First long trip - speedometer question

The speedometer in the Prius is optimistic by ~2 mph. Completely normal.

 

We both have 2010s.
"My" Prius is right on with OEM tires.
I tripple-checked it with a phone, GPS unit and by setting the trip odometer to zero and driving by 100 mile markers in the interstate.

The Garmin might be off a little bit, especially in the mountains or you may have slightly smaller than OEM tires...or (most likely) at 35,000 miles you still have the factory tires and they've worn down a bit.
There's a "fudge factor" in speedometers built-in because as tires wear down they spin a little bit faster.
2-mph...if this is really the case, is probably going to be more than "close enough."
I wouldn't bother to get it checked, but if you're really anal about mileage you can continue to use the GPS to quantify the speedo error. Using the GPS over a long period of time will give you a pretty accurate measurement for how much your speedometer is off.
Be sure and look for a speedometer change when you swap out the tires.

I look for trends with MPG, since a sudden dip can foreshadow a trip to your friendly Toyota dealership, so I usually hand calculate my MPG when I fill the car up. If the speedo is off by 5%?
It's always off by the same 5% barring a tire change so I can still monitor my efficiency accurately.


Good Luck!

 

If I'm doing 52 kmh, my Garmin shows 50. The odometer is (or should be) reasonably accurate; it's only the speedometer that's reading high. There's regulations I believe, saying that speedo can read accurate or slightly high, to discourage speeding. Most if not all manufacturers err on the cautious side, intentionally setting the speedo to read a bit high.
My 2 cents: I'd use the speedo to judge your speed, forget that it's a little high. It slows you slightly, and improves mpg.

 

FWIW, when we bought our Cruze, I had the speedometer "tested" it at the same facility that calibrates the AZ Highway Patrol cars. The tester commented that GM cars specifically, and ALL cars manufacturers in general, added a 2-mph "cushion" to speedometer readings to compensate for changes in the tyre diameter between brand new OEM and worn OEM. And, sure enough, the analog dashboard speedometer would indicate 60 mph, but my ScanguageII™ would indicate 58 mph. And, that +2-mph "offset/cushion" was consistent, from 30 mph all the way up to 100 mph, the fastest speed the speedometer shop tested our car too.

 

In my car the odo & the cruise control don't match. I find the odo is within 1mph of my nuvi. Some of it is just display rounding. Below 10mph the nuvi reports in tenths. Situation normal.

 

In both my 2012 Prius and 2012 PIP- the speedo would read 60 and Garmin (two different units) would indicate 58mph. We have a semi permanent radar "this is your speed" sign on one road I take to work every day- it would indicate 38mph while both Pri' displayed 40mph on the speedo.

 

Yes and it also means your warranty runs out sooner! And thats what they go by. Honda was made to add 2% to your mileage for the same thing. H

 

That isn't true. You're saying that our Prius reads more mileage than what we actually drive? If that is what you mean than I'm sure there would have been a class-action lawsuit. Ever try google maps and checking your mileage of your route? The Prius mileage and google maps mileage are pretty much the same.

The Prius speedometer reads 2 mph higher but if you plug in a Scan Gauge or ODB reader, it'll read the correct speed. So Toyota does know what it is doing by making the speedometer read higher than the actual speed you're driving.

 

No, the odometer does not have the same intentional offset as the speedometer. Yes, some units indeed did have enough error to cause the warranties run out very slightly early. But my Honda's odometer offset was negligible, so I got nothing while the lawyers earned a gazillion dollars 'representing' me for this uncommon 'problem'.

The lawyers then went after Subaru for the same 'problem', and earned another gazillion dollars despite my first Subaru having a tiny error in the opposite direction. Because of that, my second Subaru's odometer reads noticeably low. With winter tires it is reading 1.8% lower than actual mileage. I won't get a calibration on the OEM tires until later this month.

Both my original and current Prii odometers read 0.2 to 0.3% low, even while the speedometers read 1 to 2 mph high.

I prefer the roadside milepost markers. These were generally accurate when the road was originally surveyed and built, though beware of later realignments that cause discontinuities, and occasional offset posts. An example in my state, the 160 miles of I-90 from the Columbia River to the Idaho border is good, except for a single 'bust' at Ritzville at about MP220.

Trip meters on GPS units can also be good. My automotive unit, and my hiking unit set to run in continuous mode, read within 0.1% of the mileposts mentioned above. The hiking unit goes bad when set to 'battery saver' mode, racking up extra distance from errors as it continually wakes up and takes readings before fully re-locked on to the signal.

 

Fantastic Pam, I love it! Camping in a Prius. It certainly has enough room with the back seats down. Have fun.

 

The more you know...
(From Speedometer Accracy)
There are standards for speedometer accuracy the manufacturers must conform to. Unfortunately they allow an even larger error than what we typically actually see. The United Nations Economic Commission for Europe (UNECE) Regulation 39 specifies the most important speedometer standard. The UNEC was established to encourage economic cooperation among its member nations. It has 56 member nations, including most countries in Europe and most countries that trade with European nations. As such, it includes Canada, the Central Asian republics, Israel and the United States.
In short, the standard has two aspects:

• The indicated speed must never be less than the actual speed.
• The indicated speed must not be more than 110 percent of the true speed plus four kilometers per hour (kph) which is approximately 2.5 mph at certain specified test speeds. For example, at 50 mph, the indicated speed must be no more than 57.5 mph.

The standards also specify the details of the speedometer accuracy should be measured during the approvals process. For example, for most vehicles, the test measurements are made at 40, 80 and 120 kph (25, 50 and 75 mph). There are other speeds specified for vehicles not capable of reaching at least 150 kph (93 mph). The tests are also conducted at 23 ± 5 °C (73.4 ± 9.0 °F) measured at the speedometer.
Once a test vehicle passes the required accuracy, the regulation is actually relaxed for production vehicles manufactured to the same specification as the test vehicle to allow for variation in manufacturing process and other variables such as weight of optional equipment, brand of tire, etc. The upper limit on indicated speed is increased to 110 percent of actual plus 6 kph (3.7 mph). So your production vehicle traveling at an actual speed of 50 mph could read more than 58 mph and still be within the standard. That is an error of 16%. Similarly at 75 mph, the reading could be as much as 86 mph or almost 15%.
Individual countries can impose stricter accuracy standards, but most haven’t. The U.S., for example, only specifies that commercial vehicles, such as buses and trucks, must have a speedometer that is accurate to ± 5 mph at 50 mph.
However U.S. manufacturers voluntarily follow the standard set by the Society of Automotive Engineers (SAE), which has no force of law. This standard allows speedometers to read between one percent low and three percent high at low speed and between zero percent low and up to four percent high at speeds above 55 mph. However, these percentages are not of the actual speed, but of the maximum reading of the speedometer. Thus the allowable four percent error above 55 mph is a reading of up to 6 mph high at all speeds above 55 mph if the maximum value on the speedometer is 150 mph.
As you can see, the standards for speedometer accuracy are pretty lax. In fact, your speedometer is almost certainly well within the limits set by these standards.

 

So, would that (SAE-J862 spec) mean that a Chevette, with just a 100 mph speedometer, would inherently be more accurate than a Corvette with its' 180 mph speedometer?