Tire Calculator

[AMCKen]

Years ago I made my own tire size calculator in excel. Not difficult and works great. I also have one that tells me what speedo driven gear I need for any particular tire size and differential gears.

[philotomy]

I have that size on my car now(fenders trimmed) you'll want to change the gears in both axles to something steeper like 3.54:1,I didn't and toasted my trans(rebuilding that) the back has 3" blocks and longer u bolts a helper spring,aftermarket shackles and 2 stud lengtheners on each shock for about 5" lift,the front has those mr.Gasket spacers which were too extended,causing one to snap.A safer method is a spacer that sits in place of the top cushion,mine were 3 1/2 inch in the front (which causes cv axle noise) 2 inch max for no problems I've read here.Hope this helps

[NYEagle]

Be warned that if you go from a 75 Series tire to a 65 Series tire YOU WILL LOSE GROUND CLEARANCE !!!

I did that to my Eagle and regretted it later on. I noticed my Eagle would scrape on obstructions that it used to be able to clear. Thank goodness for the skidplate!

Was very happy when the tires finally wore out and I was able to go back to the 75's 


~ Joe in NY

[carnuck]

Go to an 85 series and you'll GAIN groundclearance plus the tire will be narrower for better clearance from the fender.

[NYEagle]

Go to an 85 series and you'll GAIN groundclearance plus the tire will be narrower for better clearance from the fender.

Now THAT sounds even better! 

Probably have to step up to a 16" rim to do that.

I wonder if an LT215/85R16 would fit in the wheelwell without any clearance problems. 


~ Joe in NY

[NYEagle]

Awww.

Just tried the Tire Size Calculator here and a warning came up that said you shouldn't increase the tire diameter any more than 3% over the stock tire or you risk brake failure.

The Diameter Difference with the LT215/85R16 tire is 12.74%

Oh well...   


~ Joe in NY

[eaglefreek]

Awww.

Just tried the Tire Size Calculator here and a warning came up that said you shouldn't increase the tire diameter any more than 3% over the stock tire or you risk brake failure.

The Diameter Difference with the LT215/85R16 tire is 12.74%

Oh well...  


~ Joe in NY

More total diameter is what will get you "more clearance". A 215/85/16 is over 30" in diameter and will need a lot of fender work or a big lift to get it to work.

[philotomy]

my oversized tires smoked my tranny,I had 30x9.5 x15's just fyi.and the front  fenders were cut by the p.o.

[NYEagle]

Yeah....  I'll just stick with my 75's.

I don't need any MORE problems with my Eagle than I already have! 


~ Joe in NY

[Sunny]

Awww.

Just tried the Tire Size Calculator here and a warning came up that said you shouldn't increase the tire diameter any more than 3% over the stock tire or you risk brake failure.

The Diameter Difference with the LT215/85R16 tire is 12.74%

Oh well...  


~ Joe in NY

The reason they state that is for liability reasons.
Anything more than 3% will throw your speedometer off [You'd be amazed how many people have installed huge tires on their car/truck, only to come back with a ticket saying its our fault.]
Secondly, is that a lot of people with 4x4's, try to cheap out and change only two tires. When you have something like huge mud terrains or swampers, the difference in worn out tread can actually equal 3% or more, causing all sorts of drivetrain issues. All of our suppliers require four tires, of the exact same brand and model to be installed on any 4x4. Most people don't realize, two tires of different brand, but same size aren't actually identical in diameter. You can get a 33x10.5R15 say, That can range anywhere from 31.5-33.5" tall [In extreme cases]. Same thing with performance tires. Nitto 555R's are a great tire, I've personally used them.. but their 275's were narrower than my BFG 265's.

The 3% rule is also  because a lot of people put big tires and rims on their vehicles, without ever upgrading their braking system. We get people all the time with jacked up SUV's who wonder why it takes a lot longer to stop, going from a stock 28-29" tire, to a 35" tire. It doesn't seem like THAT much heavier [it is lifting it, I assure you  ], but it's all rotating mass. The vehicle weight only goes up by a bit, but it directly effects braking exponentially, which creates significantly more heat, hence the possibility of brake failure. I'd say 1/10 [if that] people I see who put on significantly bigger rims or tires upgrade their braking system. We also have a few vehicles per month come in, where the older style ABS sensors trigger due to increased diameter. The newer style TPMS sensors don't matter on diameter at all.

