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Tire & Wheel Weight : Vehicle Fuel Economy

2005 Toyota Prius Wheel 15in 14lbs
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At 14lb the OEM 15in wheels on the second generation Toyota Prius was one of the lightest wheels ever produced for a mass market vehicle. The magnesium aluminum alloy that Toyota used in these wheels was traditionally used for high performance racing and sports car wheels. It turns out that lightweight performance parts can also be used to improve a vehicles fuel economy.

The combination of a low rolling resistance tire on a lighter wheel produces greater vehicle efficiency by reducing rolling resistance, sprung weight and rotation inertia (think gyroscope). Typically people experience a 2 to 10% improvement in overall fuel economy when they swap out heavy steel wheels and low efficiency tires with lighter alloy wheels and LRR tires. The fuel economy improvements from LRR tire use is maximized if the tire pressure is maintained close to the maximum cold pressure rating, typically around 40PSI.

Michelin Energy Saver A/S : An LRR Tire
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Manufacturers on the quest for 40mpgs in the real world have focused on using this time tested lighter wheel and LRR tire combo to boost the fuel economy of their more efficient models. On limited range electric vehicles like a Nissan Leaf, a lighter wheel and rim setup can extend single charge driving range, especially if the use case for that vehicle tends to be dominated by stop and go driving in and around urban areas. 

Unfortunately aluminum and magnesium are more expensive than steel. Heavy steel wheels tend to dominate the roads of today. The cost of alloy wheels can be offset by long term fuel savings, something that may add value to the consumer while also adding upfront cost increases to manufacturers. The lighter wheels and tires also give better road handling performance by reducing unsprung mass and rotational inertia, both of which contribute to better overall handling dynamics. The improved performance improves safety while also improving efficiency. It should be noted that LRR tires do not offer the best grip performance for skidpad yield in sports driving, and in certain winter weather conditions there are other tires that handle snowy and wet roads with greater grip.

Beyond Tires and Wheels

The auto industry stands to gain the most significant improvements to fuel economy by focusing on aerodynamic improvements, something that will largely benefit from computer performance improvements in CAE, especially simulated fluid dynamics. Tesla for example put a lot of design effort into making the Model S slip through the air with minimal resistance. By contrast, Nissan only focused a marginal amount of effort of maximizing the Leaf's aerodynamic performance. The Leaf, being a compact car, does not have the volume advantage of the far larger Model S to gain aero performance from smooth elongated turbulence minimizing shapes, the focus of the Leaf's design was primarily on cost performance, to build an affordable family electric car.

To give you an idea of how complex aerodynamic optimization is in an example, consider the conundrum that Boeing currently face in designing aircraft because of limited super computer performance. "Right now we have only about 10tereflops of performance and around 200exabytes of data to optimize the aircraft aerodynamic performance, but we really not more like 10x more computer power then that to simulate all of the flaps and other active surfaces for further reductions in drag and aero optimization." said the head of Boeings computational fluid dynamics program, a middle aged man with a thick french accent that I had the pleasure of speaking with briefly a few weeks ago. 

Aerodynamics are becoming a subject of focus in the long haul trucking operations of today, drag reducing side skirts are all the rage for operators and logistics companies alike. In this sector, these trucks operate nearly non-stop, logging millions of miles per vehicle. Any gains in aerodynamic performance have dramatic effects on maximizing operation cost reductions through fuel conservation realization. Better fuel economy with better aero-performance means better profits for the logistic sector. These improvements translate into better pay for drivers, better job security, and lower shipping costs. These improvement also result in air quality improvements, improving the health and well being of everyone.

In the passenger vehicle sector, a 200,000mi design life is normal for a given model. 20 years or 200,000mi, any improvements to a models aero-performance must be able to return enough fuel savings to justify the added costs of further time consuming aero-optimization in the early design stage of a given model. Lessons learned from one vehicle can be applied to others, and similarly lessons learned for aerospace can be applied to automotive.

Broadly speaking computer aided engineering, cloud computing, super computers, the internet the the whole of IT as a sector, all improving, making the cost of fluid dynamic simulations decrease, making better aero- in design more affordable over time. As computer improve as tools, our ability to design better products also improves. Improving technology is a positive feedback loop.

Tire and wheel manufacturers are also realizing gains for improved materials science, the very things used to make the wheels and tires, from new lighter stronger cheaper metal allows, to lighter stronger tire components for LRR tires.

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