Automotive Forums  Where Car Enthusiasts Go to Discuss Cars Online

I am in need of new tires on my 2001 Jeep Cherokee, and would like to get something a little larger than what I have.  The wheels are still the factory wheels that came with it, and 225/70/16 is recommended.  Does anyone have experience or an idea of the largest allowable size for that wheel?  

Go check some of the tire sites on the net. Remember a few things though, such as, What are you trying to accomplish? If its ground clearance, you will only get 1 inch more with a 2 inch taller tire ! Some states such as Pa. Does not allow thread of tire to exceed the fender with. Also there will be a limit not only by the space in the wheel well, but allow for suspension movement of the tire also, else you will rub! And you can only put so big a tire on a rim because the width of the rim needs to be in a certain range to fit certain tires or you will get a lot of problems such as poor wear etc. Alot to consider, check with a tire expert. One other thing, you will increase the gear ratio by putting on larger tires which will decrase your pick-up from a start.  

Going to a larger diameter tire will change the axle ratio.  That in turn will alter the speedometer and change the torque curve, which may result in the vehicle loosing power.  

Here is a GM TSB.  It contains some useful infor about tire size and its effect on operating conditions of the vehicle.

If you are going to change size, you must stay close to the same diameter.  Also changing tires can have an adverse effect on handling, braking and ride.  

Some tires stores won't intall tires that are not to correct size for the vehicle.  Reason?  Liability!  They are expected to be tire/wheel experts and if they install it, the courts will find them at fault if there is any legal action.  Who would take legal action? Everyone and anyone.  I was told by a tire store manager that they were sued by a truck owner because they sold him OS tires and installed them even though he came in and wanted them.  Something broke on his truck because of the large tires and wheels and he wanted someone else to pay.  


QUOTE
TSB 01-03-10-010B 2500 and 3500 series with larger than stock tires


The 2500 series trucks have LT245/75R16E tires as standard equipment.
The 3500 series trucks have LT215/85R16D tires as standard equipment.
A truck that has different tires and wheels will have Engine power/performance concerns, Transmission shift concerns, Speedometer registered speed concerns, and Cruise Control concerns.

Before any diagnosis is performed for these types of concerns, inspect the tires/wheels for the proper tire and wheel combinations.

If a truck is found to have other than the above listed tire sizes, no further diagnosis should be undertaken until the original size tires and wheels are reinstalled on the truck.

The Engine Control Unit (ECM) and Transmission Control Module (TCM) use (RPM) and (VSS) inputs as operational parameters for engine torque, transmission shifting, cruise control operation and speedometer indicated vehicle speed.

Calibrations for different tire/wheel combinations are not available.

Therefore changing of tire and/or wheel size should not be undertaken by dealerships or truck owners.

Warranty Information
Reinstalling the proper wheels and tires on the vehicle, prior to performing any type of diagnosis of the truck, would be considered truck owner responsibility and not a warranty repair.

Here is another interesting articles on OS tire....

Changing tires prompts industry action
By ALEX LAW

Increasingly, people are trying to personalize or upgrade the look of their vehicles through the addition of new (and usually larger) wheels and tires.

Driven by popular culture, installing new wheels and tires has become a huge business in North America. In 2003, Americans alone spent $3.2-billion (U.S.) to buy twice as many wheels and tires as they bought in 1993.

Unlike dangly earrings or a flashy chain, however, if the new wheels and tires aren't right for the vehicle, they can harm more than the owner's reputation for being cool. Indeed, if the wheels and tires are the wrong type or installed incorrectly or even just too big, they can affect a vehicle's dynamics, usually in a bad way.

Bad as in a harsher ride, lower fuel economy and compromised steering Really bad as in a greater likelihood of rolling over or being in a crash, potential loss of warranty and insurance coverage and maybe even increased liability in a crash.

These are serious prices to pay for a new set of wheels or "rims" or "dubs."
And that a bad installation or the wrong equipment can be a serious concern.

According to Carl Sheffer, vice-president of the Specialty Equipment Manufacturers Association, the situation is of real concern to many of the auto companies and many of the major tire firms.

Frank Taverna, GM's engineering group manager for vehicle dynamics and control systems, called going to new wheels and tires a "dilemma."

"By changing one element of a vehicle suspension system, many other elements can be impacted, affecting everything from ride and handling, to safety," he said.

To make it clear why this concern involved more than a marketing stratagem, GM put on a simple but compelling demonstration of what happens when you upsize a vehicle's wheels.

GM had arranged for the media to drive three pickup trucks that were identical except for their wheels and tires.

One was the standard set created by the company through its "holistic engineering" approach, one was a set of bigger wheels and tires that GM designed, sells and installs, and the third was a bigger set GM bought and had installed at an aftermarket shop.

The three trucks were driven over the same course to test their steering and handling and their responses to some potholes. The differences were obvious and sometimes remarkable. The stock wheels and tires delivered by far the best ride and handling, balancing comfort and agility quite nicely.

The bigger wheels and tires GM designed delivered the harsher ride that using more metal and less rubber always does, but the truck still felt quite manageable.

The non-GM aftermarket wheels and tires, however, the result was considerably more dramatic and more disturbing. There was a clear sense the truck's dynamic integrity had been compromised. It was much harder to control and a lot less comfortable to handle all situations.

The importance of the need to integrate the wheels and tires into a vehicle's design was made clear by James Cutting, the director of GM tire-wheel systems. "Wheels are the glue that bonds tires to the vehicle," he said. "They are a key component of the suspension system, and precise fit with the vehicle is critical."

For the consumer, there is virtually no way to tell what an aftermarket set of wheels and tires will do to their vehicle's ride and handling, except to say it will almost certainly compromise them. The only question seems to be how much will they compromise them, and what greater risks (safety, insurance, warranty) are involved.
__________________  

thats some good info.  I learn something new everyday.  thanks mike r