Question:

Going slightly below OEM specs

by Guest2701 | earlier

In light of energy conservation (fuel efficiency), I have considered replacing my OEM size tires to ones that are slightly narrower (tread width & sectional width) to reduce the contact area - hence rolling resistance. I know the changes in size alters the speed/odometer readings, yet, I am okay with that - within a certain range of deviation. If the load index is less, do I need to be concerned for safety if I never load the vehicle up to max? In particular, my OEM size is P205/65 R15 with a load index of 92H. I would like to downsize to P195/60 R15 with load index of 89. Rims are 6" wide.

Tags: below, OEM, slightly, specs

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Tom Reeds

First a lesson in Rolling Resistance. RR only accounts for 17% of the fuel consumed during vehicle operation. This figure will vary, of course, depending on what the driving conditions are. High speeds consume more fuel, stop and go conditions reduce the tire contribution, etc. RR is caused by 3 things - the amount of rubber in the tire (especially tread rubber), the type of rubber compound used (especially the tread rubber compound), and the amount of deflection (load vs inflation pressure). So your idea of using a smaller tire to generate a smaller contact area is mostly correct.

If you go down in size, you raise the deflection, unless you compensate for it by using a higher pressure. The net effect is almost a toss up. But there are some other considerations. Going down in tire size also is going down in load carrying capacity, which increases the risk of a load related tire failure, which sometimes have tragic results. Going smaller in diameter effectively reduces the drive gear ratio - meaning the engine will be turning faster for a given speed - and therefore consuming more fuel.

Plus, this should be all about saving money, and tires with good RR also tend to wear poorly. So I think you'd be further ahead to select a tire with good wear qualities in the stock size, and inflate it 3 to 5 psi above the placard specification and then do a regular check of the pressure. BTW, you will find that a new tire has more RR than a worn out tire due to the increased mass of the tread. So you should expect some loss in fuel economy when you buy new tires.

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