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Dear Tom and Ray

Recently, I bought a new Camry. I'd like to know what the "overdrive" is for, and when to use it. Can I only use it at certain speeds or can I just leave it on all the time? - HenryRAY: The short answer, Henry, is that you can just leave it on all the time.

TOM: Overdrive is simply the fourth gear on your automatic transmission. And since the transmission is automatic, it will "automatically" know when to go into, and out of, overdrive. So you don't even have to think about it.

RAY: There are only a couple of times when you'd want to turn off overdrive. One is when you're descending a long, steep hill, and you'd want to put the transmission is second or third to keep your speed under control.

TOM: The other time is when the transmission is hunting. That's not when the transmission gets dressed up in an orange hat and vest and goes out to bag a couple of rhino filets. "Hunting" is when the transmission is searching for the proper gear to be in.

RAY: Under certain conditions, like on some rolling hills, or in city traffic between 30 and 40 miles per hour, the transmission can be right on the edge between third gear and overdrive. And as a result, it keeps shifting back and forth between them. That's hunting.

TOM: While this is not great for the transmission, it's more of just an annoyance. So if you notice that your transmission is hunting a lot, and it bothers you, you may want to temporarily turn off the overdrive.

RAY: Just make sure you remember to turn it back on, so you don't drive all the way to Indonesia in third gear.

Dear Tom and Ray: I have a 1988 Lincoln Town Car. When I have about five to seven gallons of gas left in the tank, my gauge goes past "Full." When I have about three gallons left, it returns to a normal reading. One shop said it was the gauge. Another said it was the float in the tank. Which one do you think is right? - Harry

TOM: I'd go with the guys who said it's the float in the tank, Harry.

RAY: Me, too. The float is what sends the signal to the gauge. When the float is at the top (i.e., the tank is full) the circuit is fully "open." As the gasoline gets used up, the float moves down and the resistance in the circuit is increased. As the resistance increases, the needle on the gauge moves toward the "Empty" mark.

TOM: But in your case, there's a part of the resister (when you have between seven and three gallons of gas) that's bad. And it's not creating any resistance there. That leaves the circuit open, which is why the gas gauge reads "Full."

RAY: If the gauge itself were faulty, it probably wouldn't happen so predictably. The fact that it's tied to a precise amount of fuel in the tank makes the float a heavy favorite in my book.

TOM: So you can either replace the resister or just make the most of your unique situation. If it were my car, I wouldn't miss the opportunity to lend it to my brother every time it had about four gallons of fuel left, and then tell him to "fill it back up" before he returned it to me.