Re: TIRE SIZE CALCULATIONS....HELP!!!


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Posted by David on May 01, 1999 at 06:22:16:

In Reply to: TIRE SIZE CALCULATIONS....HELP!!! posted by Will on April 29, 1999 at 23:34:11:

A rule of thumb is to stay within 3% of original diameter when changing tire sizes from the OE. There are several reasons for this. Most effecting the mechanical operation of the vehicle. For starters your speedo reading. On most vehicles the initial information used to calculate the speed is tire revolutions and is based on that of the OE size. The overall diameter of the tire dictates the number of revolutions the tire turns in a given distance. Larger diameter less... smaller diameter more! In newer vehicles other calulations are inturn based off this info - ABS brakes, which have sensors reading revolutions at the wheel. Some vehicles can also have computer calculations the engine uses for fuel to air mix involved in mph and rpm readings, both subject and related to tire revolutions - larger lower rpm's , smaller higher rpm's at the same speed if calculated against the OE.
All this can be addressed - usualy electronicly with reprograming the computer of installing a new chip programed to use the new info.
At the end of the day all this can also be avoided by replacing the OE tires with ones that match as close as possible to the OE. And the 3% I mentioned initialy is an "unspoken" agreed upon tolerance in the idustry.

There is also a cosmetic consideration most poeple factor in. That is the amount of "gap" in the fender well - distance between the top of the tire & bottom of the wheel well opening.
There is also the performance(handling) issue - the shorter the sidewall hieght, dictated by the profile series - 60 or 65 series in your examples, usualy the crisper/better the response out of the tire resulting in quicker response from steering inputs.
All said it depends on what you are trying to accomplish by switching tire sizes. Both options you mentioned should work for you. The 195/60-14 beeing the closest in OD and with a shorter sidewall probable better steering response(depending on the tires performance rating - a completely serparate subject)but increasing the "gap" slightly. The 195/65-14, still within the 3%, decreases the "gap" decreases the rev's = lower rpm's at same speed and conversly actual speed will be faster than speedo indicates.

Again - determining what you are trying to accomplish is the first step. Recomendation in your given example would be to use the 195/60-14. Other factors involved also include the exact vehicle application. So discuss this with a knowledgable tire person in relation to your exact vehicle and desired results from the change.

Hope it helps


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