Hello everyone. I'm the second owner of a 2015 Prius Three in beautiful condition. I've noticed that the speedometer reads about 2 mph higher than speed reported by GPS at ~ 55mph, about a 3% difference. A 205/75-15 tire would offset most of that difference, but tires of that size offer very few choices. Now a 205/60-16 tire is about 2.8% taller, with much better variety in tire designs. I know that somewhere in the basement I have a set of 16x7 wheels, originally off a Subaru WRX with a 5x100 bolt circle and 45mm offset, and that weigh about 11-12 lbs. apiece. I'm thinking about trying a wheel on the Prius to see if it rubs against anything. But I've also read the following in an old thread here: "Increasing the rolling circumference in a manual car can improve mpg by reducing engine revs for a given speed. With the Prius transmission this is not the case as engine revs are likely to increase with an increase in torque load that a larger circumference will produce." So what do you think? Worth trying?
My advice: Don't worry, be happy! …seriously, though you might experiment with the tyre pressures, just remember to set the hinds at 2 PSI less than the fores (…well, that's what is recommended!) Here's a link which might be useful: Tire Size Calculator - Tire & Wheel Plus Sizing Good luck!
Why are you so sure the GPS is more accurate than you speedometer? Posted via the PriusChat mobile app.
If the speedometer indicates (for example) 50 mph, when in fact you're travelling 48 mph, what are the implications? That you're travelling 2 mph slower than indicated. That's about it. Not exactly a big problem, nor insurmountable. You could: 1. Drive 52 mph, be spot-on the speed limit. 2. Drive 50 mph.
@bwilson4web did something similar with his Gen 1 Prius, I think. He is our measurement expert and can more intelligently explain his testing.
Well, it's the -odometer- that is important accuracy wise. I suspect you will find it is almost right on. As posted above, speedometers are often set high intentionally. To keep you from getting -as many- tickets I suppose. Larger tires can certainly offset the speedometer error, but they can actually be illegal in -some- jurisdictions.
I'm not checking my facts, but I believe the purpose was to try to slow people down, reduce injury severity, and fatalities. You never want to have a speedo that reads low, deadly accurate is an impossibility, so reading a bit high was mandated.
Larger tires would fix the speedometer issue. But I believe it would then make the odometer less accurate. The actual mpg (both hand calculated and with the computer) would be ~3% lower because the car would travel further than the odometer would indicate.
