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Discussion Starter #1
This is for another car that I have acces to but the answer will be the same on any forum:

What would the effect be on any vehicle that has it's 2 rear tyres 25mm wider than the 2 front tyres? All tyres are the same size in diameter and % of width (ie 75). It is only the width that differs.

Then, what happens over a maximum distance of maybe 4 miles if my spare wheel/tyre is used in an emergency and is 1 inch larger in diameter than any other tyre.

My thoughts:

  • Nothing will happen as all tyres are using the same travel circumference therefore the speedo will still work normally.
  • Traction loss on cornering or braking perhaps?
  • Using the incorrect sized wheel for a short period to get you home will cause a raise on one side of the car with no ill effect if taken slow at a max speed of 30mph.
  • When working out allowances for use I think 3% thereabouts is the accepted standard but used tyre tread may reduce the diameter by around 15mm and the acceptable allowance sits around 4.5% (I need to research this all over again as this may be the wrong info/explanation) :eek::unsure:
All thoughts welcome and they cannot be any less stupid as my thoughts. So no worries about posting.
 

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Your question is kind of hard to understand, so I'm not sure what you are actually trying to find out.

So, 2 tires and the rears are 25mm wider, but diameter and circumference are the same:

Well there's two ways to get there. I'm assuming with both you retained proper clearance to suspension components.

1) You putt skinnier than stock tires up front - most cars sold are designed to understeer. This means they try to plow the front end in hard cornering. In this case you reduced your traction budget up front and are more likely to experience loss of traction under hard braking.

2) You put fatter tires than stock in the back. Depending on the suspension geometry, if you don't do an alignment and can get the camber right, you may experience some feathering of the outside edge of the wider tires. This would increase road noise eventually.

Regardless, by going to a fat and skinny setup, you eliminate your ability to rotate tires, and thus will increase non-uniform wear and shorten tire life.


Then you say "All tyres are the same size in diameter and % of width (ie 75) " To which I say... uhh no.

lets say I have a 205-75-17 tires and a 225-75-17 tire. that 75% in both of them means the tire wall height is 75% of the tread width. The treads are of different width, so the side wall height is different. Which means the overall diameter is different. in my example the 205 tire is 29.1" in diameter and has a circumference of 91.4" The 225 tire has a diameter of 30.3" and a circumference of 95.1". So with this statement, no it is not only the width that differs given the parameters you set, but width and diameter.

"Then, what happens over a maximum distance of maybe 4 miles if my spare wheel/tyre is used in an emergency and is 1 inch larger in diameter than any other tyre. "

This is pretty much what happens when you put a donut spare on a lot of cars. They are seldom EXACT matches. Over 4 miles the answer in most cars is nothing happens of any real consequence. Maybe some components experience a mild amount of increased stress and heat for a few miles. That's the short version. You will also have reduced traction because the donut has a smaller contact patch.

Then you start going on about 3% or 4% allowances, and I'm wondering what kind of answer you are trying to get to, and I can only speculate.

So speculation #1. The perennial paranoid warning about not having tires of differing diameters on an AWD vehicle. The renegade is essentially runnning part time AWD that fully decouples most of the time, and is running open diffs front and rear connected by a an electronically controlled wet clutch pack. It's going ot be pretty tolerant of tire replacements within the normal wear patterns of tire use. I would not replace 2 at a time and have fresh tires up front and near bald in the rear, but it should be fine if you have say 30% tread wear on a set of 4 and have to replace one tire with the same or similar tire that's new.

Speculation #2. You want to run a fat and skinny setup because it looks cool on porsches or something. Or because you want some nice AT tires but they are expensive and because you didn't rotate your tires you have 2 that are nowhere near as bad as the ones on the front. To this I say... no don't. Unless it came from the factory that way, or you are building a drag car, etc where you know why you are doing it, the costs, and aren't trying to live with it as your only daily driver, just put 4 tires the same size on that fit with appropriate clearances.

Speculation #3. You got the stupid pump and container of flat goo and would really like a spare. To this the answer, going full size or donut, is try to get reasonably close and unless it is an exact size match full size spare, don't drive around on the spare for too long. I used to have an AWD with a viscous center diff and an LSD in the rear. It came with a donut and advised no more than 30 miles.
 

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Discussion Starter #3
Raz. a very big thank you for such a well thought out presentation and I truly appreciate it. The question does not relate to any Jeep :)

I often thought that 75 meant 75 millimetres in height (ie 3 inches) height of a tyre wall but that is only 3 Inches as opposed to 75% of the tyre width. How wrong was I

My question was not meant to be hard to understand but you answered it beautifully.

