A 55-taxicab fleet test was conducted for 160 000 km to determine the effects of low-viscosity and friction-modified engine oils and rear axle lubricants on fuel economy and durability. In GM City-Suburban and 80 km/h constant-speed tests conducted at the Indianapolis Motor Speedway, fuel economy with low-viscosity engine oil and rear axle lubricant combinations was about 1 to 2 percent greater than that with SAE 10W-40 engine oil and SAE 80W-90 rear axle lubricant. These improvements were obtained after 40 000 and 80 000 km of taxicab service. Fuel economy with a graphite-containing SAE 10W-40 engine oil was about 2.5 percent better in both tests. No differences in either engine or rear axle durability were observed among the lubricants. However, oil economy with low-viscosity engine oils was about 20 to 30 percent less than that with the SAE 10W-40 oil. Adding either graphite or MoS2 to an SAE 5W-30 engine oil did not affect oil economy or oil performance, but oil filter plugging was increased in high-mileage cabs. Oil filter plugging increased with increasing engine oil viscosity, filter change interval, and cab mileage.It is concluded that the use of energy-conserving lubricants improves fuel economy for both low- and high-mileage vehicles, without adversely affecting vehicle durability. The significance of this conclusion is that vehicles on the road can benefit from the use of such lubricants.