How to determine tire compatibility [93 4runner]

Question

I have a 93 Toyota 4runner with 31/10.5-15, if I understand the conversions correctly that would be a 267/76/R15 in metric. I am looking at replacing these tires and have found a set of 235/75/R15's that fit my budget, again if I understand the conversions correctly that makes this a 28.9x9.3-15. A somewhat smaller, thinner tire. Can I use these tires as replacements on my existing rims? or is going from 10.5 down to 9.3 too much of a difference in size?

Update

I used this Tire Calculator to compare my stock tire with the ones I am looking at purchasing and it gave me the following information:

Stock Tire: 31x10.50R15

• Section Width: 10.51 in or 267 mm
• Rim Diameter: 15 in or 381 mm
• Rim Width Range: 7-9 in
• Overall Diameter: 30.97 in or 786.63 mm
• Sidewall Height: 7.98 in or 202.69 mm
• Radius: 15.48 in or 393.19 mm
• Circumference: 97.29 in or 2471.1 mm
• Revs per Mile: 671.6
• Actual Speed: 60 mph or 100 km/h

New Tire: 235/75R15

• Section Width: 9.25 in or 235 mm
• Rim Diameter: 15 in or 381 mm
• Rim Width Range: 6-8.5 in
• Overall Diameter: 28.87 in or 733.29 mm
• Sidewall Height: 6.93 in or 176.02 mm
• Radius: 14.43 in or 366.52 mm
• Circumference: 90.69 in or 2303.5 mm
• Revs per Mile: 720.4
• New Speed: 64.3 mph or 107 km/h
• Speedometer Difference: 7.266% faster
• Diameter Difference: 6.79%

There is a warning on the site that says:

When changing tire sizes, we recommend staying within 3% of the diameter/height of the original tire. Any more than this and you face the risk of brake failure.

So I guess with this information it looks like it would work, however there may be risks. When changing sizes greater than 3% what changes need to be made to compensate?

Answer 1

From the info I have been able to gather the 4runner came with 225/75R/15 or 31-10.50-15 tires. In order to determine if your wheels will work you nedd to determine the rim width. Many OEM wheels will have it stamped on the wheel. Finding it may require removal of the wheel, some stamp it next to the valve stem. You may be able to get a good estimate by eyeballing the rim with a ruler inserted into one of the wheels slotted openings. Once you have determined the width check the tire makers spec sheet they will list recomended rim widths along with minimums and maximums. Each tire maker will have their own standards. My gut feeling is that unless the rims you have are border line to big or too small for your present tires the new ones will fit. In response to your edit. Your may run in to tire clearance issues if you were going larger, since you are going smaller, for your case not a problem. Also if switching to a taller tire the center of gravity will change possibly affecting handling,particularly at high speed or during directional changes. Also not likely to occur in your case. Off road capability will be reduced with the smaller tires as your ground clearance will decrease. The smaller tire also will "float" less in deep soft mud or loose gravel/sandy conditions. As you can see by your chart the speedometer will also be affected. If you contact a transmission shop they should be able to guide you in swapping the speedo cable gear to correct the inaccurancy.