warmpancakes
New member
how much is too much?
is 6 tenths of a inch diameter too much or would 4 tenths be better?
is 6 tenths of a inch diameter too much or would 4 tenths be better?
differences in tire size
- Thread starter warmpancakes
- Start date
sytyguy
Moderated User
Re: differences in tire size
Well, let's just say you had a 245/50R16 in the front (tire diameter of 25.6) and a 255/50R16 in the rear (tire diameter of 26.0).
(245)
25.6 x 3.1416 = 80.43 (circ of tire)
63360 / 80.43 = 787.77 (tire revs per mile)
(255)
26.0 x 3.1416 = 81.69 (circ of tire)
63360 / 81.69 = 775.62 (tire revs per mile)
That means that every mile, the front tires are turning 12.15 more times than the rear. On a 100 mile trip, it's 1215 more times. And that's at .4" difference. Sounds like a lot for the xfer case to cope with to me. Tire pressure can help equalize this though.
Hood
Well, let's just say you had a 245/50R16 in the front (tire diameter of 25.6) and a 255/50R16 in the rear (tire diameter of 26.0).
(245)
25.6 x 3.1416 = 80.43 (circ of tire)
63360 / 80.43 = 787.77 (tire revs per mile)
(255)
26.0 x 3.1416 = 81.69 (circ of tire)
63360 / 81.69 = 775.62 (tire revs per mile)
That means that every mile, the front tires are turning 12.15 more times than the rear. On a 100 mile trip, it's 1215 more times. And that's at .4" difference. Sounds like a lot for the xfer case to cope with to me. Tire pressure can help equalize this though.
Hood
Re: differences in tire size
I agree... you should compare the circumference when matching tires because the diameter isn't accurate enough. You can lose .6" in tread wear.
I agree... you should compare the circumference when matching tires because the diameter isn't accurate enough. You can lose .6" in tread wear.
warmpancakes
New member
Re: differences in tire size
your logic seems well but what about going around a corner? the outter tire travels futher than the inner tire?
your logic seems well but what about going around a corner? the outter tire travels futher than the inner tire?
sytyguy
Moderated User
Re: differences in tire size
True, but that difference also plays into the differential, not so much the xfer case. Also, most corners experienced over longer periods of driving (long roads, interstates, etc...) force the vehicle to take a much less stringent line than, say, a u-turn on a city street. Then you have to consider the amount of time spent actually cornering (around a meaningful corner) versus the amount of time spent going straight and on less meaningful corners.
I've driven a full spool on the street and most "road" corners never even strain it. Take a tighter turn (45 deg or more over 100 yds) and it'll bark tires like a dog, but that shows how little the average road corner plays into tire rotation.
I'd still be tempted to try the .4 tire difference (and play with pressures), but just know the potential problems.
Hood
warmpancakes said:
True, but that difference also plays into the differential, not so much the xfer case. Also, most corners experienced over longer periods of driving (long roads, interstates, etc...) force the vehicle to take a much less stringent line than, say, a u-turn on a city street. Then you have to consider the amount of time spent actually cornering (around a meaningful corner) versus the amount of time spent going straight and on less meaningful corners.
I've driven a full spool on the street and most "road" corners never even strain it. Take a tighter turn (45 deg or more over 100 yds) and it'll bark tires like a dog, but that shows how little the average road corner plays into tire rotation.
I'd still be tempted to try the .4 tire difference (and play with pressures), but just know the potential problems.
Hood
Black Knight
I Glow Therefore I am
Re: differences in tire size
All the technical memos I've ever read on AWD vehicals say your allowed up to a 1% difference in tire diameter, I don't think I would ever run that much of a difference, but that's what is "supposably" accecptable.
Now using the sizes that Hood put up there (which aren't very different from each other), they have a 1.5% difference, so that would even be too much. Now this is assuming all is equal (manufacter, brand, air psi, tread depth)
Lets try another example you have a brand new P245/50R-16 brand "X" tire that's 25.6" tall and a completely worn out tire of same size and brand (worn out being the legal limit of 2/32nds).
Brand new on the average that size comes with 10-11/32nds of tread, we'll use 11/32nds to push this as far as we can.
So lets take 11/32nds minus 2/32nds since the bottom 2/32nds aren't usable, we get 9/32nds.
Take 9/32nds and times by 2 since we are dealing in diameter and have both sides to account for, we get 18/32nds or 9/16 of an inch or (.5625 of an inch)
25.6" minus .5625" equals 25.0375" That's an even bigger difference then the one Hood was talking about and all were talking about is just tread depth.
I'm pretty tired this morning, I hope I did this right :rotf:
All the technical memos I've ever read on AWD vehicals say your allowed up to a 1% difference in tire diameter, I don't think I would ever run that much of a difference, but that's what is "supposably" accecptable.
