Mini-Spare diameter - why not 29" or so?

[watt-man]

I just bought a '25 Bronco Sport for the wife, so I'm new here. I was surprised that the stock tire diameter is nearly 29 inches, but the donut-spare (155/70-17) is 3 inches smaller (under 26)! I have no experience with 'new car' RDU / PTU function (I'm used to Traction-Lok units from the past that wear out when you mess with dis-similar diameters) - so I wonder why Ford didn't have the donut-spare at a 155/90-17 instead. It would put the diameter much, much closer. Or a 165/90-17 to really nail the diameter, but I don't want to get into a tire-width discussion and full-size spares.

So it would seem that Ford feels the 10% difference in tire diameters is "bad enough you should repair it right away"... yet they could have just reduced this difference to 1% or 2% by upsizing the donut-spare sidewall height slightly - (in fact, the 90 series sidewall tire is cheaper at Discount than the 70 series!).

Text from the owners manual is attached. Being okay with a donut-spare width, I'm inclined to spring for a 90-series sidewall tire on the donut-rim on a '35,000-dollar-loaded-with-4WD-electronics' car. Just wondered if others have any proof that the 3" tire diameter doesn't cause damage, even though Ford thinks it's a big enough deal to put it in the manual. This seems to be conflicting advice, I think I'll rest easier with a 155/90 or 165/90.

Thanks in advance if you have some data on this.

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[Escape2Bronco]

I just bought a '25 Bronco Sport for the wife, so I'm new here. I was surprised that the stock tire diameter is nearly 29 inches, but the donut-spare (155/70-17) is 3 inches smaller (under 26)! I have no experience with 'new car' RDU / PTU function (I'm used to Traction-Lok units from the past that wear out when you mess with dis-similar diameters) - so I wonder why Ford didn't have the donut-spare at a 155/90-17 instead. It would put the diameter much, much closer. Or a 165/90-17 to really nail the diameter, but I don't want to get into a tire-width discussion and full-size spares.

So it would seem that Ford feels the 10% difference in tire diameters is "bad enough you should repair it right away"... yet they could have just reduced this difference to 1% or 2% by upsizing the donut-spare sidewall height slightly - (in fact, the 90 series sidewall tire is cheaper at Discount than the 70 series!).

Text from the owners manual is attached. Being okay with a donut-spare width, I'm inclined to spring for a 90-series sidewall tire on the donut-rim on a '35,000-dollar-loaded-with-4WD-electronics' car. Just wondered if others have any proof that the 3" tire diameter doesn't cause damage, even though Ford thinks it's a big enough deal to put it in the manual. This seems to be conflicting advice, I think I'll rest easier with a 155/90 or 165/90.

Thanks in advance if you have some data on this.

Even though mine came with a full size tire, I opted for a new take off rim off of EBay and the same optional Wildpeak tire.

The option now is no tire and an inflator. While that might be fine for a vehicle that gets a nail in the tire, it won’t help much 30 miles from the main road with a cut tire from a ledge or sharp rock IMO.

I’m sure it was all done for cost control and weight reduction.

 

[Ernest T]

Wha trim do you have? I just bought a Badlands and it’s got the same size spare, 225/65 R17, as the other 4, but it’s a Pirelli Scorpion.

 

[jkernitzki]

Wha trim do you have? I just bought a Badlands and it’s got the same size spare, 225/65 R17, as the other 4, but it’s a Pirelli Scorpion.

The Badlands Sasquatch has Goodyear Territory A/T 235/65 R17's. My spare, however, is the 225 Pirelli like yours.

 

[Dude]

Wha trim do you have? I just bought a Badlands and it’s got the same size spare, 225/65 R17, as the other 4, but it’s a Pirelli Scorpion.

My 2022 Badlands came like some ‘25s - 4 Pirelli Scorpions and same size spare 225/65 R17 but not to worry most owners get around 25k miles with those

I’m hoping owners get over 50k miles with the Goodyear Territory A/T 235/65 R17

 

[jkernitzki]

I’m hoping owners get over 50k miles with the Goodyear Territory A/T 235/65 R17

While I want a long-lived tire, I also wouldn't mind an earlier excuse to get something a little more common (and fun).

Don't get me wrong, the Goodyears appear to be a solid tire, though I've only got about 1,600 miles (all paved roads) so far.

 

[Dude]

While I want a long-lived tire, I also wouldn't mind an earlier excuse to get something a little more common (and fun).

Don't get me wrong, the Goodyears appear to be a solid tire, though I've only got about 1,600 miles (all paved roads) so far.

Lots of owners like those Goodyears! I wanted to purchase a set but my OE tires wore out too soon (didn’t find the 2025 tire without the Wrangler logo until earlier in 2025 at least 6 months after I had already replaced my OE tires)

 

[Bill G]

While changing to a taller “donut” spare would solve the mis-match affecting the AWD on-road (snow, rain, etc) it wouldn’t solve an off-road situation IMO. The “donut” spares require high air pressure which would, I think, make for a problem in the “dirt” and with rocks. If you’re going off-road I’d go with a “real” spare, if not the “donut“ would probably be fine.

 

[jkernitzki]

"Spare" is not "replacement", at least where it comes to the donuts. They are there to simply limp you to the nearest tire shop for a proper repair or replacement.

 

[Me3]

While I want a long-lived tire, I also wouldn't mind an earlier excuse to get something a little more common (and fun).

Don't get me wrong, the Goodyears appear to be a solid tire, though I've only got about 1,600 miles (all paved roads) so far.

What don’t you like about the Goodyears? They look aggressive and are fairly quiet from what I’ve read here. Not that I care for Goodyear, since they moved out of Akron. I grew up near there and they used to fly the old giant blimps around all the time. But I’ve only heard good things about them so far.

 

[jkernitzki]

What don’t you like about the Goodyears? They look aggressive and are fairly quiet from what I’ve read here. Not that I care for Goodyear, since they moved out of Akron. I grew up near there and they used to fly the old giant blimps around all the time. But I’ve only heard good things about them so far.

I don’t dislike them, but they’re a bit of a unicorn right now. There’s several other A/T’s with a broader range of sizes.

 

[watt-man]

The option now is no tire and an inflator. While that might be fine for a vehicle that gets a nail in the tire, it won’t help much 30 miles from the main road with a cut tire from a ledge or sharp rock IMO.

While changing to a taller “donut” spare would solve the mis-match affecting the AWD on-road (snow, rain, etc) it wouldn’t solve an off-road situation IMO. The “donut” spares require high air pressure which would, I think, make for a problem in the “dirt” and with rocks. If you’re going off-road I’d go with a “real” spare, if not the “donut“ would probably be fine.

Full agreement with both of you. This is a street car, the wife really likes the looks of the Heritage model (probably was an easy sell to me as my car is a 1966 as well) and we were in the market for a newer car for her. I wouldn't want to use a donut spare in an off-road situation, but I do want the donut spare to match the diameter of the others within reason. A 3" difference just seems silly.

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