HEM paper (2014) on 2010 Prius Fuel Economy data
 

Stumbled across this recent "PEMS CONFERENCE 2014 - Fuel Economy Data" slide presentation by Rick Walters, P.E., HEM Data Corp:

http://www.cert.ucr.edu/events/pems2...a/24walter.pdf

It discusses how MPG values are derived under SAE J1939, and how/why different results often occur when different input parameters are used, ie: MAF vs. MAP, etc.; and, it highlights *why* the Prius MID fuel economy numbers tend to be 5% optimistic (see pages 16-18).

Apparently Toyota calculates MPG using exhaust gas Lambda (a ratio that defines the deviation from stoichiometric (14.7:1 A/F)), vehicle speed VS (KPH), and manifold air flow MAF (g/s):

MPG ≈ Lambda × 7.107 × VS / MAF

Thus, basically, Lambda, VS, MAF, and constant 7.107 determine MPG fuel economy.

Interesting, but why is it consistently 5% high? :)

Because they lie. If any error was perfectly random, same number high as low then I would believe it is out of their control. I once told a MacDonalds employee she had given me too much change. You could have heard a pin drop in that place.

I got so tired of being overcharged 98 cents at Wendy's I keep a receipt in my wallet, so they can not charge me any more than $4.61 until they raise the price again and even then they have got it wrong just about every way you can imagine, even as high as over $8.

regards
mech

...actually, it is because although the ECM "tries" to keep the engine AFR = 14.7:1 for optimum catalytic convertor operation, there are times (warm-up, heavy load, WOT, etc.) that AFR goes richer, and this royally "screws-up" the J1939 presumption that AFR is always 14.7:1, which the Lambda number should correct, but there has never been a calibrated Lambda sensor that I know of.