F150 Spare tire question

I figured I'd ask here before I called the tire shop.

I drive a 2015 F150 4x4. It's currently equipped with tire/wheels from a 2023 so the sizes are 275/60R20. The issue is that the spare underneath is still the original 265/70R17. I get that the overall tire circumference is the most important factor here, so I used a tire comparison tool which shows the difference between the tire and wheel combo as 4.4" in circumference, and 1.4" in tire height.

I'm usually towing my 7x14 enclosed trailer 5+ hours from home, fully loaded with bike/track stuff. Would the current spare be ok to run in an emergency, or is the 4.4" difference in circumference from the other tires to great to run safely?

thanks,

 

Way too much difference in my opinion. I would want to be around half that difference.

 

To use as a spare for 20-30 miles its fine, to drive 500 miles on it, its not. You really need a spare the same size as the rest of the tires.
I'd bet theres way more than a 1.5" difference in diameter, maybe in radius....

 

I have a full size spare and just leave it in the bed of the truck. I do have a bed cover though otherwise I'm sure it could tend to walk away

 

The full size spare won't fit underneath?

 

Do you have locking or limited-slip diffs? If not, this doesn’t really matter.

Quick searches says the 2016 came w an E-lock option. You’d leave that off for freeway miles.

 

I never tried but I think I remember measuring it once and it was too big. I believe the spare is on an 18" rim and I've got the 20"

 

Wrong.

It will kill spider gears over time, cause ABS issues as well as alignment issues.

 

I have the button to toggle the rear locker, same with 4x4.

 

Get a 285/70r17 for your spare and you’re under 1% difference.

Likely close to zero when tire wear is taken into account.

 

I would go to a junk yard or marketplace and try to grab another 17" spare wheel for those trips. Later models came with a 265-70-18 spare.
And I would also check & lube the spare tire carrier. Getting a flat sucks and having to wrestle the spare from under the truck blows goats on a highway shoulder so I would do an ounce of prevention and keep the spares in the bed.

 

The best move would be get a 5th 20" rim and have the shop slap together another like tire to your current four and then put the spare into your rotation.

If you don't want to shell out for that other rim (no idea how much the OEM takeoff would be), a 285/70-17 is a size that would get you quite close for diameter and has a number of tire options available.

The strain to the drivetrain for your significantly mismatched setup is just not worth it for the price of a properly sized tire.

 

Ya like all that turning killing spiders under normal driving. How many regular miles do you think amasses enough turning to equal a 500mi trip w a 4” circumference difference? I’d bet the answer is not that large.

And even that is a minor concern if the spare is actually used as a non-permanent spare.

 

13,000 revolution difference over 500 miles with the 265/70r17

2500 revolution difference over 500 miles with a 285/70r17.

Tiresize.com gives you all the data.

 

4" of stagger is significant, Works out about right for a 175' radius corner...
Also enough to change the camber of the rear wheels.
Not a good idea.

 

So maybe a windy mountain pass is equal spider wear to driving straight with mismatched tires. Not bad. Maybe you’d really mess up the truck if you did the mountain pass with mismatched tires.

 

Driving 500 miles around a 350’ diameter circle in one direction is gonna put some wear and undue stress on the differential.
Wanna do it. Cool don’t care.

Have seen cars with worn spiders gears and pins from people running different size tires for extended periods of time.
I reckon that’s the difference between experience and theory though…

 

How many miles is “extended periods”?

We’ve all seen ppl w bald donut spares. You aren’t supposed to, but ppl still do. But if we can define the limits for the OP, maybe that will help.

Also, you likely have zero experience w all the vehicles that ran diff tires and had no issues. So it’s hard to say how prevalent the issues are.

 

So, you have experiance running 4+" of stagger on the street with no issues???
Bullshit.

 

It’s a spare. In case of emergency. People drive for weeks on fucking donuts on front wheel drive cars that have open front diffs.

this is one of those “in theory” it’s bad that gets blown out of proportion.

“Practice…..we talking about practice here….practice!”

we on a motorcycle racing forum where death and grave injury from some idiot missing his brake marker is a real possibility then I read about danger of 1.5” tall difference in size for a SPARE tire and “spider gear wear”.

I wonder what kind of wear all of us morons did doing one wheel peels in our jalopys back in the day.