Considerations for downsizing wheel diameter

Question

I have a 2011 Taurus SHO, which is All-Wheel Drive with 19 inch rims. While I admit that they look nice, that is the only advantage I see with having large wheels. I understand that they can have better handling in general than smaller wheels, but I'm not exactly racing it around the track, so this isn't a factor for me. My question is aimed at my specific vehicle, but I believe any answers are well suited for most applications.

I've been tossing around the idea of getting smaller wheels for several reasons:

• Cost: Tires with a smaller rim diameter tend to cost less.
• Comfort: Higher sidewalls mean the tire can flex more over uneven roads, reducing noise and harsh bumps.
• Fuel Economy: Lighter wheels provide better fuel economy because the car does not have to work so hard to reach and maintain speed.
• Durability: With a taller sidewall, there is more room to flex when hitting road hazards such as potholes, resulting in less chance of wheel damage. Additionally, I read somewhere (will update with link, if I find it) that larger sidewalls make the tire more durable because of that same flexibility. I believe this can be partially verified in the longer warranty periods I'm seeing on smaller tire diameters.
• Performance: While I'm satisfied with my car's performance at this point, better acceleration is a nice additional side effect.

Most of the above is demonstrated in this article that shows what happened to a VW Golf when different wheel sizes are used.

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My goal is to reduce the wheel size to 17 inches. While this is an aggressive goal, Post #4 on this forum summarizes a successful attempt to place 17" snow tires on a Taurus SHO, so it appears reasonable. Since my car is AWD, I will ensure that all wheels/tires are the same.

My car currently uses 255/45R19 tires, which results in a total diameter of 28.04". I assume this will be the target diameter for minimized effects.

Calculation:

• 255mm converted to inches is 10.0394"
• 45% sidewall height is 4.5177"
• 19" rim diameter, so adding twice the sidewall height gives the total diameter in inches.

The poster in the forum above indicated they are using 235/60R17 tires, which amounts to a diameter of 28.10". This is about a 0.06" or 0.2% difference from OE.

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My question is focused on covering all the implications of a reduced wheel size, especially one as drastic as a 2 inch difference. My research has turned up the following:

• Speedometer: Unless the total diameter of the tire does not change much, the speedometer could have a significant error. I believe my goal of using the tire size above indicates this isn't a significant issue.
• Fit Around Equipment: The new wheel must fit around all of the components, such as brake calipers.
• Handling: The article referenced above shows that the car's skidpad g-force was lower with smaller wheels. Again, I don't plan on racing anyone, but it could make a difference in an emergency maneuver.
• Width: This question indicates that changes in width can cause stability issues, but I believe this only applies to that specific case where the OP was asking about replacing one wheel.
• Suspension Damage: This article indicates (towards the bottom of the page) that a significant wheel diameter difference can result in suspension damage. I'm not sure how this could be, if the car is riding at the same height with a new wheel. It's possible that this is referring to a 2" increase, where the tire can't absorb as much shock as before, so the suspension must absorb more of the energy.
• Tire Pressure Monitoring System (TPMS): Besides ensuring that the new wheel is compatible with the TPMS, it's likely that the pressure requirements for a different tire might trigger the TPMS unnecessarily. I wonder if this can be calibrated or if, perhaps, it could be disabled altogether (I check my tire pressure regularly).
• Total Tire Diameter: Using the calculation above, you can avoid some issues by ensuring that the new wheel is compatible with a tire that brings the new total diameter close to the old diameter.

What other considerations for a wheel reduction are required? In my specific scenario, assuming I've taken the above into account, would it be unwise to continue with a reduction from 19" to 17"?

Answer 1

When considering tire size, don't use mathematics to figure it out. You'll figure out it's not going to equate between tires types and companies. The sizes you see on the side of the tire are more general than exact (except the wheel size, which needs to be correct).

Also, ensure you are using the tread width to discover the sidewall height ... but still, refer back to my first point.

Those things said, when looking between two tires, the better figure to use to discover how different a tire is from another, especially when changing rim size, is to look at the rotations per mile (RPM). The company cannot fudge or round the count here, so it is a more accurate number which relates to the rolling diameter of the tire itself. If you have a tire which has an RPM of 932 and you are looking at a tire which has an RPM of 930, you know you are right in the ballpark of where you need to be. The two numbers don't have to be exact, but the closer you get, the less discrepancy in size there will be. This will get you a lot more accurate results than trying to figure things out mathematically.

Speedometer:

If you get a tire which has pretty much the same RPM as the tire you currently have, the speedometer will not be affected.

Fit Around Equipment:

The wheel needs to fit around the brakes and suspension parts. Don't neglect the suspension in this area, because if the suspension is change different than the stock Taurus, you may run into fitment issues. When looking at the wheel, you need to look at offset as well as the diameter. If the offset is wrong, it can come too close to the brakes or to the suspension, which is of major concern here.

Handling:

The diameter of the wheel does affect the handling (as you pointed out). The fact there will be more give in the sidewall can account for most of this, not the rim itself. You need to match the rim width to the tire you plan on putting onto the rim. If the tire is too wide for the rim, this can cause issues in handling. While you probably won't find any real issues with using too wide of a rim for a given tire, you'll run into two issues: wear patterns; just plain butt ugly (sorry, the second is strictly my opinion :o).

Width:

The biggest issue with reducing width (and I'm assuming you are talking about the width of the tire, not the rim, necessarily) is the skinnier the tread width the less contact patch there is with the ground. This can affect your handling to an extent, but can also affect how well the tire performs while stopping and on the skidpad. I know you said the skidpad is of less concern to you, so that is really not too big of an issue. I don't have a magic formula for how much of a difference the width would make in either case, but do know there is a difference.

Suspension Damage:

I'm not really sure how suspension damage might occur. As long as the wheels/tires are not coming into direct contact to the suspension, I'm not seeing as how damage might occur. There may be something, but I'm not seeing it.

TPMS:

You've about got it covered. I'm sure there must be ways to calibrate the TPMS. I'm also pretty sure the tires should be running about the same pressure from one size to the other. You'll have to check this to ensure there's no issues.

Total Tire Diameter:

As stated ... don't use the math calculations. You need to know the exact physical size and using the math won't cut it in most cases. Use the sizes already suggested to ensure the best fit. As stated on the web page you provided, using size checkers like you'd find on TireRack.com will help you immensely in both getting the right fit as well as getting an idea of how a new wheel will look on your Taurus.

Realistically I think you have most of it covered. A few minor things to consider:

• Vehicle Resale: While you may not want the 19" wheels, if you ever sell your car, you may want to retain the original wheels (store them) just to ensure you get full resale value out of your vehicle.

(I thought I had more ... guess that's it.)

In my opinion, I'm not seeing a huge downside to downsizing rim sizes other than the performance aspect. If you can live with the reduction in performance, then I think you'll come out ahead. I don't think you are going to take a huge performance hit, but there will be some. Even with putting new rims on the car (as long as you don't get stupid expensive ones), I would bet the difference in tire cost will pay for the rims within two tire replacements. You are right, the cost of tires goes up exponentially with the size of the rim. You'd think with less rubber would come less cost, but not a chance. Realistically with what you ask, this is something you'll have to figure out if it's something you want to do or not.