Now using the sizes that Hood put up there (which aren't very different from each other), they have a 1.5% difference, so that would even be too much. Now this is assuming all is equal (manufacter, brand, air psi, tread depth)
Lets try another example you have a brand new P245/50R-16 brand "X" tire that's 25.6" tall and a completely worn out tire of same size and brand (worn out being the legal limit of 2/32nds).
Brand new on the average that size comes with 10-11/32nds of tread, we'll use 11/32nds to push this as far as we can.
So lets take 11/32nds minus 2/32nds since the bottom 2/32nds aren't usable, we get 9/32nds.
Take 9/32nds and times by 2 since we are dealing in diameter and have both sides to account for, we get 18/32nds or 9/16 of an inch or (.5625 of an inch)
25.6" minus .5625" equals 25.0375" That's an even bigger difference then the one Hood was talking about and all were talking about is just tread depth.
I'm pretty tired this morning, I hope I did this right :rotf:
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Re: differences in tire size
The way I understood the tollerance difference is that 3-4% is acceptable. However on todays AWD systems it is less affected. But, for our trucks.....
In your tread wear example it comes out to about 2.3-2.5% depending on how accurate you make the numbers. This is well within the 3-4% tollerance and the reason normal treadwear doesn't affect it that much, but it is possible to run into trouble on some tires. The 1% tollerance that you read about is on the original tire size, the remaining 2-3% is to account for tread wear. Does that make sense? The average person is too stupid to account for treadwear much less know how to calculate it.
In short, you are allowed 3-4% tollerance including treadwear and 1% if you just go by tire size.
If you take the 245/50 and the 255/50 mentioned above and compare them....
Diameter - 25.65 vs 26.04 = .39 difference
That's 1.38% - 1.52% depending on which you choose to take the % of
the 255 is 1.52% taller and the 245 is 1.38% shorter
Anyway that is out of the common knowledge tollerance of 1%
TOO MUCH... probably not a good idea to do it.
Now... if the 255's only had about 30% tread left on it and the 245's were new...
you might be ok...
Hope this all makes sense
The way I understood the tollerance difference is that 3-4% is acceptable. However on todays AWD systems it is less affected. But, for our trucks.....
In your tread wear example it comes out to about 2.3-2.5% depending on how accurate you make the numbers. This is well within the 3-4% tollerance and the reason normal treadwear doesn't affect it that much, but it is possible to run into trouble on some tires. The 1% tollerance that you read about is on the original tire size, the remaining 2-3% is to account for tread wear. Does that make sense? The average person is too stupid to account for treadwear much less know how to calculate it.
In short, you are allowed 3-4% tollerance including treadwear and 1% if you just go by tire size.
If you take the 245/50 and the 255/50 mentioned above and compare them....
Diameter - 25.65 vs 26.04 = .39 difference
That's 1.38% - 1.52% depending on which you choose to take the % of
the 255 is 1.52% taller and the 245 is 1.38% shorter
Anyway that is out of the common knowledge tollerance of 1%
TOO MUCH... probably not a good idea to do it.
Now... if the 255's only had about 30% tread left on it and the 245's were new...
you might be ok...
Hope this all makes sense
Black Knight
I Glow Therefore I am
Re: differences in tire size
LMFAO :rotf:
Hey Jon not that I don't believe you, but where did you get that number from I've just never heard of any AWD having that high of a tollerance :dunno:
I've seen newer Subaru's eat tranny/transfer case because some Jack-me-off wasn't paying attention and only put two tires on it, and they were the same size's. I've also seen many never have any issues, so it's kinda hard for me to say what is truely acceptable, so I just say 1% :lol:
SonomaGTlover said:
LMFAO :rotf:
SonomaGTlover said:
Hey Jon not that I don't believe you, but where did you get that number from I've just never heard of any AWD having that high of a tollerance :dunno:
I've seen newer Subaru's eat tranny/transfer case because some Jack-me-off wasn't paying attention and only put two tires on it, and they were the same size's. I've also seen many never have any issues, so it's kinda hard for me to say what is truely acceptable, so I just say 1% :lol:
Re: differences in tire size
Didn't get it from any specific source... that is the reason I started my post with...
A brand new tire compared to the same tire worn slick is very near the 3% difference which pushes the tolerance threshold. You proved that in your calculations so even tires of the same size may not be a good idea. 3% considering the treadwear is about the same as 1% based on tire size alone.
If you buy two tires of the exact same size from different manufacturers, I would almost bet that there is at least a 1% difference already. Not trying to plead my case, just offering a different viewpoint. I may be totally wrong but 3% is only about 12 rev/mile depending on the tire. I don't think that will cause too much excess heat to build up.
Anyway, the moral of the story is that the closer you match the tires the better off you will be... AND dont' forget to rotate them occasionally to avoid one wearing more than others.
