Watch your gas mileage

The first Plymouth Voyager I purchased in 1991 had a nifty car computer that gave you a running tally of fuel economy, instantaneous and averaged over the current tankful, and an estimate of miles left to go before you had to fill up. By the time I was ready to replace that Voyager, Chrysler Corp. was setting you up so that if you wanted the nice options you had to either go to Dodge or Chrysler. I liked my dealer so Dodge was out, and the Chryslers were too expensive so I reluctantly had to give up my car computer.

Since 1996, just about all cars sold in the U.S. were required to have an On Board Diagnostics II computer, which would essentially determine if you were going to pass your vehicle emissions inspection. This computer keeps track of such things as your rate of fuel use (hmmmm), speed (hmmmm), coolant temperature, ignition timing and anything else pertaining to the emissions control of the vehicle. Numerous third parties have sprung up offering interfaces to the OBDII computer that a consumer can use, that can either save data or show the information on a computer (PC, mobile PC or Palm) screen or a custom display.

I finally got myself a Scangauge II which gave me back all the things I had with that Voyager car computer (except the compass) and much more. It's like three devices in one. The first screen lets you determine if there are any emissions control issues that will cause you to fail your inspection, so you can get them attended to before you actually take the test. The second screen allows you to select 4 values for continuous display, including battery voltage, current MPG, coolant temperature, manifold pressure, speed, a calculation of engine load and throttle setting. The third screen is your trip computer, which keeps track of cumulative fuel mileage for your current trip or your current tankful, how far you have left on your current tankful, the maximum coolant temperature during that time and distance traveled.

The first thing I did was attach it to my wife's PT Cruiser, which I always suspected of having lousy fuel economy but could never prove it because I couldn't drive it far enough to get a reasonable result. My commute consists of the first half on secondary roads with slightly rolling hills then the rest on interstates like I95/128 (55mph limit) and I93 (65mph limit). I held to the speed limit with cruise control and otherwise tried to drive as lightly as I could (no problem at midnight there's no other traffic to upset by my driving that way) and managed a 30mpg total for the 95 mile round trip. Not as bad as I suspected. I feel part of the problem is because there's only a 12 gallon tank there's an illusion of bad economy as fillups are frequent.

The past few days I put the Scangauge in my 4 cyl manual transmission Honda Accord. I know that if I drive fairly conservatively I can get 1 week out of one tank (about 500 miles). I've always averaged around 31-33 mpg with the car on this commute so I wanted to see how the gauge corresponded to that. Sure enough, the first day I averaged about 33mpg keeping to the speed limit and using cruise control. Good. Yesterday I filled the tank just before I got to work and set the trip computer, then really went to town on the way home to stretch as much as I could - coasting to traffic lights and slowing on the hills, again there was no other traffic to contend with after midnight. I managed 35mpg for the commute home. I'm going to finish this tank trying to conserve as much as possible, then do the next one driving to keep more with traffic and I'll see how much difference it makes.

One thing I noticed that it appears that the engine load is reported differently in the two cars. The Scangauge manual does state that the number may reflect either the percentage of total power available or the amount available at that particular engine speed. It looks like the PT Cruiser reports percentage of total power (around 15 - 25%) while the Honda reports power at RPM since I got readings regularly in the 70% range.