Fuel Consumption over 80 MPH

Some of you already know this, but thought this would be good information for those that are not aware. Ever noticed how much more gas you use when you drive fast, say over 80 MPH? Your cars fuel management system uses the oxygen sensor to adjust the AFR for the highest efficiency, this is know as Stoich and indicated as 14.7 on an AFR gauge. During cruising or idle this is the ideal AFR.

You may have heard the term Open Loop (OL) and Close Loop (CL) which refers to the oxygen (O2) sensor. Your engine management (ECU) uses the O2 to maintain 14.7 in CL at RPM's below 4200. Above 4200 RPM or when you accelerate you go OL and the O2 no longer feeds back to the ECU to maintain 14.7 AFR.

What this means is if you cruise over 4200 RPM, which is around 80 MPH your injectors will be open longer making you run excessively rich. Your ECU is still pulsing the injectors, but at a much longer duration.

We recently drove back from Florida in a 92 Miata and our first run of speeds around 85 MPH delivered 21 MPG. That sucked, only about 200 miles to the tank! Next fill up we held the speed to around 80-82 MPH which on this car is about 4200 RPM. Our gas consumption went to 31 MPG, that is about a 33% improvement! Big difference for 5 MPH.

From what we have seen on all the Miata years 90-05 the OL is around 4200 RPM. With different final drives and transmissions 4200 RPM could be faster or slower than 80 MPH. This might be something you want to consider next time you are on a high speed trip somewhere.

Gary

That's why we adjust the AFM in NA SM's, not for mileage but efficiency and power.

Thanks for the info. I wondered where the closed loop/open loop switch happened. Now I know to keep it under 4200 rpm.

It would be pretty easy to verify this with a scan tool hooked up as you went down the road........just in case anybody has a scan tool and is bored!!
PS take a passenger and let them watch the scan tool!!