Driving with mismatched tire widths on same axle

[Sukhoi]

I just bought a new Chevy Colorado with P255/65-17 tires and a bulbous P265/70-16 temporary use spare. I need to replace the spare with something more durable for dirt road use, and ideally less bulbous as the spare restricts the departure angle. Besides dirt roads, ideally this spare will allow me to drive 1000 mi home from a road trip without having to repair the punctured P255/65-17 tire.

I have seen mention of one person using an LT215/85-16 as a spare on this truck (LT for stiff sidewalls on thin tire) as it noticeably improves the ground clearance when stowed but is the same OD for driving. I'm sure dirt road driving would be fine, but what about 1000 mi on pavement? Would I have dangerous steering, braking, or pulling issues with P255/65-17 on one side and LT215/85-16 on the other side? What about only running the spare for long distance on the rear axle to remove the steering component?

 

[jlee]

If you plan on running a spare for 1000 miles I would get a matching size spare. If you just need to get out of the woods to a tire shop, anything of the same rolling diameter is fine.

 

[herm0016]

why not just get the tire fixed and be on your way? you must be passing a tire place sometime closer than 1000 miles. by "new" do you mean new to you? if it has a the g80 locking rear differential you could blow it up by using tires of different diameters on the rear axle. look on the door of the glovebox to see if it has the RPO code G80.

 

[BurnItDwn]

Are you going on the Al-Can highway for a trip? I believe they have built it up to the point where there is a service station at least once every 200-300 miles

 

[Sukhoi]

If you plan on running a spare for 1000 miles I would get a matching size spare. If you just need to get out of the woods to a tire shop, anything of the same rolling diameter is fine.

You annoyingly are probably right.

why not just get the tire fixed and be on your way? you must be passing a tire place sometime closer than 1000 miles. by "new" do you mean new to you? if it has a the g80 locking rear differential you could blow it up by using tires of different diameters on the rear axle. look on the door of the glovebox to see if it has the RPO code G80.

Are you going on the Al-Can highway for a trip? I believe they have built it up to the point where there is a service station at least once every 200-300 miles

Heh yes there are tire places within probably 100 miles of anywhere I'd likely get a flat. This is mostly southern Utah. But depending when and where the flat happened I'd rather just be able to drive all the way home to CA than take a half day off my trip to repair the flat.

If the ground clearance wasn't an issue I'd 100% get the matching spare, but thought I'd investigate the idea of using a thinner spare for more stowed clearance. Good point about the G80 locker, and I do have it, but everything I am looking here is just skinnier with the same diameter.

 

[herm0016]

utah? there is a place to patch a tire in every town for a reason. i have traveled much of the dirt in southern utah.

points that i would try to hit are the moki-dugway road, natural bridges national park and i really like the route south from capital reef along the notom-bullfrog road. you can cut west on a beautiful drive through grand staircase and down into boulder from that road. if you will be in moab at all i recommend horse thief campground out by island in the sky entrance to canyonlands. the dirt track north after you enter arches has some cool hikes that are not as crowded. stop at coral pink sand dunes state park north of kanab and spend a day volunteering at best friends animal shelter in kanab. Coral pink sand dunes is really something special. great little campground, generally not crowded. goblin valley is quite a cool place as well. I think i have hit every state and national park/monument in utah except for the salt flats.

i carry a plug kit and an air compressor.

like this:
https://www.amazon.com/Quality-Tool...t=&hvlocphy=9028921&hvtargid=pla-571637933708

and
https://www.amazon.com/Viair-00088-...words=vair&qid=1561493308&s=automotive&sr=1-4

 

[Sukhoi]

Yeah there are so many good spots there. Already had a trip planned for September before buying the truck, so I'm trying to decide how much to modify the itinerary to add 4WD roads. Old car is a 2009 RAV4 AWD so I've been on most of the 2WDHC roads in the area but not much any rougher. Like on the Notom-Bullfrog road now we can finally do the side roads to Hall Creek and whatnot. We actually are planning on staying at Horsethief if we can get a spot, so glad to hear you recommend it.

