As I was thinking about going to bigger size tires on my Sport Trac it was bugging me not knowing what effects it would have on the rest of the drive train. I knew it would change things but I was not sure of what changes were being made. I searched the internet and came across some good information that I thought could be useful to other people in the same situation. Here are some things that are effected by changing tire size:

Lets say that you change the stock tires (255/70R16) to bigger tires. First you have to convert your metric tire sizes to inches. You can do that by clicking here. Then use the formula below to figure out your New Final Drive Ratio.

This is the formula to find your new final drive ratio:

So, the final verdict is if you change your tire size you are also making changes that you may not have thought of. You may be thinking, "What effect do these changes have"? (Feel free to correct me if I am wrong) Here are my thoughts:

Going from our rear differential ratio of 4.10 to a lower ratio may increase gas mileage but may not be as good for towing trailers or driving up serious hills (which I don't think is much different from towing a heavy trailer on level ground, at least mechanically speaking). Also you may have a loss of power with the bigger tires and lower gearing in the rear diff. Another thing that is effected is the braking; there may be a small loss in braking power. How big of a change in tire size will determine how much the above things will be effected. The bigger the change in size the more dramatic changes you will notice. Small changes in tire size may go somewhat unnoticed.

Going to really big tires can really lower your rear differential ratio into the low 3.xx's. If you went to bigger tires for off-road driving you may be defeating yourself from the onset. Bigger tires are better for off-road driving but a lower rear differential ratio is NOT. You may need to change the gears (Ring & Pinion) in your rear differential to make up the difference and get you back to your original setup.

Here is the formula to do that:

If you have changed tire size on your 4x4 and are now lacking power because of the larger tires, you could correct the problem by making a gear change. It's easy to figure what gear you should be using based on the stock vehicles tires and gears. Here's how you do it:

For instance, lets say that you went from stock 28" tires to 33" tires and the Ring & Pinion (R&P) ratio on your rig is 3.73. To find the new R&P ratio you should be using to keep the stock performance you would do the following:

You should change the Ring and Pinions to a 4.88 gear set.
{Thanks to Mikes 4x4 pages for the ring & pinion info!}

John Brancaccio, Jr.