A couple of months ago I replaced my 2006 Prius with a 2017 Prius. My mileage to work in the 2006 Prius was always 27.2 miles but now in the 2017 Prius is comes in at 26.8. This past weekend I checked the mileage on the highway using the mile markers and after 100 miles the odometer read 98.8. So it seems it is off by slightly more than 1%. Is this something to be concerned about? Could low tire pressure alone account for this? I guess I would be more concerned if the odometer was reporting more mileage, this way my warranty lasts a little longer.
It’s always a good idea to check that your tires are properly inflated, but low tire pressure would tend to cause the odometer reading to be too high, not too low. If the tires are underinflated, their circumference is smaller than normal, so each revolution of the wheels carries the car a shorter distance than registered by the odometer, which is calibrated for tires of the expected size. An odometer error of 1% or so is well within industry standards. NIST Handbook 44 – 2017, Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices, allows ± 4% for odometers used in commerce (see section 5.53). There are also SAE standards for odometers, such as J2976 (± 2.5%) and J1226 (± 4%), but compliance with these is voluntary. See the fascinating discussion of odometer accuracy in Lopez v. Nissan North America, Inc., 201 Cal. App. 4th 572, 135 Cal. Rptr. 3d 116 (Ct. App. 2011). The odometer and the speedometer use data from the same wheel speed sensors (collected by the Skid Control ECU), so if the odometer is wrong, the speedometer may also be affected. If you have a GPS receiver or a smartphone that indicates speed, you could check this yourself; for accurate results, use cruise control to drive at a constant speed on a long, tangent section of roadway. The speedometer could also be checked by driving the vehicle on a calibrated dynamometer, though I wouldn’t expect Toyota to pay for this service under the warranty without a much larger error than you’ve described.
