Prudent service personnel should be cautious when discussing fuel-consumption issues with customers.
Ultimately, the root causes of customers' complaints about their vehicles' gas mileage may be beyond the control of a tire dealer and/or service shop operator.
During my travels, I have witnessed dozens of those gas mileage conversations between concerned motorists and service personnel. Some service writers and technicians have handled the questions very effectively and represented the business very well. However, others bungled badly by making assumptions and taking a customer's pronouncements as facts.
Please consider these factors and observations. If nothing else, I hope they save you from overpromising and under-delivering gas mileage numbers to customers.
First, recognize that the people whining to you about fuel consumption may not have been monitoring gas mileage at all—let alone measuring it accurately. According to Lord Kelvin (he created the centigrade/Celsius temperature scale), measurement is the basis of all knowledge.
Second, experience shows that many motorists don't even attempt to measure a vehicle's fuel consumption until the cost of fuel frightens them. Or, the cost of fuel isn't a concern until their monetary situation changes. For example, the driver in question loses his or her routine overtime income or is scaled back to part-time status at work.
Third, years of observation have convinced me that you should never jump to conclusions. In reality, you don't know what gas mileage a vehicle actually delivers until the driver establishes an accurate baseline. To me, several tanks' worth of fuel-mileage measurements is just the beginning of a valid baseline.
On the other hand, I put much more credence in the numbers when a car owner shows me a detailed gas mileage log that dates back several months. To me, the log is still more credible when I see notations such as air conditioner operating, car pool week, delivered newspapers, etc.
And don't forget tire pressure as an element that can affect gas mileage, as well. Hopefully, the driver also notes where he or she purchases fuel, which can be a factor.
Failing all else, automotive reviews from a variety of magazines may provide you and your customer with believable fuel-economy numbers for a particular vehicle. (Prior to the Internet, many people I know would research fuel consumption by digging through back issues at the local library.)
Fourth, I prefer to see separate baselines for local driving and interstate driving. Recognize that in some instances, the customer's driving patterns are so inconsistent that it's extremely difficult to establish a reliable baseline of consumption. The fact that a vehicle is driven by several different people in a week also may complicate matters.
Fifth, myriad factors you don't see may—I emphasize may—impact fuel consumption. A key detail is how carefully and consistently the driver fills the fuel tank. Preferably, the same driver refuels the vehicle every time. Hopefully, he or she refuels it slowly and stops when the pump clicks off; hopefully, he or she refuels at the same location all or most of the time.
Remember that the amount of alcohol in the fuel may vary a great deal from one filling station to another. (It's likely to vary even from one tank to another at the same station.) To grossly simplify the discussion, adding alcohol to the fuel reduces tailpipe emissions but also impacts fuel mile-age because it reduces the potential energy of the gasoline.
Discussions of fuel mileage are great opportunities to sell needed maintenance and repairs. Well-maintained vehicles always use less fuel than poorly maintained ones do. However, an uninformed motorist's fuel-mileage expectations may far exceed whatever a competent technician can provide at any price.
All too often, the vehicle in question just wasn't purchased with fuel economy in mind. Some motorists are very reluctant to admit that critical fact to you.