Mileage Mystery

Aug 16, 1997

RAY: I forget the kind of car he owns. But it could be any car.

TOM: '39 Studebacker.

RAY: No, not one of those.

TOM: 1912 Humborg.

RAY: Let's say he owned a 1983 Dodge Aries. Okay? Anyway he had the car tuned up in the fall. He brought it in for the fall tune-up. The car was running pretty well, but it was time for a tune-up.

TOM: It was like October, November, huh?

RAY: Well in New England it was like -- July. The car had been running well and he had no complaints. He asked us to do a major tune-up on the car in preparation for the harsh winter he knew was coming. In fact, we did that and charged him an outrageous price and he left grumbling a lot, but he left nevertheless.

TOM: Our motto: We may be slow, but we're expensive.

RAY: Right. We make our money the old fashioned way. So we performed this tuneup, and this is a car on which he had been accustomed to getting somewhere on the order of 25 miles per gallon. And he called up after 3, 4, 5 weeks, in the fall when the cold weather has arrived, and said, "Gee, my mileage is way down. You must have screwed up the tune-up." He said, "To save you trouble I didn't want to come all the way to your shop because it's too far away. I went to my local mechanic and had him check up on you guys. He went through the entire tune-up all over again and checked everything you did and he said it was perfect and he couldn't figure out..."

TOM: How that could be since you guys did it!

RAY: " ...why I was getting poor mileage. In fact, my mileage has dropped from 25 on the highway down to about 16."

TOM: Oh my god! What I wouldn't do to get 16 mpg.

RAY: Gee I don't know. Anything else you can tell me? He said, "One other thing, I have noticed since it's gotten cold, that my heat doesn't work so well."

TOM: Sounds like a hint to me!

RAY: Ha ha! We know what's wrong with the car. If you give us another couple hundred bucks we'll fix it. And for about twenty bucks the problem was fixed. What was it?

RAY: Here's the answer: What was wrong was that the thermostat was opening up too soon. It was in fact broken, it was never closing. The engine was never reaching operating temperature. Instead of the water getting up to 190 degrees, which it was supposed to, it was probably going up to about 150. You wouldn't notice this in September and October, when you have limited use of the heat, but you will notice this in December and the frost is on the pumpkin. With the operating temperature way down like that you would get poorer mileage, and you would notice it in the cold weather because it will never get up to 180. But in July it would get up there and you wouldn't know anything is wrong.

Get the Car Talk Newsletter