Didn't get it from any specific source... that is the reason I started my post with...
I've had 2 other awd vehicles and it was a number that just stuck with me over time. Not saying that I am right but that is the way I learned it then.
A brand new tire compared to the same tire worn slick is very near the 3% difference which pushes the tolerance threshold. You proved that in your calculations so even tires of the same size may not be a good idea. 3% considering the treadwear is about the same as 1% based on tire size alone.
If you buy two tires of the exact same size from different manufacturers, I would almost bet that there is at least a 1% difference already. Not trying to plead my case, just offering a different viewpoint. I may be totally wrong but 3% is only about 12 rev/mile depending on the tire. I don't think that will cause too much excess heat to build up.
Anyway, the moral of the story is that the closer you match the tires the better off you will be... AND dont' forget to rotate them occasionally to avoid one wearing more than others.
sytyguy
Moderated User
Re: differences in tire size
Preface: If I was in a position to do so, I'd ride with a .4 difference. So, I'm not telling anyone not to try it, just saying that the transfer case won't be happy.
Not only is it the potential damage to the transfer case, but also the drag it'll produce in trying to overcome the difference. I did a test about 8 years ago with tire pressures to see what pressures resulted in the lowest driveline drag. The test was to get up to 60 mph and let off the gas at mailbox #1 and record the speed at mailbox #2 (which was around 1000' away). I can't recall the exact speeds, but there was a gain of over 4 mph from 32 psi all the way around to 42 in the front and 32 in the rear.
That example doesn't apply directly to the question at hand, but it does show the transfer case's performance difference when the F & R rotational diameter is closer to each other (it also show how much front weight bias there is on the Syclone). Now, knowing all of this, I would easily try the 245/255 example I gave above, but by no means would I try using something as different as, say, OE 17/18 ZO6 wheels.
And while we're at it, let's do the math on the OE ZO6 example.
17" Front (265/40/17 - 25.346" dia)
25.346 x 3.1416 = 79.63 (circ of tire)
63360 / 79.63 = 795.68 (tire revs per mile)
18" Rear (295/35/18 - 26.13" dia)
26.13 x 3.1416 = 82.09 (circ of tire)
63360 / 79.64 = 771.84 (tire revs per mile)
That's 23.84 more rotations over a mile on the front. Every oil change, that's 71,520 more rotations.
Hood
Preface: If I was in a position to do so, I'd ride with a .4 difference. So, I'm not telling anyone not to try it, just saying that the transfer case won't be happy.
Not only is it the potential damage to the transfer case, but also the drag it'll produce in trying to overcome the difference. I did a test about 8 years ago with tire pressures to see what pressures resulted in the lowest driveline drag. The test was to get up to 60 mph and let off the gas at mailbox #1 and record the speed at mailbox #2 (which was around 1000' away). I can't recall the exact speeds, but there was a gain of over 4 mph from 32 psi all the way around to 42 in the front and 32 in the rear.
That example doesn't apply directly to the question at hand, but it does show the transfer case's performance difference when the F & R rotational diameter is closer to each other (it also show how much front weight bias there is on the Syclone). Now, knowing all of this, I would easily try the 245/255 example I gave above, but by no means would I try using something as different as, say, OE 17/18 ZO6 wheels.
And while we're at it, let's do the math on the OE ZO6 example.
17" Front (265/40/17 - 25.346" dia)
25.346 x 3.1416 = 79.63 (circ of tire)
63360 / 79.63 = 795.68 (tire revs per mile)
18" Rear (295/35/18 - 26.13" dia)
26.13 x 3.1416 = 82.09 (circ of tire)
63360 / 79.64 = 771.84 (tire revs per mile)
That's 23.84 more rotations over a mile on the front. Every oil change, that's 71,520 more rotations.
Hood
ashman
------
Re: differences in tire size
just to throw another wrench into the equation...the example you gave would only apply to a wheel/tire that was NOT touching the road. the actual equation would be (radius x 2) instead of diameter to find circumfrence. radius would be measured from the centerline of the axle to the road surface.
sytyguy said:
just to throw another wrench into the equation...the example you gave would only apply to a wheel/tire that was NOT touching the road. the actual equation would be (radius x 2) instead of diameter to find circumfrence. radius would be measured from the centerline of the axle to the road surface.
turbodig
Active member
Re: differences in tire size
If the .4" larger tire was in the front, it'd be workable with air pressure.
If the other way, I'd say no.
Keep in mind, advertised heights usually aren't correct within .4".
Really paranoid? Put the taller tires on the rear, pull the front propshaft, and submit the size difference to the air.
It's always best to get the rotation ratio (front/rear) to be the best it can. Few people play with air pressure enough to achieve this.