Definitely need to get another plug kit and compressor as the old ones are staying in the RAV4. Though the one dirt road flat tire I've had, which was on Hole In the Rock Road about 10 yr ago, was patchable but not plugable.

Now I'm thinking of splitting the difference and getting a P235/75-16 spare. It's only 7% narrower than the reg tires, but will still get me close to 2 cm more clearance along the departure angle.

 

[NutBucket]

Space saving spare on a truck? That's pretty crappy. I'd just get a full size spare and call it a day.

 

[Sukhoi]

I guess maybe something I'm missing in this decision process is how much diameter difference is "in spec"? Even if I have a matching spare if it's not in a 5-tire rotation am I still not going to want to drive 1000 mi on it because a 0.5" diameter difference is going to fry the differential over 1000 mi?

I see that the G80 acts as an open diff until the tires have 120 RPM difference, and per my math at 60 mph there's only a 22 RPM difference even with a 1" diameter difference. So I guess the more likely 0.5" diameter difference is no issue.

 

[NutBucket]

If you're going to buy a new tire anyway then why not just do it "right" and get one the same size?

 

[Sukhoi]

Because I want more ground clearance with the spare tire stowed. This is all a balance of ground clearance vs driving dynamics when the spare tire is in use.

 

[mindless1]

WTF? Ground clearance? This is a trivial difference and if you're going to rub, better the spare than something else.

Don't go off-road, you don't have any sense about it.

Further,

I need to replace the spare with something more durable for dirt road use

You're off in the twilight zone somewhere, there is no durability issue on dirt roads, only a traction issue.

Please, quit trying to think about this. You are failing constantly including the math that a small diameter difference is not an issue over 1000 mi.

Either you are trolling or just ignorant. It's the same old thing, that there are the known correct things to do, then someone comes along and tries to minimize, yet even then maximize asking about 1000 mi.

Hey, it's your vehicle and your accident, tire or tow bill. Maybe just quit trying to do ridiculous things and follow the advice proven over decades?

I don't even understand how this topic exists, it's like you have an IQ of 40 yet you are able to post on the internet.

Your response is excessive. The Garage is not OT or P&N.

Perknose
Forum Director

 

[Sukhoi]

WTF is wrong with you?

 

[mindless1]

^ Feel free to completely dismiss me as an internet kook. It is just one opinion.

 

[deadlyapp]

Here's a good tool I've used in the past :
https://www.wheel-size.com/calc/

I've heard a 1-3% rolling circumference difference could be acceptable, however with a locked diff you may have less tolerance there.

Personally, if I were you and seriously thinking about being off-road with possibility of losing a tire, and want the extra ground clearance, I'd get a full size spare and mount it either in the bed of the truck or the roof of the truck in a rack. Alternatively, there are swing away tire mounts that mount to a trailer hitch, however it would similarly affect your angles in the rear.`

 

[NutBucket]

I do recall reading about the smaller spare to increase departure angle. Seems kinda ridiculous if you ask me. And I definitely agree that having the spare rub isn't that big of a deal....and if you're concerned about inches when off roading maybe you should consider a lift.

 

[jlee]

Some food for thought here - if you're concerned enough about gaining that little extra departure angle, this may not be a trip you want to run solo. If you're comfortable running it with no other vehicles, then you shouldn't have any problem losing a degree or two of departure angle (because if it's important enough to be a concern, you're pushing far too hard alone).

If you do have other vehicles with you, then you'll have people to help you stack rocks / spot you to make sure the slight loss of clearance under the bed isn't a problem.

 

[dave_the_nerd]

You have a Colorado - can't you just stow the spare in the bed when you're offroad? Eliminates the ground clearance/departure angle issue completely, don't it?

 

[Sukhoi]

Here's a good tool I've used in the past :
https://www.wheel-size.com/calc/

I've heard a 1-3% rolling circumference difference could be acceptable, however with a locked diff you may have less tolerance there.

