How complicated is it to fix a minivan's speedometer?
July 19, 2019 1:35 PM   Subscribe

My Sienna's speedometer isn't quite right. Judging by the various "your speed" signs, it says that I'm going 65 when I'm going 60, for example. I know that the speed signs aren't accurate, but they've all shown that, at various speeds, in various states -- the speedometer isn't precise. This is true for drivers of various heights.

Because of Reasons That Are Boring, it's important that my speedometer show the correct speed. How complicated and expensive is it to get this fixed? I'm in the USA. Do I go to my usual garage where I get oil changes, or do I have to go to the dealer where I get the countless recalls taken care of?
posted by The corpse in the library to Home & Garden (23 answers total) 1 user marked this as a favorite
 
Most countries have regulations that say that a speedometer must never show an incorrectly low speed, but may show up to x% over + a bit more. In the UK I believe this is 110% + 6.25mph. The reasons for the first part are obvious. And if you're actually not going as fast as you think you are, that's not such a bad thing for accident statistics.

Speeds as shown by a GPS are pretty accurate, but tend to lag behind, and are only an average. So if you're on a long straight road travelling at a steady speed, a GPS is way more accurate than a typical speedometer.

You can get your speedometer calibrated by any half-decent car mechanic. They need to be able to do this when someone changes their wheel size, for example.
posted by pipeski at 1:53 PM on July 19, 2019 [4 favorites]


Are you sure you’re running on the right sized rims and tires? That can have a significant effect.
posted by spitbull at 2:03 PM on July 19, 2019 [16 favorites]


This is true for drivers of various heights.

Er. What now?

"Judging by the various "your speed" signs, it says that I'm going 65 when I'm going 60, for example."
This is not a good metric at all to judge the accuracy of your car's speedometer.

There are two methods to establish if you even have a problem, because it is not at all definite that you do:
1: GPS units will show speed. Put your car on cruise and drive for 10 minutes and see what your speedo says versus the GPS. If there is a significant variation (>5mph) your car may have an issue.

2: Drive a known distance. By which I mean an accurate distance. Drive the distance at a constant speed (cruise control, not human controlled) and time it with a stopwatch and distance in miles /time is your speed in miles per second. Multiply by 3600 to get miles per hour.

As for actually adjusting it, there should be something in the manual about doing this or, failing that, a Toyota specialist or dealership will be able to do it. Any mechanic should be able to do this, but they all work differently so it may be easier to get a dealership to do it.
posted by Brockles at 2:05 PM on July 19, 2019 [3 favorites]


Also, as mentioned: Judging by the various "your speed" signs, it says that I'm going 65 when I'm going 60, for example. The tolerance band that manufacturers are required to comply with means that speedometers should never UNDER-read and so are required to be accurate or +x% over. So it may be perfectly correct for your car to be reading about 10% over, unless I am reading that wrong and your car is saying 60 when the roadsigns are saying 65. That could be a tyre size issue, as also noted.
posted by Brockles at 2:07 PM on July 19, 2019 [1 favorite]


The only sure-fire way to know how much your speedo is off is to drive for a known distance at a known speed (cruise control, flat terrain) and see how closely they match. If you set the cruise at 70 MPH, in one hour, you should have traveled 70 miles. On the interstates, the miles are marked. In Missouri, every 0.2 mile is marked on the interstates, as a special bonus to a former governor's friend. GPS works, but not as well.

Changing the speedo setting is another story. Depending on the vintage of your Sienna, it can be anywhere from exasperatingly expensive to outright impossible. Changing the output of the transmission sensor can also mess up your ABS.

Big question is, why is it not accurate? The "your speed is..." signs may be lying to you. Or your tires are not the ones that the engine computer expects (look on the driver door frame).
posted by notsnot at 2:14 PM on July 19, 2019 [1 favorite]


Years and years ago when our family Ford Winstar's speedometer started acting up, my dad drove up to a bored cop parked in a speed trap and asked him to clock his speed. This included a few times with the gun thingy as well as "drive in front of me and flash your lights at 45mph." It helped us figure out exactly what kind of wrong it was.

I think it must have been fixed a few months later at a regular service appointment, because I know it was doing all right when I was learning how to drive.
posted by phunniemee at 2:33 PM on July 19, 2019 [4 favorites]


I don't know why it's important for you to have the correct speed, but if additional technology solves your problem, then it's cheap and easy to get a GPS speed display, which, as it's GPS, accurately measures your speed using the Doppler effect of the GPS radio transmissions. As it's not reliant on the size of your tyres, it's also consistently accurate.
posted by ambrosen at 2:54 PM on July 19, 2019


Waze displays your speed as its open, ride and experiment with a passenger who has the app
posted by tilde at 3:01 PM on July 19, 2019 [2 favorites]


Second getting a GPS speedo app and driving at a steady speed on a straight road while a passenger uses the app to check how accurate your car's speedo is.
posted by EndsOfInvention at 5:06 PM on July 19, 2019


FYI OP, here’s a thread on “justanswer.com” where a claimed Toyota-certified mechanic (who seems to know what he’s talking about) describes the factory-authorized troubleshooting test procedures for the Sienna speed sensor in great detail. Also yourmechanic.com suggests the replacement cost for the driveline sensor runs between $180 and $600, although such sites are often wrong. That sounds like the range from “just the part” to “part plus about two or three hours of labor.”
posted by spitbull at 3:38 AM on July 20, 2019


And here is the relevant section of the official Toyota Sienna service manual.

