Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - OBD-II vs FI+VSS

peterj · 05-16-2008, 09:42 PM

I have just recently begun investigating the specifics of hypermiling and the use of gauges to help develop good driving habits. I would like to summarize what I have learned (or at least what I think I have learned) in the hopes that others can comment and/or correct me where I have mis-learned.

It seems as if reading the fuel injector (FI) cycle and vehicle speed (VSS) is the only way to accurately determine instantaneous fuel usage. This method will work on just about all gasoline-based fuel injected vehicles, is not overly complicated, expensive, or invasive, but *does* require some gear-head knowledge for installation. There are some inexpensive homebrew solutions, but all commercial equipment is rather expensive.

Reading information from the OBD-II port is limited to post-1996 cars and does not (generally) provide specific information for fuel consumption. The amount of information available through OBD-II varies greatly, and the only information available on all vehicles relating to fuel consumption is mass airflow (MAF). Based on a 14:1 air:fuel ratio, MAF can be used to obtain a rough estimate of fuel consumption. The level of accuracy will vary depending on vehicle type, and will always require some degree of calibration. On the upside, commercial equipment is relatively inexpensive and installation is as simple as locating the OBD port and plugging in a cable.

It seems as if some implementations of OBD provide FI information, so in theory, cars with this data should be able to provide much more accurate information.

The most popular OBD device is the ScanGuage II, but I have been unable to determine which OBD parameters it uses to calculate instantaneous mileage. It seems clear that it must use MAF, but it could bias that based on other values. It is also not clear if it makes use of optional parameters such as FI if they are available. (Any clarification here would be helpful.)

Clearly the FI+VSS method is superior for tracking exact fuel consumption and instantaneous mileage, and will provide the best feedback on driving style. What I am unsure about is how close OBD data will come to the FI+VSS method. Are they close enough such that most drivers in most situations will be able to obtain most of the information they need with OBD in order to maximize mileage?

Many thanks for any addition and/or clarification on these issues.

05-16-2008, 09:42 PM	#1 (permalink)
peterj EcoModding Lurker Join Date: May 2008 Posts: 5 Thanks: 0 Thanked 0 Times in 0 Posts	OBD-II vs FI+VSS - Request for Comment I have just recently begun investigating the specifics of hypermiling and the use of gauges to help develop good driving habits. I would like to summarize what I have learned (or at least what I think I have learned) in the hopes that others can comment and/or correct me where I have mis-learned. It seems as if reading the fuel injector (FI) cycle and vehicle speed (VSS) is the only way to accurately determine instantaneous fuel usage. This method will work on just about all gasoline-based fuel injected vehicles, is not overly complicated, expensive, or invasive, but does require some gear-head knowledge for installation. There are some inexpensive homebrew solutions, but all commercial equipment is rather expensive. Reading information from the OBD-II port is limited to post-1996 cars and does not (generally) provide specific information for fuel consumption. The amount of information available through OBD-II varies greatly, and the only information available on all vehicles relating to fuel consumption is mass airflow (MAF). Based on a 14:1 air:fuel ratio, MAF can be used to obtain a rough estimate of fuel consumption. The level of accuracy will vary depending on vehicle type, and will always require some degree of calibration. On the upside, commercial equipment is relatively inexpensive and installation is as simple as locating the OBD port and plugging in a cable. It seems as if some implementations of OBD provide FI information, so in theory, cars with this data should be able to provide much more accurate information. The most popular OBD device is the ScanGuage II, but I have been unable to determine which OBD parameters it uses to calculate instantaneous mileage. It seems clear that it must use MAF, but it could bias that based on other values. It is also not clear if it makes use of optional parameters such as FI if they are available. (Any clarification here would be helpful.) Clearly the FI+VSS method is superior for tracking exact fuel consumption and instantaneous mileage, and will provide the best feedback on driving style. What I am unsure about is how close OBD data will come to the FI+VSS method. Are they close enough such that most drivers in most situations will be able to obtain most of the information they need with OBD in order to maximize mileage? Many thanks for any addition and/or clarification on these issues.