Conclusively as a spare wheel with the wrong size of diameter wheel/tyre it will be ok for around 4 miles providing I adhere to low road speeds.

However fear not. I am on the search for a couple of a tyres and a wheel for a vehicle as yet not discussed o_O;)(y)
 

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Discussion Starter #4
Your question is kind of hard to understand, so I'm not sure what you are actually trying to find out.

So, 2 tires and the rears are 25mm wider, but diameter and circumference are the same:

1) You putt skinnier than stock tires up front - most cars sold are designed to understeer. This means they try to plow the front end in hard cornering. In this case you reduced your traction budget up front and are more likely to experience loss of traction under hard braking.

2) You put fatter tires than stock in the back. Depending on the suspension geometry, if you don't do an alignment and can get the camber right, you may experience some feathering of the outside edge of the wider tires. This would increase road noise eventually.

Regardless, by going to a fat and skinny setup, you eliminate your ability to rotate tires, and thus will increase non-uniform wear and shorten tire life.


Then you say "All tyres are the same size in diameter and % of width (ie 75) " To which I say... uhh no.

lets say I have a 205-75-17 tires and a 225-75-17 tire. that 75% in both of them means the tire wall height is 75% of the tread width. The treads are of different width, so the side wall height is different. Which means the overall diameter is different. in my example the 205 tire is 29.1" in diameter and has a circumference of 91.4" The 225 tire has a diameter of 30.3" and a circumference of 95.1". So with this statement, no it is not only the width that differs given the parameters you set, but width and diameter.

"Then, what happens over a maximum distance of maybe 4 miles if my spare wheel/tyre is used in an emergency and is 1 inch larger in diameter than any other tyre. "

This is pretty much what happens when you put a donut spare on a lot of cars. They are seldom EXACT matches. Over 4 miles the answer in most cars is nothing happens of any real consequence. Maybe some components experience a mild amount of increased stress and heat for a few miles. . You will also have reduced traction because the donut has a smaller contact patch.

Then you start going on about 3% or 4% allowances, and I'm wondering what kind of answer you are trying to get to, and I can only speculate.
The car is an old Mitsubishi Shogun and the factory fitted wheels I believe are 235/75/15 but someone changed them to 255 tyres and the front 2 are all but bald. (UK law means they are illegal though it just made its MOT) Finding used 255 tyres as runners is proving quite hard or expensive but 235 tyres are readily available both used and new.

That's why I was asking about the compatibility issues of using different sized tyres by width. I see now that I really need to pay attention to that.

As you have explained there is a difference in wall height and essentially in overall diameter but I am led to believe there was a tolerance of around 3% of that overall circumference difference between both different sized tyres that allow the tyres to work. If the measurement is within this allowance then it's ok to drive but it gives me a headache trying to understand the calculation.

If I am correct I need to use the figures:

75% of 255mm = 191mm and 75% of 235 = 176mm.
That means the diameter of a 15" wheel ie 375mm + 191mm+191mm= 757mm X 3.14 for the circumference = 2376.98mm
and the smaller width means the the diameter of a 15" wheel ie 375mm+176mm+176mm=727mm X 3.14 for the circumference =2282.78mm
Therefore subtracting 2282.78mm from 2376.98mm = 94.2mm.

If that 94.2mm falls within 3% of the overall circumference then it should be ok. So 3% of 2376.98 = 71.3mm. In this case its not compatible :eek::cry::cry::cry: as its 4%

Hopefully I have not lost anyone just yet but have I calculated that properly.
This figure falls by the way side if the overall tolerance of 3% is wrong.

I only have another 2 weeks to wait until I can get near this truck to see if the 235's fit the rim and if so I will then look for another pair of the same sized tyres to keep the 4 of them the same. It may be a that I buy 2 new tyres for the sake of an extra £50 as opposed to runners but that means the tyres will still be there when the Shogun dies as it only does around 200 miles a year for use at a very wet, muddy and rocky launch site at the low tide mark and I aint putting a new(ish) Renegade into salt water deeper than its tyre wall for fear of corrosion to the brakes.

However I do appreciate the replies and I do understand that this is a Jeep forum before anyone shouts that out but it is the experience that you guys offer that made me consider wether to post in here or go to a Shogun forum. (y)(y)(y)
 
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