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the OBD2 dont lie, it shows actual fuel used, period.
just look. I use palmer software. you can plot it to your hearts content.
it can do the math real time or after you get home. any why you want.
on some cars the compression is really high and the throttle plate is closed fully at decel.
so they open the ISC to let in a letter air. (ECU code)
so this too can very , car to car model and year.
do not do real coasting , and worse engine off ,until you are sure the ignition will not go to lock, I know one friend that drove offroad , on freeway doing that stunt. car was totaled. Him ok. IGNITION LOCKS are DANGEROUS. (not in operators manual)
on old cars cut fuel leaks a little gas to keep CATCON lite off.
on newer cars it is heated and cut fuel , cuts way more fuel than idle consumption.
Check it out?
you can test this theory
drive down hill X
log data in decel mode , coasting in gear, and out of gear.
repeat same hill , same speed , neutral with motor off ( find a safe way to do it)
now examine your logs.
end mystery.
Running good OBD2 software, the kind that shows real fuel consumed in all modes is important.
some Cars will not allow you to capture real time injector pulse width.
for those cars I use a scope. (DSO is best)
attached to the injector you can watch the real thing. Nothing better.
federal OBD basics dont alllow that , you need extended diagnostics for that.
MOST Off the shelf device readings:
the MPG and the like are actually , just MAF/vss derived.
eg: (typ. code)
Hear are the steps to make the conversion:
1. Divide the MAF by 14.7 to get grams of fuel per second
2. Divide result by 454 to get Lbs fuel per second
3. Divide result by 6.701 Gals fuel per second
4. Multiply result by 3600 to get gallons per hour
The expression for GPH is: MAF * 0.0805
To calculate MPG divide the MPH by GPH.
The final expression for MPG will be: VSS * 7.718 (speedo scaling, is crude)
those scan guage readings are mostly bunk.
ok , in closed loop, never ok , coasting.
IMHO.
BEFORE assuming, take real measurements ,directly and compare.
trust me...
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