Mismatched tires

[zxcvbob]

• My new truck has 285/75/16 tires on 16x6.5" rims.  The recommended rim size for those tires is 16x7.5" or x8".  The tires are in good condition and they are wearing okay.  I don't seen any weird bulging at the sidewalls near the bead.  I've talked to a couple of mechanics and they've told me there shouldn't be any problems from having wide tires on narrow rims and I should leave them alone.
  
• The spare tire is 245/75/16 and looks really old.  I haven't pulled it down yet to look at the date code because it's raining this week.  (and we need the rain)
  
• I have 4 brand new but couple of years old 215/85/16 tires in my garage.  I'll never haul anything heavy enough where the less payload capacity will make any difference.  215/85 and 245/75 tires are essentially the same diameter.  215 tires will be happy on 6.5" wide rims.

If I don't do anything, and someday I have to put the spare tire on, it needs to go on the front so I don't ruin the differential, right?  Or else drive *really* slow if I can't do that and it's on the back.  (the truck is RWD)

I kinda think I should leave the road tires and wheels alone.  Sell the 215's.  And buy a used 235/85/16 tire for a spare.  When the 285s wear out, replace them with 235/85s.  235/85 is a little smaller in diameter than 285/75 but they are pretty close; 16 revolutions per mile difference.  At 60 MPH or less and short distances that shouldn't matter.  What say ye?

[Bogie]

Where you REALLY have to worry is if you have an AWD drive car such as a Subaru... If you lose a tire, you need to replace all four, because having one smaller or larger will trash the powertrain, and void your warranty.

[Hawkmoon]

The general rule-of-thumb is that tire width (at the sidewall, not the tread) shouldn't be more than 2 inches wider than the rim width, but people exceed that routinely. A 285 is 11.22 inches, so those tires in American off-roading parlance are equivalent to 32x11.22.

Most Jeep Cherokees (the original Cherokee, not the current Cherokee) and Comanches came with 7-inch rims. It's very common for people to run 31x10.50 tires on the 7-inch rims, and it's not unheard of to run 32x11.50s. I think you'll be fine running those tires on the 6.5-inch rims, but I would go a bit smaller when you replace them.

If I don't do anything, and someday I have to put the spare tire on, it needs to go on the front so I don't ruin the differential, right?  Or else drive *really* slow if I can't do that and it's on the back.  (the truck is RWD)

Does the truck have a limited slip differential? If so, then keep the two tires on the back the same size. If you don't have a limited slip, it doesn't matter whether or not the two rear tires are the same size.

[zxcvbob]

The general rule-of-thumb is that tire width (at the sidewall, not the tread) shouldn't be more than 2 inches wider than the rim width, but people exceed that routinely. A 285 is 11.22 inches, so those tires in American off-roading parlance are equivalent to 32x11.22.

Most Jeep Cherokees (the original Cherokee, not the current Cherokee) and Comanches came with 7-inch rims. It's very common for people to run 31x10.50 tires on the 7-inch rims, and it's not unheard of to run 32x11.50s. I think you'll be fine running those tires on the 6.5-inch rims, but I would go a bit smaller when you replace them.

Does the truck have a limited slip differential? If so, then keep the two tires on the back the same size. If you don't have a limited slip, it doesn't matter whether or not the two rear tires are the same size.

That's a good point about limited slip.  I don't think it has that but not sure.  (and I don't have an owner's manual)  How to check...  Cut the steering wheel all the way to one side and drive in a little circle and see if the rear wheels try to hop?  My previous truck had limit slip and if I recall correctly it did that.  (was annoying sometimes when trying to park)

[Brad Johnson]

I have 4 brand new but couple of years old 215/85/16 tires in my garage.  I'll never haul anything heavy enough where the less payload capacity will make any difference.  215/85 and 245/75 tires are essentially the same diameter.  215 tires will be happy on 6.5" wide rims.[/li][/list]

Stick with the 245/75. The extra inch or so of tread width doesn't sound like much, but it's enough to make a difference in ride quality, durability, and general usefulness. Besides, it's the factory size for the truck so likely fewer strange or unanticipated issues. Also, being the factory size for a super-popular truck probably means a ton more options and likely lower prices.

*edit to add* looked at Discount Tire and sticking with 245/75 gives twice the choices. Prices start noticeably lower, too ... $87 vs $127.

Brad

[Hawkmoon]

That's a good point about limited slip.  I don't think it has that but not sure.  (and I don't have an owner's manual)  How to check... 

Jack up one rear wheel. Put it in neutral with the parking brake off -- so BE SURE TO CHOCK A FRONT WHEEL.

