Thursday, August 17, 2006

How fast are you driving?

After jokingly being called a slow poke by our pastor, I began to wonder if our Saab 900's speedometer was off. The Mentor Police Department helped out by placing a mobile speed indicator on one of the roads I was traveling. I found that I had to drive about 33 mph to make the sign read 25 mph. But would that translate into a straight 8 mph discrepancy at all speeds or was there an incremental ratio involved. In other words, would freeway speeds be even more off than a standard 8 mph? Sharon and I found out this evening.

After taking the kids to Lake Erie with some friends and stopping to chat with Pastor Chris Anderson in Madison (BTW he gave me his two cents), we took I-90 home. This was the perfect opportunity to use the mile markers as a gauge. So, with cruise control set, Sharon timed the amount of seconds it took to travel one mile. For the first mile, I set the cruise at 80 mph. It took us 58 seconds! (It takes 60 seconds to travel a mile at 60 mph.) The second time, I set it at 85 mph. It took us 53 seconds! It seems that we are off a bit, eh? But how much?

It has been a long time since I took high school algebra. So, I called on Bob Shaffer, a retired math teacher from our church and Christian school. He reminded me that distance (D) equals rate (R) multiplied by time (T). With that in mind, the formula would be D = R * T. That was exactly what I needed to know. Here's what we came up with.

1 mile = R * (58 sec/3600 sec) or R = 3600/58 = 62

When the speedometer says 80 mph, we are actually travelling 62 mph.

1 mile = R * (53 sec/3600 sec) or R = 3600/53 = 68

When the speedometer says 85 mph, we are actually travelling 68 mph.
What does all that mean? First of all, it means that my normal driving speed is a bit slower than I expected. I've been driving with the speedo pointed toward 75 mph. That was a prety good guess but not quite right. While this may be due to switching to the P205 60 R 15 tires from the Saab 9000, we had similar problems with the P195 60 R 15 and P185 65 R 15 (standard size) tires we had used at other times.

The second thought could have been discouraging, but instead it made me smile. Since purchasing the car in June 2005, I have been boasting about the great fuel economy. My maintenance software says that I have averaged about 26 mpg over the last 14 months. That includes a low of 19 mpg during the winter and a high of 36 mpg on a long trip. This new data indicates that the speedomoter discrepancy affects the odometer as well. So, depending on the rate of speed (which varies by that elusive incremental ratio) my fuel economy is a bit overrated! Oh well.


Doug said...

I found out this week when Lori and I were on vacation that my speedo is off by 5 mphs. I set my cruise at 71. We passed one of those speedo reminders that police set up, and the reading was 66 mph.

Andy Rupert said...

Do you remember the article in the Dispatch years ago which compared the accuracy of the speedometers on new cars. They compared major brands like GM, Ford, Honda, etc. and found that most were off by several miles at 70 mph. I found that interesting and understand better the leniency of police officers when someone is over the limit by a few.