Add-On Fuel Economy Computers

When it comes to on-board fuel economy computers that help you achieve the best mpg, automakers aren’t the only ones in this game. There’s a healthy aftermarket for this as well.

In fact, when testing vehicles, we often use an aftermarket gauge to more accurately monitor fuel economy. Our favorite is the ScanGauge II from Linear Logic ( This easy-to-use device simply plugs into a car’s On-Board Diagnostic (OBDII) port on 1996 and newer models and pulls information directly from the vehicle’s computer. The OBDII port is usually located behind the dash on the driver’s side. It’s plug-and-play with just a single connection needed. You can mount the gauge panel anywhere it’s easy to see.

Fuel economy isn’t the only function of the ScanGauge II. It also reads diagnostic codes so you can troubleshoot problems with your vehicle before going in for service. If you have the skills you can also reset the code and fix the problem yourself. The gauge can display up to four functions at once. We generally set ours for instant fuel economy, average fuel economy, throttle position, and mph.

Throttle position is a great measurement that shares how far the throttle is open, with the lower the better for fuel efficiency. There are a total of 15 gauge functions to choose from and you can cycle through them by simply pressing one of the device’s four white buttons. Enter the vehicle engine size, fuel type, and gas tank size and the ScanGauge II becomes a trip computer with valuable information like distance to empty.

Linear Logic has also added a performance monitor function that allows you to test acceleration and power output. A smaller unit called the ScanGauge-e is also offered. It doesn’t have some of the functions of the ScanGauge II, but does include a mpg bar graph and calculates CO2 tailpipe emissions.

PLX Devices ( offers an even smaller solution called the Kiwi MPG that also plugs into the OBDII port. A more advanced design, simply called Kiwi, upgrades to a small color LCD display with graphic information. Taking the concept a step further, PLX Devices also has wireless transmitters that plug into the OBDII port and allow you to use smart phone apps to monitor, graph, and log information pulled from the car’s computer. On iPhone, iPad, and iPod Touch Apple platforms, the connection is Wi-Fi. On Droid devices the wireless connection is through Bluetooth.