Q A car dealership told me to use only the recommended octane fuel for my vehicle. They said that using a higher octane level actually had a negative effect on the performance because the engine could not fire at its optimum level. My '05 Mercury Montego can use regular 87 octane, but when I use a midrange 89 octane I get better performance and better mileage. What is the truth? Is there a break-even point to using higher-octane gasoline if it is beneficial?

A The rule of thumb is to use the lowest-octane gasoline that will operate the engine efficiently. For the vast majority of modern cars and trucks, that means regular unleaded, which is typically 87 octane. The octane rating of gasoline is a measure of the fuel's resistance to ignition, meaning the higher the octane rating, more heat and pressure are required to "start the fire." If the recommended octane gasoline provides proper starting, power, drivability and efficiency, there's no benefit to using higher-octane fuel.

What engines might benefit from higher octane? Engines designed and built for higher performance, featuring higher compression ratios or turbochargers and more aggressive fuel and ignition mapping. These engines will produce less power, mileage and efficiency when operated on lower-octane gasoline because the engine management system reduces the combustion temperatures by richening the fuel-air mixture or reducing ignition timing.

You'll have to be the judge whether 89 octane gasoline provides more performance and mileage to offset its higher cost. It could, but in most cases won't.

In my case, using midrange 89 octane in our turbocharged VW engine provides good power, mileage and drivability -- even though the manufacturer recommends premium. Pay your money and take your choice.

Q My 2002 Mercedes has a garage door opener built into the rearview mirror with three buttons that will open three different garage doors. The first button has failed, and Mercedes says a new one costs around $1,200. Can it be fixed, or can I use one of the other buttons to open my garage door?

A If only the first button has failed, just program one of the other two buttons to open your garage door. If the entire system has failed, use an old-fashioned remote push-button opener and clip it to the visor.

Q I have a 1940 Cadillac LaSalle that needs an electric fuel pump to help with starting and to prevent vapor lock. The mechanical pump must drain back or be too slow to start the car sometimes. Can you tell me how to plumb it so I can turn the electric pump off after starting? Someone said I need to run a line parallel to the old line and install check valves.

A I suspect the higher volatility of today's gasoline might be part of the problem. The simplest answer is to completely bypass the mechanical fuel pump by installing a low-pressure electric pump at the rear of the vehicle near the fuel tank. Adding a fuel-pressure regulator under the hood would allow you to fine-tune fuel pressure for the carburetor on your wonderful old Cadillac.

Motoring note

Do I detect that wonderful new-car smell in the air? The 39th annual Twin Cities Auto Show opens Saturday, March 10, at the Minneapolis Convention Center.

In addition to the 400 new models from 35 carmakers around the world, several new attractions highlight this year's show. The Biodiesel Green Room features the latest in hybrids and flex-fuel vehicles, and some vehicles will be available for test drives outside the Convention Center. And the newest feature: Rock music's Blood, Sweat and Tears will perform live at the Convention Center auditorium on 8 p.m. Saturday, March 17. "Spinning Wheels" at the auto show -- perfect.