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Old 11-23-2019, 06:29 PM   #1 (permalink)
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225/50R17 -> 215/50R17?

Vehicle is 2012 Honda Accord sedan. Car comes from the factory with 2 tire sizes, 215/60R16 and 225/50R17 and mine has the 225. It is about time to replace tires so I thought I might as well try 215s out, but there aren't quite the right size out there. My options were 215/50 and 215/55, conveniently being 1.5%(circumf.) shorter and longer than OEM size, respctively.

I am inclined to go with 215/50 because I noticed how the car likes to slip out of the lockup clutch & sometimes downshift especially on an interstate where I am not legally allowed(speed limit) to store enough kinetic energy to help me up the hill in advance

I listed some things I could think of but would like to run them by ecomodders before I pull the trigger.


Pros
- 1lb lighter around the outside of wheel assy (unsprung too!!)
- $30 cheaper per set (give or take)
- 6/32" lowering (approx.)
- 1.5% more torque available at a given vehicle speed to help with climbing
- less rolling resistance (less sidewall, width)
- less air resistance (width, lowering of vehicle)

Cons
- odometer inflation (100,000 -> 101,500)
- less maximum traction available
- 6/32" more wheel arch gap increasing air resistance

Please let me know if I am missing anything.

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Old 11-23-2019, 10:37 PM   #2 (permalink)
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The taller the tire the less rolling resistance (of course everything being the same). There are many factors when it comes to rolling resistance of tires so it is hard to say for sure if that would count.

Typically you would want a taller tire for better fuel economy (less rpm). Is it hilly enough in your area to make smaller tires worth it since their only benefit is on hills when you need to speed up? The rest of the time you will be at higher rpms for the trip.
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At 55 mph, a 10% drag reduction translates to a 5% increase in fuel economy. At 70mph,a 10% drag reduction translates into a 4% increase -Phil Knox (Aerohead), Aerodynamics Seminar #2

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Old 11-24-2019, 04:50 AM   #3 (permalink)
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Huh, I did not realize that taller tire would have less rolling resistance!

I just tested it out and in 5th @ 46mph with TC locked my car turns 1500rpm.
When TC unlocks because of an incline (cruise control for accuracy) it flares up anywhere between 2200 to 2600.
I was thinking 1.5% more rpm @ 1522 won't be as bad as unlocking the torque converter every other hill I encounter.

I also have an Acura in the family which shares many component with the Honda, but with slap shift so I can select which gear I want to be in, and using OBD port my car was actually getting 10-15% better mpg in 4th with TC locked compared to 5th unlocked, going up same hill, same speed, similar wind (tried back to back). And 4th locked was turning 300-500rpm more than 5th unlocked.
Now I cannot do that in the Honda, I may have to look into wire-rigging the gear selector, but I digress.

I usually let the speed fall about 5-15mph going uphill, to recover on the downhill.
And the car likes to hold it in 5th once I hit 42+mph and down to around 32mph. But half the times I can accelerate it back to over 40 while holding the TC locked but other times it's TC unlock and with some head wind, car shifts down to 4th with TC unlocked and exceeds 3000rpm.

I also do mixed city/highway driving that my actual MPG is 1-2 above EPA mixed rating(27-28 vs 26mpg). I may not be very good at coasting but I do try to use as little gas pedal as possible by looking far ahead and using hills to my advantave, and don't drive over 65mph.

Would you say the 1.5% more torque & rpm making a positive difference in city driving is insignificant?

Sorry I'm just throwing my thoughts out there, too tired after work but I tried my best.
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Old 11-24-2019, 09:07 AM   #4 (permalink)
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First, the differences between tires (meaning make and model) is much larger than the differences due to tire size.

A larger tire has more load carrying capacity and a tire shop is unlikely to mount a smaller tire because of the legal issue.

A larger tire generally has better RR - all other things being equal - but, again, the differences between make and model are much larger.

The air resistance is a smaller tire is negligible.

So the best use of your time is to research tires of the specified size.
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Old 11-24-2019, 05:41 PM   #5 (permalink)
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I may not have many posts on this specific forum but I am not stupid nor am I new to cars as I was comparing same model of tires especially when I was comparing prices. You don't compare apples and oranges and say one fruit is cheaper right?

Honda specifies load rating of 93 for my vehicle. For the models I was looking at, 215 was 95 and 225 was anywhere between 94 nad 98. I checked beforehand but since you mentioned load rating I wanted to make it clear.

I already have a shop I've been going for 10 years because it's the only place that will do alignment according to my wishes instead of what the machine tells them. And they know I don't ask for unreasonable (i.e. 215 on 10" rim) jobs. They have put 215/50R17 winter tires on my car that normally uses 235/45R17. So I think I'm good there.

I will do what I want with my time whether it may seem productive to you or not.

But thanks for confirming that 1cm narrower tire has negligible aerodynamic benefit.
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Old 11-24-2019, 08:53 PM   #6 (permalink)
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Gen 1 Volt OEM Tires are among the lowest rolling resistance 17” tires on the market. AKA Goodyear 215/55r17

To get good wear you will need to bump up pressure
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Old 11-24-2019, 10:49 PM   #7 (permalink)
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I try to keep tire pressure (cold) 80% or above of max sidewall, not sure yet if it is worth the bumpier ride for me to go more.

And for Gen 1 Volt tires, are they Goodyear Assurance Fuel Max?

Under the Assurance family there are;
- All-season
- Comfortred Touring
- Fuel Max

- Maxlife
- Weatherready


but Maxlife and Weatherready has different font on the sidewall. The rest of them look almost identical and I am trying to identify it by looking up photos that show the tread but they are hard to come by so far..

AND if it is the Fuel Max, there is a 22lbs one and 23lbs one, I'm guessing the lighter one.
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Old 11-25-2019, 12:05 AM   #8 (permalink)
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OEM Volt Assurance tires are 19lbs

If you go to tire rack you will find different tread widths, the narrowest tire is the OEM Volt tire

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