I noticed on a recent trip that my GPS unit shows a slower speed than does my speedometer. For instance, when speedo set to cruise at 75, GPS shows 73 mph. Am I getting better or worse fuel economy than what the MFD shows?
If the GPS is correct then the car thinks it has gone further than it really has so its computed miles per gallon is too high (I think )
That would be the obvious answer to me, too. But I'm waiting to see if anything else might negate this.
Speed is not a factor in determining mpg. Distance traveled and fuel used are the only factors involved.
But speed is calculated by distance over time so if the speed is incorrect the distance is probably incorrect also (assuming time measurements are more or less accurate)
You would have to measure your odometer against a known distances travelled to determine that as the speedometer and odometer are separate devices. In the UK (and the EU) a car's speedometer is allowed by law to read 10%+ a constant above the actual speed but never allowed to read under, I've seen posts where Prius owners have tested that the odometer is much better than the speedometer in accuracy wise.
While your indicated speed and your odometer likely pull data from the same source, how that data is then manipulated and displayed to you in each gauge is not the same. Basically - your speedo can be off, but that doesn't necessarily mean your odometer is off, as well. I'm pretty sure MOST car (and motorcycle) manufacturers intentionally have their speedos showing slightly higher than actual speed. This is a bit of a buffer - imagine if your speedo indicated 10% too slow! You'd get speeding tickets rather easily... which would lead to litigation for the manufacturer. IMTO (in my technical opinion), it's a safety/liability measure engineered into the instrument. That said, I doubt your odometer is off by very much. Maybe 1% or so. Again, if your odometer was off by a significant amount, it could lead to litigation. There's no real way to tweak the odo to make you safer, either.
The UK Prius adds exactly 10% to real speed over ground. This can be seen by comparing the speedometer to the speed seen in the Vehicle Signal Check screen of the MFD diagnostics. The odometer records the correct distance travelled. The car measures real distance travelled by multiplying real MG2 rotation speed - which is very accurate as the motor control circuitry has to know exactly where the rotor is in its rotation to apply the correct waveforms to keep it moving in the right direction - by the assumed circumference of the tyre. Tyres only have nominal rolling circumference, and that circumference changes with load and pressure, and with tyre wear. Tyre wear itself can account for about 2% difference in distance travelled. There's a good article on this at TireRack.com. I'm not sure whether GPS accounts for actual terrain or only measures speed and distance over the reference geoid, which is an approximation to the Earth's surface.
I'll answer the question and then address the why: GPS reads higher than trip meter - actual MPG mileage is better, a greater value. GPS read lower than trip meter - actual MPG mileage is worse, a lower value. HOW TO ADJUST MPG actual_MPG = indicated_MPG * (GPS_miles / tripmeter_miles) LOGIC The tripmeter, the car's distance measurement, it tied to the number of tire rotations. However, tires with identical specifications, 175P65R14, can have different revolutions per mile. Usually these differences are just a couple of % and even as the tire wears down, the actual diameter changes due to the lost tire tread. For normal speed management, it doesn't matter. But for MPG when you are in the 50 MPG range, 2% is the difference between 49 MPG vs. 51 MPG. So if you put tiny, smallest possible tires on all four wheels, they would spin much more rapidly and indicate a greater distance than actually traveled. The actual speed would be less than indicated. So if you wanted to 'sandbag' a mileage contest, put four donuts on the car and run the course IF they are using an MPG indicator in the car. The irony is that tire physics indicate a larger diameter tire will give lower rolling resistance and better mileage but it also leads to lower than actual speed indication. So the indicated MPG looks as if mileage has gone down ... exactly what is reported for brand new tires with fresh and thick tread. Since I use gas receipts versus trip meter, I use a calibration constant from the GPS-to-trip meter to adjust the actual miles. This gives accurate MPG. On the road, I set my cruise control to a target speed and then use the GPS speed to 'trim' to true speed. GOOD LUCK! Bob Wilson
I have did the road mile marker tests also and be careful. The crews must have been drunk on some of these roads. LOL But a cross reference to use with your GPS and odometer to measure distance traveled. On a long trip your GPS might be a good check of your odometer.
Keep in mind that your map database can be off too. All of the highway exits in Utah are off by two miles in my map database. Their physical location is correct, but the exit numbers are shifted by two. Presumably the State of Utah changed their numbering datum at some point after the database was created. Tom