If you can turn the rear wheel by hand, you don't have limited slip. If you can't turn it, you have limited slip.

Driving in circles is not a reliable test. A limited slip in proper operating condition should NOT hop or skip when you drive in a circle.

[Hawkmoon]

Stick with the 245/75. The extra inch or so of tread width doesn't sound like much, but it's enough to make a difference in ride quality, durability, and general usefulness. Besides, it's the factory size for the truck so likely fewer strange or unanticipated issues. Also, being the factory size for a super-popular truck probably means a ton more options and likely lower prices.

^^^ Excellent advice.

[Brad Johnson]

Driving in circles is not a reliable test. A limited slip in proper operating condition should NOT hop or skip when you drive in a circle.

Depends on the LSD type and what kind of surface you're on.

Brad

[Jim147]

Just put on the brakes, don't pump it. Hammer the gas and see what the mark or marks look like. It's to hot to be jacking stuff up.

[dogmush]

That's a good point about limited slip.  I don't think it has that but not sure.  (and I don't have an owner's manual)  How to check...  Cut the steering wheel all the way to one side and drive in a little circle and see if the rear wheels try to hop?  My previous truck had limit slip and if I recall correctly it did that.  (was annoying sometimes when trying to park)

Find you some dirt or gravel, power brake it up, and drop the hammer.  Then check for one or two tire spins.

[230RN]

Where you REALLY have to worry is if you have an AWD drive car such as a Subaru... If you lose a tire, you need to replace all four, because having one smaller or larger will trash the powertrain, and void your warranty.

+Same thing if one tire leaks a tiny bit and you don't keep track of it and get it re-inflated regularly.  One of these days I'll make an appointment and get it bubble-tested and fixed.  One of these days... 

[Doggy Daddy]

Depends on the LSD type and what kind of surface you're on.

Brad

What if it's 'shrooms instead?   

[Brad Johnson]

What if it's 'shrooms instead?   

Then you'll still be driving in circles, only it'll be on a polka dot dragon.

Brad

[tokugawa]

Where you REALLY have to worry is if you have an AWD drive car such as a Subaru... If you lose a tire, you need to replace all four, because having one smaller or larger will trash the powertrain, and void your warranty.

 Is this really true? Some tire shop put on a very slightly smaller rear tire on our Subaru after a non repairable puncture. The next time I was in another tire shop, they pointed it out as bad ju ju. i figued the differential would take care of any slight rotational difference, but I can see if it was a LS type maybe the thing would grind itself to pieces over time. Probably 5000 miles on it so far like this. Maybe I should do the Limited Slip check and see.

[JTHunter]

• My new truck has 285/75/16 tires on 16x6.5" rims.  The recommended rim size for those tires is 16x7.5" or x8".  The tires are in good condition and they are wearing okay.  I don't seen any weird bulging at the sidewalls near the bead.  I've talked to a couple of mechanics and they've told me there shouldn't be any problems from having wide tires on narrow rims and I should leave them alone.
  
• The spare tire is 245/75/16 and looks really old.  I haven't pulled it down yet to look at the date code because it's raining this week.  (and we need the rain)
  
• I have 4 brand new but couple of years old 215/85/16 tires in my garage.  I'll never haul anything heavy enough where the less payload capacity will make any difference.  215/85 and 245/75 tires are essentially the same diameter.  215 tires will be happy on 6.5" wide rims.

If I don't do anything, and someday I have to put the spare tire on, it needs to go on the front so I don't ruin the differential, right?  Or else drive *really* slow if I can't do that and it's on the back.  (the truck is RWD)

I kinda think I should leave the road tires and wheels alone.  Sell the 215's.  And buy a used 235/85/16 tire for a spare.  When the 285s wear out, replace them with 235/85s.  235/85 is a little smaller in diameter than 285/75 but they are pretty close; 16 revolutions per mile difference.  At 60 MPH or less and short distances that shouldn't matter.  What say ye?

If those "brand new tires" were what the truck came with from the dealer - GO TALK TO THEM !!  Have them explain the difference in your rim size vs. the recommended size.
Years ago, I had a 4WD p/u that had 15" x 6" rims.  The original tires were bias-belted H78 x 15 that were replaced with 225/75/15.  Later, I got some 8" x 16" rims and put 31/10.50/16 mud/snow tires on it.  Not only did my MPG change, so did my speedometer - by over 5 mph !
If you go down in size to the 215s, it is likely that you will find that your speedo will read higher AND your total miles will go up faster as well.  This will make your vehicle become a "high mileage" trade when it really is not one.