If the .4" larger tire was in the front, it'd be workable with air pressure.
If the other way, I'd say no.
Keep in mind, advertised heights usually aren't correct within .4".
Really paranoid? Put the taller tires on the rear, pull the front propshaft, and submit the size difference to the air.
It's always best to get the rotation ratio (front/rear) to be the best it can. Few people play with air pressure enough to achieve this.
warmpancakes
New member
Re: differences in tire size
these are the tires Toyo TIR
rear 295/35/18 26.0 Tall
front 275/35/18 25.6 Tall
front 265/35/18 25.4 Tall
Just contacted Toy they have a 255/40/18 25.9 tall problem solved
these are the tires Toyo TIR
rear 295/35/18 26.0 Tall
front 275/35/18 25.6 Tall
front 265/35/18 25.4 Tall
Just contacted Toy they have a 255/40/18 25.9 tall problem solved
Re: differences in tire size
I had no idea this would get into this type of conversation. I just did a search and found basically nothing, just a couple comments. It seems like we need to get some real answers just so we know.
Maybe I can do this when I get my new wheels/tires for the track? I'll do the 60mph mailbox coast test and possibly get creative with something else. won't be until fall though.
I was going to use 245/50-16 and 275/45-16 but I think they will be too close to notice anythng. The diameter difference is .09 different, that's less than 3 rev per mile.
Note to those less interested: If you want different tire sizes... this is probably the best combination available.
If you take the prop shaft out then tire size won't matter... even at extreme difference.turbodig said:Really paranoid? Put the taller tires on the rear, pull the front propshaft, and submit the size difference to the air.
I had no idea this would get into this type of conversation. I just did a search and found basically nothing, just a couple comments. It seems like we need to get some real answers just so we know.
Maybe I can do this when I get my new wheels/tires for the track? I'll do the 60mph mailbox coast test and possibly get creative with something else. won't be until fall though.
I was going to use 245/50-16 and 275/45-16 but I think they will be too close to notice anythng. The diameter difference is .09 different, that's less than 3 rev per mile.
Note to those less interested: If you want different tire sizes... this is probably the best combination available.
sytyguy
Moderated User
Re: differences in tire size
True, but given the speed and tire temperature, the tire may grow back to closer to original (unloaded) height. You want wrenches thrown in, throw in pressure and temp gains coupled with rotational diameter and change in circumference. Then compare it all to sidewall compression under weight for our particular trucks. We do a fair amount of tire math when we're at the Road Course, but it's usually on RWD cars, so we usually just adjust after testing.
This is all subjective, I agree, until someone actually does some testing (to what extent and benefit, I'm not sure). I have some axle counters that I could slap on my DD Jimmy (with some mismatched wheels tires) and record the actual rotations vs. speculative. Though it would be useless data until we knew what was too much (if the math predicted 1200 more rotations over 100 miles, and the actual was 700, is someone really gonna jump up and say, "Great I feel so good about 700 more rotations that I'm gonna go drop $2000 on wheels and tires!").
Although, it would be nice if we knew the change in transfer case drag as the rotation gets farther out of sync. Not that I expect anyone to do such extensive testing on a transfer case, just saying it would be nice.
Hood
ashman said:
True, but given the speed and tire temperature, the tire may grow back to closer to original (unloaded) height. You want wrenches thrown in, throw in pressure and temp gains coupled with rotational diameter and change in circumference. Then compare it all to sidewall compression under weight for our particular trucks. We do a fair amount of tire math when we're at the Road Course, but it's usually on RWD cars, so we usually just adjust after testing.
This is all subjective, I agree, until someone actually does some testing (to what extent and benefit, I'm not sure). I have some axle counters that I could slap on my DD Jimmy (with some mismatched wheels tires) and record the actual rotations vs. speculative. Though it would be useless data until we knew what was too much (if the math predicted 1200 more rotations over 100 miles, and the actual was 700, is someone really gonna jump up and say, "Great I feel so good about 700 more rotations that I'm gonna go drop $2000 on wheels and tires!").
Although, it would be nice if we knew the change in transfer case drag as the rotation gets farther out of sync. Not that I expect anyone to do such extensive testing on a transfer case, just saying it would be nice.
Hood
Re: differences in tire size
I don't forsee any issues with that, a near perfect match. :tup: (but my combo is closer...:rotf: )
Nice combination, the 255 in front and the 295 in rear is just over 3 rev per mile difference.warmpancakes said:
I don't forsee any issues with that, a near perfect match. :tup: (but my combo is closer...:rotf: )
warmpancakes
New member
Re: differences in tire size
Im not worried is the Wifes truck if it breaks she can fix it
sytyguy said:
Im not worried is the Wifes truck if it breaks she can fix it
Re: differences in tire size
That's the spirit :tup:warmpancakes said: