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Checked my miles per gallon today after resetting it after my last fill up, and it told me that I'm getting 25 mpg. I've driven only city miles so far, and I've never gotten this good of gas mileage before. Does anyone know how accurate the trip computer is? 25 mpg seems very optimistic considering I haven't driven on the highway at all
Cars generally read slightly more optimistic MPG numbers than in reality. If you relly want to know how accurate it is, fill up your car, drive it until empty. Fill it up again and see how much fuel you put in. Calculate your MPG according to the number of miles you put on. Compare actual vs. displayed MPG.
It's only accurate if you have the original recommend tire/wheel specifications. Proably within a couple percentage bc the mph has to be acurate within 1 to 2 mph average throughout the mph range.
Based on 292 tanks I'm calculating that the MPG in the display there is 1.6% optimistic over the long haul, which means on average it reads about 0.4 - 0.5 MPG high. However I've found the variation to be quite large - over several months it can be +/- 1% on top of that (0.6% to 2.6% high). That's just my car though. I haven't found any rhyme or reason to the variation - seasonality or anything like that. It could be dependent on driving style too - city vs highway, or aggressive vs mild.
And depending on what your city driving is like, 25 MPG is certainly possible. My lifetime average is 28.84 MPG, which includes quite a few highway trips, and in the time I've owned the car I've only had a handful of tanks that are 23-25 MPG. Most are 28-30.
I have a feeling the reason for the discrepancy is the error in calculating the amount of fuel used, since miles are easy to count. When at the gas pump, you know exactly how much gas you put in, but when the car is counting each squirt of the fuel injectors, theres probably a bit of variation here and there
The MPG indicator on my car is about .5 off from the actual reading I get by doing the division, which is the only way to know for sure what the MPG is
I'm not sure if the technology used on my 2016 RDX is different, but after recording 30 fill-ups on a spreadsheet, the average reported by the RDX is 11.2 L/100 km (21.00 MPG) and the average doing the math is 10.8 L/100 km (21.78 MPG). Those numbers are within 3.6% of each other.
I believe the value reported by the RDX is the most accurate. It's somewhat like how inkjet printers measure each picolitre of ink used to determine when the cartridge is empty.
Since I always top up my tank to the nearest dollar, the fuel which is actually used is never measured very accurately. For example on two identical trips I may have obtained the same mileage, but because the price of gas dropped a few cents, I might squeeze in an extra 1-2 litres into the tank to bring the cost to the nearest dollar. That results in a 3% error in the manually calculated value.
Fuel injector pulse width will give you a "good enough" estimation of how much fuel is consumed, but the exact amount is going to be dependent on fuel temperature, density, viscosity, absolute fuel rail pressure, and ambient air pressure. During closed loop operation everything gets trimmed by the oxygen sensors. I'm not sure how many of these factors are taken into account by the ECM when trying to calculate fuel usage. The manufacturer really has no need to accurately totalize the amount of fuel used over time as the oxygen sensor makes sure the mixture is stoichiometric the vast majority of the time, and the only reason to totalize fuel consumption over an extended period would be for display to the driver.
12-29-2015, 02:40 PM