Personally, if I were you and seriously thinking about being off-road with possibility of losing a tire, and want the extra ground clearance, I'd get a full size spare and mount it either in the bed of the truck or the roof of the truck in a rack. Alternatively, there are swing away tire mounts that mount to a trailer hitch, however it would similarly affect your angles in the rear.`

Interestingly all the media about the G80 declares it an open diff until locked at 100-120 RPM slip, but I found Eaton literature stating it's actually low-bias LSD until locked. These tires are roughly 95" circumference, and a 0.5" diameter difference to the spare tire (anything closer would be dumb luck) is then ~1.5" circumference difference, which is also ~1.5%. And that is probably too much to risk driving long distance unless the spare is on the front.

I do recall reading about the smaller spare to increase departure angle. Seems kinda ridiculous if you ask me. And I definitely agree that having the spare rub isn't that big of a deal....and if you're concerned about inches when off roading maybe you should consider a lift.

Some food for thought here - if you're concerned enough about gaining that little extra departure angle, this may not be a trip you want to run solo. If you're comfortable running it with no other vehicles, then you shouldn't have any problem losing a degree or two of departure angle (because if it's important enough to be a concern, you're pushing far too hard alone).
If you do have other vehicles with you, then you'll have people to help you stack rocks / spot you to make sure the slight loss of clearance under the bed isn't a problem.

Those are good points. None of this would be so far away from trafficed areas I'd starve and die, but I'm not going to have a buddy with me to toss on a tow strap and resolve the situation in 5 minutes. Looks like there are three options I'll have to decide between:

1. Buy new rim and TPMS and go full 5-tire rotation. Most expensive and complicated to implement.
2. Full-sized spare. 50% chance the flat is on the front and I can drive all the way home, and otherwise just need to move spare from rear to front in a parking lot.
3. Thin spare for a little ground clearance but no long distance driving.

Thanks for the help everyone. Leaning toward #2.

You have a Colorado - can't you just stow the spare in the bed when you're offroad? Eliminates the ground clearance/departure angle issue completely, don't it?

Going to be camping in the bed. Could put it on the camper shell roof but I suspect that would impact MPG heavily.

 

[deadlyapp]

With the right rack and wind shielding, you'd probably avoid much issue. Alternatively the cheaper option is keep it in the bed and throw it on the ground every night before you sleep just don't forget to put it back in!

 

[ElFenix]

Going to be camping in the bed. Could put it on the camper shell roof but I suspect that would impact MPG heavily.

that's mostly going to depend on how fast you're going. on barely improved roads keeping below 40 mph, it's not going to be much.

 

[Sukhoi]

Heh I guess that is an idea to just toss it outside before sleeping. I'll have to see how our interior setup is, but will keep that in mind. Guess I won't totally write off the roof thing either. I could just put the tire up there when going on more serious dirt roads.

 

[BoomerD]

Be more concerned about the rolling circumference/diameter than the width. You CAN get tires of different widths that will be "close enough" to the same diameter for ise as a spare...but, I also agree with everyone who's said " just get the bad tire fixed/replaced once you get back to some form of civilization.

 

[dave_the_nerd]

I would expect the odd-width tire to negatively (dangerously) effect handling characteristics when you're on the highway driving to/from your campsite. If you're doing some 80mph panic stop and your front right tire only has 3/4ths the contact area of the front left, you're pretty well f****d.

So I would very strongly encourage you to get a full size spare and figure out stowage later.

 

[Sukhoi]

I would expect the odd-width tire to negatively (dangerously) effect handling characteristics when you're on the highway driving to/from your campsite. If you're doing some 80mph panic stop and your front right tire only has 3/4ths the contact area of the front left, you're pretty well f****d.

So I would very strongly encourage you to get a full size spare and figure out stowage later.

Actually from what I understand at the same diameter and pressure the contact patch area is relatively independent of tire width. Wider tires have a wider contact patch that is shorter from front to back. This increases lateral grip, but at the expense of longitudinal grip. The thin spare may actually panic brake better (not necessarily a good thing), and would be inferior in the corners.