Richie Roads

I typically fill up once a week, reset the trip odometer, and the next time I fill up I check the TSX's calculated MPGs for the trip, and I compare them with my calculated MPGs (gallons needed to re-fill up/trip odometer reading). The TSX's calculated MPGs are consistently 1-2 mpg higher than what I calculate. Today, my calculations showed 24.93 mpg, and the TSX calculations showed 26.90 mpg. What's gives?

WheelMcCoy

2012 TSX Wagon here, and I've been observing this phenomenon since the car was new. It's always 1-2 mpg more optimistic than my own measurements, even though I use the same gas pump repeatedly. I can only speculate as to what's behind the discrepancy. I think the trip computer dynamically calculates the burn rate for X miles and gives you the average mpg. It's accurate, but is only a snap-shot of the last X miles.

When measured manually, it is per tank, and pump variations, along with weather, introduces errors. I keep expecting the mpg to average out over time though (been measuring every fill up since the car was new), and that eventually they would come closer. But the difference of 1-2 mpg seems to be constant.

fbt

I think that's fairly typical. At least on my 2014 Sportwagon and my previous 2007 Audi A3

opboarding

I always figured it was the pumps giving us the max allowed rounding error legally allowed. 1.0 gallons listed on the pump is probably only around 0.97 gallons in real life.

TexasHonda

My understanding is the instantaneous MPG figure is calculated directly from calculated mass air flow from determined fuel//air ratio. Avg MPG is simply a time integrated value based on instantaneous MPG. A fudge factor is probably used to insure an "optimistic" MPG value. I've always found about 5% overestimate.

See following link for details:MAP- and MAF-Based Air/Fuel Flow Calculator

good luck

WheelMcCoy

But if I always used the same pump that measured to the 0.97 gallon, I should still be able to calculate an accurate average mpg.

Richie Roads

This past week I made a conscious effort to go easy on the gas pedal, especially on surface streets, and I calculated 27.1 mpg, vs. the on-board computer's 28.0 mpg for the week.