to a regular tire. This helps maintain traction despite the smaller footprint. They have a very limited lifespan in terms of miles before they are completely bald. People that run them for an extended period usually find this out the hard way, on the side of the road, again, with a flat spare that had the inner belts poking through what's left of the tread. Then they learn that their car has no room for a full size spare, and freak out when they discover the cost of a new space saver.

I believe your Ranger actually came with 245/70/16. 245/75/16 is very odd.

The difference in absolute rolling radius between a 245/70/16 and a 245/75/15 is nearly nil.

The wheel/rim diameter is the least relevant thing in this equation :

245= tread width in millimeters
75 (or 70) is the aspect ratio. That ratio is measured as a percentage of sidewall height from bead to tread in relation to tread width. (An aspect ratio of 75, means the the sidewall height is equal to 75% of the tread width). Therefore, a 255/70/16 would be "shorter" than a 265/70/16 because even though they are both fit a 16" wheel, and one is 10 mm wider, the 265 is also thereby 7mm taller.

So, if the standard tires are 265/70/16 and the spare is 245/75/15, the final rolling radius is very close despite the smaller wheel diameter because 75% of 245 is about the same as 70% of 265... make sense?

Now, a 235/75/15 has a noticeably shorter rolling radius than a 265/70/16 and would be a bad choice for a spare.

Even on a 2 wheel drive truck, you would never want to put the spare on the back. The shorter rolling radius means that the smaller tire has to complete more revolutions to cover the same distance as the larger tire. As such, it will destroy a differential in no time. Limited slip or not. Even on the front, where the wheels roll independently from each other, it can cause big trouble on a modern vehicle with the now mandated traction control. Speed sensors at the corners count the number of revolutions each tire makes. When the computer senses that one wheel is turning faster than the other 3, it activates the traction control and tries to compensate.

It's ALL about rolling radius.
Click to expand...


Door Jam Sticker.jpg


Sidewall.jpg


So, as I was saying:

245/75R16=9.65" wide, 7.24" sidewall (x 2) + 16" for the rim = 30.48"
235/75R15=9.25" wide, 6.94" sidewall (x 2) + 15" for the rim = 28.88"

Difference in rolling height = 1.6"
 
As for speed sensors and additional differential wear, I'm not sure it's going to make much difference. I mean, the only time all four wheels are doing the same rpm is when you're moving in a straight line. There aren't many straight roads in WV.
 
Prages said:
As for speed sensors and additional differential wear, I'm not sure it's going to make much difference. I mean, the only time all four wheels are doing the same rpm is when you're moving in a straight line. There aren't many straight roads in WV.
Click to expand...
The wheel sensors are programmed to allow for the differences encounters during normal driving and things like making turns etc. A prolonged instance of one wheel reporting a significant difference can and will trigger the system.

As for the diff... roll the dice. An open diff won't be affected. A limited slip diff will eventually way itself and become shrapnel.

A locked diff will bark and chirp and shudder and fight with the straggler tire until one of them says uncle. It's almost always the diff that taps out. Unless you're running a corporate 9 bolt that's been Lincoln-Locked, the match won't go to the judges. I've only seen this about a thousand times. Just trying to offer some advice from the perspective of someone who has pretty substantial experience and first-hand knowledge. Just trying to help.
 
Addendum. "Odd" is still odd. I appreciate the pictures, but I never said it was impossible, just unusual. The stock size on "most" Rangers and Explorers from that Era was either a 255/70R16.

I only sold about a bazillion of them during the Firestone debacle. Having one of the rarer options is what it is.

The number of revolutions per mile on a 245/75/16 = 662.3
For a 235/75/15, = 698.7

How many miles do you suppose the average LSD will travel down a straight hiway with one wheel turning 36 more revolutions per mile than the opposite will go before something breaks?
 
Well, I don't have a limited slip or locking dif, and I sure as hell don't have traction control (2000 Ranger).

Still, I wouldn't want to put too many miles on it with the smaller spare.

I guess I'm just getting argumentative today. Sorry. :embarrassed:
 
  • Like
Reactions: OGG
Sure am glad my spare will be same size as all the rest :grin:

My truck also runs 16" wheels. All S10s run 15s except those with the ZQ8 suspension (Xtreme and SS) and mine does have a posi.
 
I've done 40 or so miles on a mini spare before. I wasn't worried but was able to avoid highways completely. My CR-V is the older version with the full sized spare mounted on the back. I much prefer that arrangement.
 
On my front wheel drive cars if I had a flat on the front axle I would always put the donut on the rear axle and move the regular tire from the back to the front. If I had a rear flat, regardless of FWD or RWD it stayed on the back until I could change it out. I never drove with them on the front axle.
 
You must log in or register to reply here.
Back
Top