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.