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.
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.
- 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.
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"?