Q The owner's manual for my new vehicle recommends gasoline with 87 octane or greater. On a recent trip in Montana, I noticed their regular is 85.5 octane and their next level up is 88 octane with 10 percent ethanol. I used a half-tank of 85.5 and noticed no difference, but after reading the owners manual about 87 or better I used the 88 octane for as long as I was in Montana. Why does Montana rate theirs at 85.5, and would it have made a difference in the long run if I used that rating?

A As I said in my response to a previous question [March 3] about octane, a key measure of octane is the fuel's resistance to pre-ignition or detonation. In other words, higher-octane fuels require more heat and pressure to ignite. At higher elevations, air is less dense, meaning fewer air molecules per cubic foot of air that is drawn into an engine. Less dense air effectively lowers the compression ratio of the engine, meaning the cylinder will develop less heat and pressure during the compression cycle. That means a lower-octane fuel can be used without pre-ignition and/or detonation.

The Montana Weights and Measures Bureau has this to say: "While the minimum antiknock rating in Montana is 85.5 for regular unleaded fuel, that is not necessarily what is found. Due primarily to competition and/or multiple suppliers of refined products, regular unleaded gasoline found in western and northeastern Montana usually has an octane rating of around 87.0. Central and southern Montana's fuel octane rating is generally found to be between 85.5 and 86.0."

Since your vehicle apparently operated well on the lower-octane fuel at those altitudes -- no worries. Today's computer-controlled engine management systems can and will adjust fuel-air mixture and ignition timing to safely operate on a variety of octane ratings.

And remember this axiom: Operate your vehicle on the lowest-octane fuel that provides good performance, driveability and fuel economy. Any additional octane is unnecessary and a waste of money.

Q I have a 2009 Ford Focus with 50,000 miles on it. Yesterday, the interior dome light came on while I was driving, and now it does not turn off. It does not turn off with the headlights or with the manual on/off switch, and it does not dim after a few seconds upon leaving the car. It finally turns off after about 10 minutes of leaving the car parked. What could be wrong?

A The body control module (BCM) has extensive self-diagnostic capability that a scan tool can read as fault codes. Check for codes from the door-ajar switches or the smart junction box. Right now, the interior lights are finally being turned off by the battery-saver function.

Q I have a 1998 Honda Civic with 47,600 miles. When I straighten the steering wheel after making a left-hand turn I occasionally hear a "ping." Is this a cable in the steering column? What is the problem, and how much would it cost to have repaired? I'm now unemployed, so I've put off having this checked out.

A The most likely cause of a "ping" or "clunk" in the steering/suspension is a worn component such as a strut bushing, control arm bushing, ball joint, steering rod end or upper strut bearing. A loose steering rack or a worn U-joint in the steering column might cause noise when straightening the steering wheel. There are no structural cables in the steering system.

There's no way to know what's causing the noise or the potential cost of repair without having the steering and suspension checked by a professional. I'd suggest making an appointment at a reputable tire shop or a dealer for an inspection and estimate. This could be a safety problem, so don't put this off.