Speedometer shows 2 MPH faster than actual speed

2 miles per hour fast compared to what?

 

The actual speed.

Almost every car speedometer reads a higher number then the actual. This is normal and is across all mainstream car brands.

 

Ken@Japan provided a great resource in Post #2 of the thread Accuracy of Speedometer for TCH. He links us to the UN requirements which bias all speedometers on the high side. I believe the purpose would be so that people do not unintentionally exceed speed limits.

BTW, is it only on the 2010 Prius V with Package #8 that you get those 16" wheels?

 

They're a special dealer option to help close the sale for those people who can't decide between the 15" and the 17".

 

Mine reads 1 mph high at 35 mph and 65 mph when gps says 34 and 64. No biggy to me.

 

I had read somewhere recently that Toyota claims that their odometer logic computes the distance/speed without error. I will see if I can find that again.....

In the meantime, what is your tire size? I assume the V is calibrated for the stock 17" tires. If the 16" tires have a larger circumference than the standard 17" tires, then you will travel farther than expected per wheel revolution.

With that said, 2 mph seems like a lot unless you drive at > 100 mph. For comparison, dropping down to the stock 15" tires (195/65R15) would also make you cover more ground per revolution (adding about 1 mph at 70 mph). But the stock 15" and 17" are not particularly well matched. It turns out that 185/65R15 would be a closer match to the 215/45R17 and that was what I was planning to get for snow tires.

As an aside, I had assumed that those that change wheel/tire sizes just had the dealer reprogram the odometer to compensate. Is that not the case?

 

My Prius is 1 MPH off. My GPS and my radar detector with GPS for photo/speed cameras shows the MPG. I just went on a 3,200 trip. I watched all three. My Prius IV with 15" rims is 1 MPH off 100% of the time.

 

Speedometer Scandal! - Feature - Auto Reviews - Car and Driver

Related chart: http://www.caranddriver.com/var/ezf...lication/1e1b33f85f31df61e4830f32f82a5a71.pdf

 

Mine is doing 54 mph at 55 mph on the speedo.

 

55 mph=55 mph on my car according to GPS

 

Bah, my old Skoda's speedo readings were 14% off! When I was going 50km/h (31.25mph) it would show about 57km/h (35.63mph). I knew that I could go 150km/h (93.75mph) indicated speed on the highway, because in reality it was only 130km/h (81.25mph) (which is a legal limit here). And yes, it *was* on the stock wheels, so just the right size the speedo was supposed to be calibrated to at the factory. Hell, that's what I call an inaccuracy...

 

Speedometer calibration requirements vary by country. Most places in Europe call for a wider margin between displayed and actual speed.

The accuracy of an individual speedometer is also affected by the circumference of the tires. Changing wheels and tires can change the accuracy.

As for odometer accuracy, that is separate from speed and is not overstated. However, tire circumference comes into play just as it does for speed.

Tom

 

The police have set up one of those digital speed readouts beside a residential street in my neighborhood to slow down drivers. Of course this is at 25 to 35 mph, but my speedometer on my 2010 Prius III reads exactly what the police readout displays.

 

Prius speedometers on both Gen2 and Gen3 (and probably Gen1 as well) read a bit fast, usually about 2 MPH at 70 MPH. They do it on purpose, if you want to know why read post #4 on this thread and check Ken@Japan's post which is referenced.

The computer in the car knows how fast it is really going. If you call up the MPH display on a ScanGauge you will see that the speed is exactly what is displayed by your portable GPS. The radar displays along side the highway are sometimes set to read fast because it is believed that will slow you down and keep you safe so your can't rely on them to check the speedometer.

 

The Italian car magazine "Quattroruote" (one of the most authoritative on the italian car magazine market), has recently tested the new Prius with 17" wheels/tires and measured a +4.5% delta in reported speed at 130km/h (i.e. the car was 4.5% slower; this is quite common on all cars in sold in EU).
If you run the math, that equals in MPH in the same delta, equivalent to:
130km/h = 80.78 mph
130/1.045 = 124.40 km/h = 77.3 mph

If the delta were constant at all speeds (which is not, quite likely), at 55 mph, your real speed would be 55/1.045=52.63 mph.

 

We have a Prius III press car (with the optional 17" wheels) for review and we noticed that it indicates a surprising 10% higher speed than our Prius II fleet car when driving side by side (131 vs 120 km/h), and 10% more too than timed at constant speed on the freeway (120 indicated vs 110 real). Have yet to make a long term distance test with a GPS, but I find it weird because it defacto improves indicated gas mileage by 10%. Any similar experiences with 17" cars ?

 

No, not with mine. It's off about 2 MPH at 70 MPH, less at slower speeds. Your 17" wheels must be a port or dealer option? All the Prius IIIs leave the factory with 15" wheels, only the Vs have 17" wheels. What size tires are on it? 215/45-17 would be the correct size.

 

I too see a 1-2 MPH difference depending on speed (seems a percentage). I have 15" wheels - you mention 16" which I do not believe are standard (maybe they are???). If you've changed your wheels, the MPG and speed will change accordingly... If it came stock, then you are set fine.

I would not be surprised if this has something to do with the "MPG" being off by a given percentage thread (I'm not quoting anything - as it seems many have differing accounts of how far off MPG rating really are). So if speed is off, MPG is also off.

Some posts also point towards a "cruise control" functionality - where on cruise it will always be 2MPH fast... Not sure how that works as it seems to always be a bit fast...

Guess a corporate responsibility of safety (which Toyota clear both has, and lives by - regardless of what we think) - is a decision that while some may not like, I personally don't mind. It's a hassle, and we may not like the decisions, but there is normally a workaround (all-be-it not always easy).

Hey - they had a weird backdoor on our 2005 Lexus GX - you could press a few buttons, and be able to navigate on the fly (change destinations). Once that came public - they removed the backdoor... They believe it dangerous (assumption) - so they change the design. I tend to respect a company that has a position, and goes the extra mile to enable the position (there *is* a real $$$$ cost to just removing a dumb back-door password - and it is not insignificant).

I tend to always see 1-2 MPH over my GPS (speed dependent - as makes sense)... The GPS is aftermarket - but GPS data is GPS data. If you think it is an issue, know the issue, and drive with that knowledge...