Some manufacturers will also void a warranty on the vehicle if you go out of 3%, regardless if you change all four or not. I know we had an issue with an Acura MDX, customer came in, wanted bigger tires, Acura has a note in our system to contact the dealer because they do not want anything larger or smaller installed. Called the dealer, they basically said any mechanical failures are the responsibility of the client, Acura does not condone tire size changes with their AWD vehicles.  

Also anything over 3% will change the ride and handling of the vehicle. Too tall of a sidewall and the vehicle will feel 'squishy' compared to stock, and it will generally made handling a lot less responsive. Too short of a sidewall, and I hope you know a good back surgeon. The vehicle will respond quicker but at a massive sacrifice to ride comfort.

One of the problems we also see a lot, is putting putting super wide tires on their cars/trucks, and then not being able to drive through rain or snow. In wet weather, a narrower tire will do better as it has more pounds per square inch of force pushing down and makes it harder to hydroplane. In dry weather a wider tire generally causes more traction under heavy acceleration. In 'thick' mud, a wider tire is generally better. On ice, with a good ice tire you also want a wider tire as it has more surface area and a bigger contact patch. Most often, it's not worth the loss in snow traction to get a little bit better ice traction, and this doesn't apply to 'general snow tires' Only specialty ice tires. So it's best to find a good all around balance in size.

All of that being said. Too heavy of a tire can/will cook your brakes during a hard stop from highway speeds. Stock braking systems are rarely designed to handle an excessive increase in tire weight, which can cause your brakes to absorb more heat than they are designed for, causing brake failure. It can also cause an excessive strain on your transmission and engine if your vehicle does not have the power to handle it. Putting significantly bigger tires on something with a weak transmission is never advised.

Basically, it's not 'unsafe' as long as you realize your braking and handling could be adversely effected and you stay within reason. It's there so the guy running 37" Super Swamper's on stock tiny brakes can't come back and complain when his brakes are a smoking pile from repeated hard stops from highway speeds, or wonder why his transmission has no first gear anything, trying to push 37's up a hill from a stop every day.

Sorry for the incredibly long winded response haha.. I've dealt with this question a billion times, and I've spoke to some of our suppliers about it in detail.. So I figured I'd share what they have to say.

[carnuck]

It isn't a weight factor doing in the brakes. It's the lever principle. Bigger tire diameter = smaller brake effectiveness to stop the big tires.

[Sunny]

It isn't a weight factor doing in the brakes. It's the lever principle. Bigger tire diameter = smaller brake effectiveness to stop the big tires.

I was sent to a seminar for Bridgestone and they touched on this subject, because their Potenza line was getting a ton of people complaining about increased braking distance. They had the results from a test using 3 identical cars, same size tires, and three different styles of rims. Light weight race rims, standard rims, and heavier aftermarket rims of the same size. There was a pretty noticeable difference in heat and braking distance in each increment. Combine that with the fact that Potenza's are one of the heavier tires in their class is probably why they were getting so many complaints. Their point was people were upgrading to aftermarket rims and Potenza's, and complaining it didn't stop as well, even with the same diameter tires. Their focus wasn't rotor heat, but it was included in the test. In every increment the weight went up of the tire/rim package, the measured temperature of the rotor went up. I'll see if I still have the 'book' they gave us. It was included in there, if I can find it.. I'll scan it and post it up.

I always just assumed that it was a diameter issue as well, it's simple physics larger diameter takes more to stop from a smaller brake, but according to their study It also very much has to do with weight. Which, also makes sense if you think about it.

You are right, but I was trying to cover all the bases, and I remembered that seminar I had to attend and that was their explanation for it. I also wrote it after I had just woken up [still in bed haha] from being out until 5AM  ,

I also think the complaints were just because Potenza's are horrible tires.. hah.

[carnuck]

LOL! The tire size increase however does the same thing to the brakes as it does to the axles. Taller tires = higher gear ratio (the bad way. It takes away engine power and bottom end) It does the same things to brakes. Longer to take off, and longer to stop.

[Sunny]

LOL! The tire size increase however does the same thing to the brakes as it does to the axles. Taller tires = higher gear ratio (the bad way. It takes away engine power and bottom end) It does the same things to brakes. Longer to take off, and longer to stop.

Yep. Totally true. That's another complaint we often get. "Well the car has less power now" with significantly bigger tires. Even though it was explained about differential ratios vs tire diameter prior to the install.
You can never please people.  I'm sure you've had your fair share too hah.

Tire size affects a lot more than people realize.
There is so many variables to consider.

[kalve]

I got 215/70/R15 an no rub so far looking to go with better set of tires this year.