Tire Diameters..tire manufacturers decide height?? Hunh?

[Arnold]

WHEN did this start? I feel like I have been asleep! The  more people I talk to about this the more I realize THEY know and I don't.
 Maybe I am just too old. I USED to think that a tire's diameter was..
set by the width of the tire..then the aspect ratio..and the wheel diameter.

 What STARTED off as..(what I thought) a VERY simple..I need 4 snow/or aggressive tires for my truck..ends here.
  I need 4-245/75/16 tires for my 4x4.
  I luck out and the spare is new..I luck out again and I find another new spare on a rim. Same tire. I see another new tire..different manufacturer/different model..similar tread pattern. It is near my buddies house..he brings it up to me.
 It is more than an inch shorter!! WTF??

  Then it really begins..I cannot tell you just how much time I have spent at this..used tires..new tires..used tire dealers..new tire dealers..wreckers.

  There is a 1 1/2" difference in height between NEW 245/75/16 LT load range E tires. Some worn 235's are taller than some new 245's!

  The 3 guys(anyway) that own a large wrecking yard..2 of their employees..2 other people there..they were talking to me like it is common knowledge that the tire manufacturers decide how tall the tire will be. This..talk..that the tire is this wide..the aspect ratio..(height) is just all talk NOTHING else. They say this is done because there are so many 4x4's/awd..pretty much all cars have ABS..and that when you need 1 tire..you are now forced to buy 4..IF yours aren't close to new..and providing that model is still made. Indeed..I saw tires on sale that are being discontinued.The new models are an inch shorter.

  Now I find in ads..and in talking to people that they too could not match their tire height.

  I guess I a just determined that I am not going to have to drop a bare minimum of $1,000 FOR 4 new tires when I only need 2.

  Rant over.

[enjenjo]

It's always been that way. If a tire is bigger, each part of the tread touches the road less times per mile, leading to better tread wear. so premium tires are generally bigger for the same nominal size. On big trucks the difference can be as much as 4" with the same size marked on the side.

[wayne petty]

245/75/16


the 75 is the aspect ratio.. or percentage of height to width...


so a 245/75 tire section is about 183MM high.  according to direct calculator calculation...

a 235/75 tire is going to be both narrower and shorter to maintain the aspect ratio...  the direct calculation is 176MM section height...

lets take a peek over at tirerack

http://www.tirerack.com/tires/TireSearchResults.jsp?customSizeSearch=&width=245%2F&ratio=75&diameter=16

each tire selection has a SPEC tab...

it will show you info for that exact tire.. and also info on comparable sizes..

its a great reference source...


whats interesting... i just selected a 2006 avalanche snow tire and rim combos..  as the size tire you selected comes as an option on that truck...

http://www.tirerack.com/snow/WinterTireCompare.jsp?autoMake=Chevrolet&autoModel=Avalanche+4wd&autoYear=2006&autoModClar=&startIndex=0&search=true&performance=W&sortSize=16&snowSortCode=19442&winterType=AB

[Arnold]

It's always been that way. If a tire is bigger, each part of the tread touches the road less times per mile, leading to better tread wear. so premium tires are generally bigger for the same nominal size. On big trucks the difference can be as much as 4" with the same size marked on the side.

 Hasn't ever been that way for me..at least here in Canada..Ontario..Toronto area. Unless I have been extraordinarily lucky.
 I have forever..mixed/matched whatever
the same size stamped tires on the side of the tire..and they have always been the same..or very,very close.
 Mind..I have not really played around with this stuff seriously for a # of years.
 I spent from about 1985 to about 1993 in taxi maintenance in Toronto
205/215/225 15 tires were all pretty much identical in height respective to their size. Any manufacturers tires were exactly the same height as all the others. Competition for taxi tire business was fierce..so there  were piles of different manufactuers tires bought. A 205/215/225 was just grabbed and stuck on.Matter of fact they were racked in such a fashion that one just grabbed one from this or that rack and they were the same height. It was glaringly obvious when a wrong tire was placed in there.
 Differences in tire height could not be tolerated as almost all taxis were ex Police Cruisers with posi or some other kind of locking rear end.
 Even the years 1975 to 1985 when I bought/played around with/sold
GM pickups..I did not encounter variances in tire height.
 I rememeber when someone needed a tire..they just got that size and that was that. Clearly not anymore.

 LOL..4" difference in height!! in large truck tires. WOW! lock the axles in mud/snow/ice..get traction on dry pavement. Somein's gonna give.

 Wayne..Wow! The fact the Tirerack lists the actual tire diameter..is a clear indication that you can  indeed check the actual tire diameter. Thanks for that! I had not..obviously gone to their website. Nor have I noticed it on other tire places. The tire places I went to..seemed to just shrug it off. The wreckers I went to was very aware of it!

  Anyways..Thanks Guys!

  Tirerack..I can get a list of tires and their actual diameters now for new tires.THANKS

[Inprimer]

just reshod Caddy SRX , 2 diff sizes  fronts are 235/60 R 18 rears are 255/55 R 18 all unidirectional and being all wheel drive, I wonder how the front to back gear ratios co exist ......just wondering been told some Benz and lexus do the same...

