There are two size differences - diameter and width. Since this thread is concerned with diameter you may be referring to that.
The diameter difference is a result of the difference in width. The difference in width required a different aspect ratio to get the rolling diameters as close as possible, but not exactly the same.
As to the reason for different tire widths front and rear - that is done for several reasons. The primary one being to induce under-steer. That topic has been discussed frequently before. Related to that is the fact that the front is lighter and that the front tire need not be spec'd to carry as much weight.
Thanks for the answer.
The next logical question would be: If one were to carry an emergency spare, which size is the better choice to cover both front and back?
You all pose questions that are based on different issues. You should NOT put tires that have a DIAMETER difference that is too great because it will put faults into the ABS, Traction Control, and other monitoring systems.
Aggressiveness is more a perception of width of the tire tread and even the RIMS size.. but not the overall height or diameter of the tire. go to tirerack.com and look up all the info on sizing and the plus2 method.
BobDiaz asks about witch size to carry for an emergency. Once again a loaded question. Define your emergency solution! Are you going to change the damaged one and continue for 500 miles? 1000? or just to get you to a tire center and get the damaged tire fixed or replaced? If your trek is a timed rally from LA to NY, take two rears with you or a pit crew. If you just need to have something to get you to a tire center in the next exit or next town... the fronts are your best option... but think of this Bob how many spares do you carry on your Honda motorcycle???? and when was the last time you had a flat in your other vehicles? I have had one flat in the last five years with an approximate 300,000 miles driven... the odds just dont add up to be a real concern.. AAA card in my wallet will suffice for now.
You bring up several good points. My use of the word "emergency" is in the context of worst case use until the next major town/city with repair/replacement tires.
Now 95% of the time, I'm within a city and getting help is no big deal. However, every year or so, I do travel from LA to Idaho and drive some very open stretches where there's no cell phone service and lots of miles and miles of nothing. I don't recall the maximum distance to the next potential town, but it's likely to be over 100 miles.
On the other hand, of the 20 or so trips I've made over the years to Idaho (1,100 miles one way), I've never had a flat tire. So, the chance of that happening is VERY small.
The next logical question would be: If one were to carry an emergency spare, which size is the better choice to cover both front and back?
I carry a front size wheel/tire for a spare. It's lighter and thinner and fits well behind the passenger seat.
The intention is to have a tire that can be used in an emergency - and that means that it will not be used indefinitely, just long enough to have the the damaged tire repaired or replaced.
The difference in diameter between the OEM front and rear tires is 3/10's of an inch. Since my car now has all four tires identically sized, if the flat occurred on a rear tire, a front would be moved to the rear and the spare put on the front. That would avoid mechanical problems with different diameters on the rear.
By the way, my car now has 195/50-15 tires on all four wheels. The difference in diameter compared to the original 175 rears is only greater by 1/10 of an inch. That however, makes it close to a half inch difference to the original fronts. It would be OK, I believe, to have that difference temporarily on the two fronts in the event of a flat but not on the rears since it is a rear wheel drive. Changing would just require removing a good front wheel and replacing it with the spare then removing the rear and putting the good front that was removed on the rear. The car is light and has only three lugs on each wheel so this procedure would be simple and quick enough to do.
This thread has totally lost sight of my original request! (Jwight) gave me some facts to work with but most other comments have zip to do with the limitations of the ESP system.I'm well versed on section width's,aspect ratio's and so on.Can anyone shed some solid info on how much tire diameter difference or ratio from front to rear these onboard computers can tolerate before they go bonkers? Or as silly as this may seem, what if you just remove the 40 amp fuse marked ESP under the dash? would it render it inoperable and the car would'nt care what you ran for tires?
Kind'a like cars use to be!!
Or as silly as this may seem, what if you just remove the 40 amp fuse marked ESP under the dash? would it render it inoperable and the car would'nt care what you ran for tires?
It would not care; at that point you're sans ESP plus ABS - don't recommend it for the street.
How does the smart fortwo perform in the snow?
The smart fortwo standard active safety systems - Electronic Stability Program (esp®), traction control, cornering brake control, anti-lock brakes, electronic brake distribution and brake assist - help drivers stay in control on snowy and icy terrains.
The majority of the smart’s weight (engine and transaxle) is over the driven wheels resulting in better traction overall. Wider rear wheels and tires allow for better grip and traction in marginal weather conditions. Narrower front wheels and tires allow for less traction loss by penetrating into snow and ice deeper for improved steering control. All smart fortwo vehicles come standard with all season tires.
MY .02:
Lots of folks here seem to be happy with big wide stickies up front, but I'm inclined to stick with the factory wisdom. The front end of the car is pretty light, and last winter's experience in snow makes me think "pizza cutters" up front are a good idea.
To your original point, the stock fronts differ mostly in width, and about a tenth of an inch in diameter(radius). Never heard of problems with stock rears up front, too small of a difference.
...To your original point, the stock fronts differ mostly in width, and about a tenth of an inch in diameter(radius).
BARNEY O knows the difference in diameter between front to rear OEM's - and it is more than the 1/10th you mentioned - more like 3/10's.
As to what the tolerance would be to not aggravate the ESP program, well that is only speculation at this point since no one has come forward with definitive information. What is known is that 1 1/2 inches is not acceptable but 1/2 is.
There is a way to do a little experimentation if anyone is willing - that is to deflate the front tires which would reduce the rolling diameter. If any accurate measurements could be taken that could be done in stages until a problem presents itself with the ESP.
The AutoGuide.com network consists of the largest network of enthusiast-owned enthusiast-operated automotive communities.
AutoGuide.com provides the latest car reviews, auto show coverage, new car prices, and automotive news. The AutoGuide network operates more than 100 automotive forums where our users consult peers for shopping information and advice, and share opinions as a community.
1976 Rover Mini
2003 Chevrolet Corvette Z06
2007 Mercedes-Benz E320 Bluetec (SOLD)
2009 smart Passion (Gone)
2011 smart Passion
Today
Linear Mode
