Understanding Fuel Economy

Overall vehicle fuel economy for a given car is determined by a ratio designed into the vehicle called the N/V ratio, where N = engine speed, and V = vehicle speed. To get the best fuel economy, you want the N/V ratio to be as close to 1 as possible. For the layperson, this is achieved by getting to the highest transmission gear (say 5th or 6th) as quickly as possible. The N / V ratio is set by the transmission gear ratio as well as the axle or differential ratio.

So, for instance the N/V ratio cruising down the highway in overdrive is typically 2000 RPM / 65 MPH for a N/V of 30.77. Now, take for example driving around town in heavy traffic in first gear, for a N / V ratio of 2000 RPM / 10 MPH = 200.0

As yet another example, if your vehicle has a "Instantaneous Fuel Economy" readout in the information center, get going 55 MPH and note what the readout is showing. Now, carefully shift the transmission into neutral and note again what it says. More than likely it will be "pegged" or much greater than the readout can display. What this essentially showing you is
a very low N/V ratio: 600 RPM (engine idling in N) / 55 MPH = 10.91 (BTW this is also a technique for saving fuel called "Hypermiling").

So in supporting what Dale has mentioned below, lower speed limits actually WASTE fuel, as your engine cannot operate in the highest gear, without "lugging" the engine, forcing the automatic transmission into a lower gear, or you to shift your manual transmission into 3rd or 4th gear, rather than the preferred fifth gear.

This is also quite clear when you purchased the vehicle, and looked at the window sticker with the EPA mileage - city mileage is ALWAYS lower than HIGHWAY mileage for this exact engineering reason.

So, the next time you're cruising down the road, check your N/V ratio and try to get it as low as possible!