[Arnold]

just reshod Caddy SRX , 2 diff sizes  fronts are 235/60 R 18 rears are 255/55 R 18 all unidirectional and being all wheel drive, I wonder how the front to back gear ratios co exist ......just wondering been told some Benz and lexus do the same...

 Interesting question! I don't know what the height difference if any is?

  Used to be the on a 4x4 ALL tires had to be the same height. They DO if there is any locking between the front and rear axle causing them to turn at the same speed. In I think 1973..to take advantage of the demand for
people that wanted "full time 4x4"..as in leave it in 4wd all the time..GM started making "full time" 4x4's by using a centre differential. Previous transfer cases had no "differential" in them(other than I think Quadratracks..but that is a whole different story). In the "full time" transfer cases to eliminate the binding that of course occured if the front and rear diffs turned at the same speed..the differential in the transfer case fixed that problem! The front and rear axles could turn at different speeds. You could run different height tires on the front and rear. Only problem with that is that you could never "lock" the transfer case. You never really had to do that unless you were stuck anyway.
  There would still be some binding on the highway even with a full time
4x4..even with a centre diff. There would of course be tire wear that was different etc. GM planned to sell a billion of these things and knew that was coming..SOoo..they ran different gear sets in the front and rear of full time 4x4's. 4.09 and 4.10. Even some odd ball ratios.
  When awd's came out..(I think the Eagle was the first?). Any slippage
caused clutches etc to engage/disengae(I think in the Eagles case the special fluid thickened)
  I was thinking of buying a GM 4wd min van..these are awd setups where
they are basiclly fwd..until there is wheel slippage..then the rear axle is driven.
  GM's sort of awd system..the one that came out around 2000 in their 1/2 and up pickups etc is not really an awd..no centre diff. What happens in those(when the awd button is pressed) so they can be driven in 4wd on the highway is that when there is wheel slippage..4wd is automaticaly engaged..locked..(transfer case locked) and when the slippage goes away..it shifts itself back into 2wd.
  A common problem on GM awd vans is that their abs system is a 2 way thing. It detects differences in wheel speed in  braking AND acceleration.
ANY Abs problems..and the awd system gets disabled.
  My buddy said that any Denalis have a true awd clutch transfer case that can also be locked. I don't know.

   4x4,awd sure has gotten complicated! Some of these systems that
automaticaly shift power from side to side or diagonally!

[wayne petty]

inprimer

check the tire placard on the car...   if those are the correct sizes then it is probably set up to work ...

improper sizing will also turn on your ABS and TCS lights...

if you take a photograph of your ... i forgot what they call it...

its a 4 x 5" white plastic label with row after row of 3 digit letter combos... those are RPO numbers identifies what parts your car was assembled with...

the label could be in the glove box..  on a door jam.. under the hood...  in the trunk.. down in the spare tire well..

looking at a proper year RPO list usually turns out to be interesting to most car guys... sees what other options were available for their car...

warning... that might lead to the giant PUN..

WOW... i could have had a V8...



inprimer.. and other caddy owners... want to have some fun...

engine off.. key on... foot off the brake pedal...

2 fingers...  \ |,,|    push and hold the OFF and the warmer buttons on the ac control panel.. until all the dash board lights up.. usually about 5 seconds...

sometimes its the passenger side warmer button as an UP arrow..

once it lights up.. release...   you can look for codes to be displayed on one of the panels..

if you knew the proper buttons to push.. caddys are equipped with on board scan tools.. display live data from multiple systems..

[Arnold]

inprimer

check the tire placard on the car...   if those are the correct sizes then it is probably set up to work ...

improper sizing will also turn on your ABS and TCS lights...

if you take a photograph of your ... i forgot what they call it...

its a 4 x 5" white plastic label with row after row of 3 digit letter combos... those are RPO numbers identifies what parts your car was assembled with...

the label could be in the glove box..  on a door jam.. under the hood...  in the trunk.. down in the spare tire well..

looking at a proper year RPO list usually turns out to be interesting to most car guys... sees what other options were available for their car...

warning... that might lead to the giant PUN..

WOW... i could have had a V8...



inprimer.. and other caddy owners... want to have some fun...

engine off.. key on... foot off the brake pedal...

2 fingers...  \ |,,|    push and hold the OFF and the warmer buttons on the ac control panel.. until all the dash board lights up.. usually about 5 seconds...

sometimes its the passenger side warmer button as an UP arrow..

once it lights up.. release...   you can look for codes to be displayed on one of the panels..

if you knew the proper buttons to push.. caddys are equipped with on board scan tools.. display live data from multiple systems..

  Good points as usual Wayne..
  Duh..I never thought of that..but an improper tire size can shut down the awd(abs light on comes on as soon as different tire speed is detected)
  I find those option sheets with a quick Google search as to where they are most likely to be. I like the UPSIDE down ones in the glove box haha.

  My buddy had a blower Benz and he was NEVER supposed to engage the diff lock button with the wheels spinning.
   SOoo he took me out and said it was under warranty(it was brand new) and lit it up pretty good! THEN hit the dif lock. Hooked up pretty good!

[enjenjo]

My 2000 Pontiac interprets a change in wheel speed on one wheel as a flat tire, lighting the tire pressure light. If you run two different size tires, it also lights the tire pressure light.