I have read somewhere here that the speedometer under reports the actual speed by 2+% and that to compensate for this you need to add 2-3% to your calculated mileage to compensate. I just don't believe that so I ran a little test with the best tool I have. I used my GPS to track the total miles driven through and entire tank of gas and compared it to what I got on the trip computer of my CX5. I know the GPS may not be that accurate but still that would just mean that the likelihood of them being close would just be less. So here it is.
Filled the tank with 12.35 gallons
CX5 trip computer was 414.9 miles = 33.595 mpg
Garmin Nuvi GPS was 417.1 miles = 33.773 mpg
That is just a .5% difference, which I find pretty amazing.
Yeah, I know it could have been coincidence that they ended up so close but its better than just assuming that just because the speedometer may be off 2% that the trip computer is off that much too. In fact I don't know how anyone can determine that the speedometer is off anything because its a big fat needle dial gauge with printed numbers in the background. How do you get an accurate reading with that?
My average speed was 54 mph
The computer was calculating that I was averaging 35.4 mpg (its always this optimistic).
I have a 2013 sport with the stock 17" wheels.
I also for the hell of it filled that tank with 93 octane vs the usual 87 I use all the time. Right now my mileage is pretty consistent from tank to tank driving the same 69 miles one way each day. I always set the cruise to 69 mph and rarely encounter traffic. The commute is 95% highway. My last tank I got 33.8 MPG using 87 octane and I didn't feel any difference in power nor did the engine sound any different. I also filled up at the same gas station using the same pump.
Filled the tank with 12.35 gallons
CX5 trip computer was 414.9 miles = 33.595 mpg
Garmin Nuvi GPS was 417.1 miles = 33.773 mpg
That is just a .5% difference, which I find pretty amazing.
Yeah, I know it could have been coincidence that they ended up so close but its better than just assuming that just because the speedometer may be off 2% that the trip computer is off that much too. In fact I don't know how anyone can determine that the speedometer is off anything because its a big fat needle dial gauge with printed numbers in the background. How do you get an accurate reading with that?
My average speed was 54 mph
The computer was calculating that I was averaging 35.4 mpg (its always this optimistic).
I have a 2013 sport with the stock 17" wheels.
I also for the hell of it filled that tank with 93 octane vs the usual 87 I use all the time. Right now my mileage is pretty consistent from tank to tank driving the same 69 miles one way each day. I always set the cruise to 69 mph and rarely encounter traffic. The commute is 95% highway. My last tank I got 33.8 MPG using 87 octane and I didn't feel any difference in power nor did the engine sound any different. I also filled up at the same gas station using the same pump.
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