Idle Consumption Data
 

This isn't really scientific or anything, just some numbers from a recent drive in a friend's car.

2010 VW Jetta Sportwagen 2.5L 5-cyl

Idle fuel use numbers found by keeping a constant speed down hill in neutral. I let each set stabilize for while and used the brake to keep the speed constant.

200 MPG @ 56.5 MPH
145 MPG @ 40 MPH
72 MPG @ 20 MPH
50 MPG @ 15 MPH

Average is 3.525 Hours per Gallon

Comes out to 0.284 GPH or 0.00473 GPM (Gallons per Minute)

Thought that might be kinda useful, I'm sure other people do this for other cars...

Warmed up and idling at 850rpm my 1.8L gasser (B18B1 from a 90's Integra LS/DA) uses about 0.36GPH, but when the car is rolling my idle roams between 850 and ~1400rpm, so I end up with an average rolling idle consumption somewhere around 0.45, give or take a few hundredths.

Roaming rolling idle seems like it's not an uncommon problem in Hondas, I wonder if it's due to alternator load or something? I have the coolant lines disconnected from my IACV, I thought they were only to prevent seizing in sub-freezing weather, but maybe I'll try reconnecting them.

Some relative modern vehicles seem to increase idle speed when car is moving, especially when brake is applied, something to be aware of when testing.

I did press the brakes to see if the idle went up, because it does on my CRX, but it didn't change the idle speed during these tests. I didn't pay attention to what the idle speed was when it was actually stopped though... I'll try to find out if it's any different when rolling.

Sounds about right, actually. I remember the on-boards on our old Focus TDCi showing about 0.9 to 1.1 liters per hour whenever we were stuck in traffic. (0.9 with the AC off, 1.1 with it on)

Niky,
An interesting post.

The Australian built Ford Territory (SUV) had a 4 litre engine and Ford claim it uses 1.5 to 2.0 litres per hour at idle for the unleaded petrol variant.
The variation depends on options and use.

There is a diesel version too but they are not making any statements about the idle consumption for that one.

Peter.

@IsaacMTSU, there is a professor at MTSU that has an interest in fuel efficiency. Dr. Cliff Ricketts....do you know him? I have not spoken with him in a long time (only over the phone)

Yeah, Dr. Ricketts runs the solar car team and some of the alternative fuels projects! They made a bio diesel Corvette an some other cool stuff. I didn't really help with the building of the cars, but I've talked to him a bunch and I know some people who were on the solar car team.

I used the bar graph instant fuel economy gauge (factory) in my CVT 2002 Insight and my CVT 2008 Altima to calculate idle fuel consumption. Coasting in neutral, with the engine idling, the Altima got 60 MPG (max gauge reading) down to 15 MPH which works out to .25 GPM. The Insight got 150 (max reading) down to 15 MPH which worked out to .11 GPH.

regards
Mech

Idle RPM with an ECM/PCM is not constant like on older engines. When parked yes it will be near constant. Things like airflow and coolant temps (which vary at speed) are built into your tune, time and % from letting off throttle spark/fuel changes, and of course startup vs @ operating temp, all allow for an adaptive idle, auto transmission idling circuits are more involved than this.