78 tires vs 70 series tires - Fuelly Forums

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Old 04-04-2008, 05:00 AM   #1
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78 tires vs 70 series tires

Has anyone done a comparison on tread width for mileage? Given both are filled to max sidewall pressure. Seems narrow tire would gain better FE but would like some real world feed back before taking the plunge.
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Old 04-04-2008, 08:27 AM   #2
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It's a definite that the narrower tire is better for FE. Not only do they generally have a lower rolling resistance, they definitely have a smaller silhouette for aerodynamics.
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Old 04-04-2008, 09:08 AM   #3
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There would be a balance between supporting the weight of the vehicle and passengers and rolling resistance... go for a tire that's enough under spec and it's gonna roll like a tank track and be substantially unsafe.
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Old 04-04-2008, 10:50 AM   #4
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I haven't seen a 78-series tire for decades...

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Old 05-23-2008, 02:00 PM   #5
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Sounds like I should go for the narrowest tire the car manufacturer recommended for better FE. Narrower tire would have a little worse in handling but as long as it's on the list, it's OK?
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Old 05-23-2008, 02:08 PM   #6
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I recently switched my front tires from 195/70R14 to 185/75R14.It seems to turn easier than before. They are only slightly more narrow.
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Old 05-23-2008, 04:25 PM   #7
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Quote:
Originally Posted by Biffmeistro View Post
It's a definite that the narrower tire is better for FE. Not only do they generally have a lower rolling resistance, they definitely have a smaller silhouette for aerodynamics.
Not entirely true. The aerodynamic part is true, and a narrower tire will be lighter weight. Rolling resistance, OTOH, works opposite.

Given that everything else is equal (construction, materials, tread pattern, air pressure), the wider tire will have LESS rolling resistance. Rolling resistance comes mainly from sidewall deformation -- look at where the tire meets the road, it's partially flattened instead of perfectly round, and the sidewall bulges to make that happen. Comparing most car tires, tread squirm should not make much difference in RR, though knobby off-road truck tires might have more.

Now, on to why wider tires have less RR. The contact patch size is determined by the weight on the tire and the pressure in it. At 50psi (pounds per square inch) with a 500 pound load, that's a 10 square inch contact patch. If that tire is 5 inches wide, it will have a 2 inch long contact patch -- so only 2 inches of sidewall must deform. If the tire is 2 inches wide, it will have a 5 inch long contact patch -- so 5 inches of sidewall must deform. The narrower tire in that extreme example will have far worse RR.

The other advantage for wider tires is better cornering, so you can carry more of your momentum through the corner.

In Hateful's case, talking about 195/70R14 vs 185/75R14 there's just not much difference all around. The 195 is 10mm wider than the 185, that's slightly less than 1/2 inch difference.

According to http://www.miata.net/garage/tirecalc.html the 185 is .6 inches taller resulting in a difference of 6 revolutions per mile and a speedometer error of .4mph at 60mph. Again, a tiny tiny difference. It's all pretty much a wash, and if it were my car I'd go with the wider tire.
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Old 05-23-2008, 04:51 PM   #8
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The .6" taller is better to fill in the wheel well space; have tape around the front well edges. The small bit of narrowness would help to reduce contact with the back wheel skirts.I'm planning to switch those soon. I wasn't expecting much resistance reduction.
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