[zxcvbob]

If those "brand new tires" were what the truck came with from the dealer - GO TALK TO THEM !!  Have them explain the difference in your rim size vs. the recommended size.
Years ago, I had a 4WD p/u that had 15" x 6" rims.  The original tires were bias-belted H78 x 15 that were replaced with 225/75/15.  Later, I got some 8" x 16" rims and put 31/10.50/16 mud/snow tires on it.  Not only did my MPG change, so did my speedometer - by over 5 mph !
If you go down in size to the 215s, it is likely that you will find that your speedo will read higher AND your total miles will go up faster as well.  This will make your vehicle become a "high mileage" trade when it really is not one.

The brand new 215/85 tires in my garage were pulled off of my half ton truck 2 years ago when I scrapped it.  They are the same diameter as the 245/75s the new truck was originally equipped with, and they are the same diameter as the 255/70s the half ton started out with.  When I switched to LT215/85/16 tires on the half ton my mileage went up about 2 mpg and my speedometer did not change.  This was my second set of them, and the truck crapped out a couple of weeks after I bought the tires.

The new truck had 285/75 tires put on by the previous owner.  Its spare tire is a somewhat worn 245/75 and might be one of the originals.

When I check my fuel economy, I adjust for the speedometer error (calculated using GPS.)  With the current 285 tires I multiply the miles by 75/70 because when GPS says I'm going 75 the speedometer says 70.  I should find out how to calibrate the speedometer; that might have to be done by a GM mechanic thru the OBD port.

[Jim147]

It's been a while since you could just do the math and go to a GM dealer and pick up the right gears for the speedometer.

[dogmush]

Is this really true? Some tire shop put on a very slightly smaller rear tire on our Subaru after a non repairable puncture. The next time I was in another tire shop, they pointed it out as bad ju ju. i figued the differential would take care of any slight rotational difference, but I can see if it was a LS type maybe the thing would grind itself to pieces over time. Probably 5000 miles on it so far like this. Maybe I should do the Limited Slip check and see.

Some all wheel drive systems are more sensitive to it then others. Scoobies are known to have pretty fragile drivetrains, so they are one of the sensitive ones.  BMW says tire circumference should be within 1% on their drive systems.

The issue is as tire circumferences diverge, something has to slip to make up the difference, and constant slipping in the diffs or, especially the transfer case will lead to premature failure.  Front to back differences are worse than just one corner being smaller.

Also, in newer AWD systems excessive slip will pop a driveline code, and may drop the vehicle into limp mode.

[Jim147]

A friends son just put a new front differential in after running different size tires fron and rear. Not sure how long he ran it like that.

If you have to run differently size tires on a 4x4, make sure they are spinning. Running mud bugs and mud trucks we always had the fronts turning faster than the back. But they were spinning the whole time.

[Hawkmoon]

I should find out how to calibrate the speedometer; that might have to be done by a GM mechanic thru the OBD port.

I would wait until you get the right size tires on it.

How worn are the oversize tires that came on it?

[zxcvbob]

I would wait until you get the right size tires on it.

How worn are the oversize tires that came on it?

It's pretty low on my priority list.  Making sure I have safe tires is pretty high. 

I haven't measured the tread depth yet, but there's a lot of it.  And they sing on the highway but I couldn't hear that until I got a new muffler.

[Jim147]

Turn the radio up.

[Boomhauer]

I wouldn’t have any problems keeping the current ones on

I’d swing by a tire store and buy a good used tire for a spare they are typically cheap.

I also wouldn’t waste time on trying to change the speedometer

[zxcvbob]

I wouldn’t have any problems keeping the current ones on

I’d swing by a tire store and buy a good used tire for a spare they are typically cheap.

I also wouldn’t waste time on trying to change the speedometer

I'm pretty sure that's exactly where I'm headed. I want to inspect the spare I got first and haven't got that far :)

Turn the radio up.

The radio worked on the test drive, but did not work when they delivered the truck.  Then it started working again.  Then it stopped.  (The head unit thinks it's working but there's no sound)  I need to pull it out and check the connections at the back.  Then try to test the speaker wires somehow.  Then buy an aftermarket radio that has Bluetooth and a good tuner and not much else.

[zxcvbob]

I just pulled down the spare tire.  I think it's one of the original tires from when the truck was new.  Bridgestone.  Might even be the original spare, but it has some wear to it. The DOT code is 2007.  (twentieth week of 2007)  Some checking on the sidewall.  So I might as well not have a spare.  I'll take care of that this week.