You’ll note the driveline speed sensor ECM is also providing data for the ABS and traction control systems (ergo, “getting out of speeding tickets” is not the “only reason” one should care about speedometer accuracy on a modern car, and also tires aren’t “everything” that can affect it). It’s a computer reading a sensor. Computers and sensors fail. Safety is implicated.

Step one is getting it hooked up to a computer diagnostic rig. I’d get it diagnosed at a Toyota dealer or dedicated Indy specialist Toyota shop first. You’re not the only one to report this issue on Siennas, although it isn’t among the more common complaints.
posted by spitbull at 3:47 AM on July 20, 2019


Also one key thing to observe and report to your mechanic if you want to do some diagnostic driving yourself following the real-world distance driven or GPS methods: is the speedometer always apparently off by the same numerical value? Or does the apparent error get smaller or larger as speed increases? For whatever reason, in my own car, the speedometer and cruise control (which also needs speed sensor data, obvs!) always show one mile per hour higher than he car’s own built in GPS (I often wonder randomly why the two systems couldn’t interact like the airspeed or AOA sensors on jet planes...oops). Always. I could be going 9mph on the speedo and it’s 10 on the GPS. Or 78 on the dash and 79 on the phone and nav system gps.

My phone GPS and my OCD-ish real world calibration (I’m a musician, there’s a lot of driving, you get bored) generally concur with the higher speed, although the difference is small enough to be within margins of error for higher speed mileage checking). My tires are OEM spec and it’s done this through two sets of different branded tires now. So in my case it’s a systematic calibration difference that weirdly errs to underestimating my speed on the dashboard by 1mph in any situation. That’s gotta be in software as any physical variation in measurement should produce a speed-sensitive value, as I understand it (a percentage of whatever speed you’re at).

If instead you see the size of the discrepancy grow or shrink as you go faster or slower ( i.e., your dash and GPS disagree by a fixed percentage and the numerical spread increases or decreases with speed) that’s more likely a hardware issue like tire size or a misaligned sensor.
posted by spitbull at 5:04 AM on July 20, 2019


Sorry should have said my speedo shows one mile per hour LOWER than the GPS.
posted by spitbull at 5:15 AM on July 20, 2019


Are you including the driver height because the viewing angle of the needle and numbers can vary? (the "shadow" of the needle of the speedometer on the dial with the numbers?).

Have a friend drive at an agreed upon speed, follow them at a fixed distance, and read your speed. Be on the phone with them and try several different speeds.
posted by at at 5:37 AM on July 20, 2019


Yup, the height because of the viewing angle. No, it’s not about speeding tickets.
posted by The corpse in the library at 6:46 AM on July 20, 2019


Assuming you have cruise control, does that have a digital display of the speed you set?
posted by spitbull at 7:44 AM on July 20, 2019


One option, depending on the type of speedometer you have: Speedohealer discussion. I have no experience of these devices.
posted by StephenB at 7:47 AM on July 20, 2019


No, the cruise control doesn’t have a digital display. The minivan is a 2011, if that’s any help. But it sounds like a trip to a Toyota specialist is what’s needed.
posted by The corpse in the library at 8:01 AM on July 20, 2019


I may have made this all sound more mysterious than I meant to. The boring reasons are to do with a new, teenage driver, and the teenager's uptight mother (me) wanting everything to be Just Right before launching him into the world.
posted by The corpse in the library at 8:16 AM on July 20, 2019


If that is your reason, and your car speedo OVER reads, then I'd leave it exactly like it is and don't mention to them that it is wrong. No harm in having new drivers going slower than they think they are. If they DO know about it, I'd just tell them I got it fixed.

There is no need whatsoever for the car to be accurately reading as long as it is UNDER the speed limit you are trying to drive at.
posted by Brockles at 9:02 AM on July 20, 2019


Well, it is easy to get a speedometer calibrated or repaired. Just takes money. Of course, that's modulo the problem of finding an honest mechanic. Just remember that the call them "stealers" for a reason--a good independent shop is a treasure.

As for the "new teenage driver" thing, part of driving is problem-solving. Car won't start because the steering wheel is locked? Windshield wipers aren't cleaning the windshield adequately? Oncoming drivers are flashing their high beams at you? What do you do about something that wasn't covered in drivers' ed?

You can let your new driver help troubleshoot the problem--they may have some good and creative suggestions and maybe they'll learn something along the way. Good luck--letting the babies out to fly is the hardest part of being a parent, I think.
posted by Gilgamesh's Chauffeur at 9:05 AM on July 20, 2019


I doubt you have a faulty speed sensor; the failure mode for something like that isn't going to be slight inaccuracy, it's going to be a dead speedometer and a check engine light illuminated on the dash. Given that you haven't mentioned a check engine light, concerns about ABS and traction control are IMO totally unfounded.
posted by Larry David Syndrome at 9:11 AM on July 20, 2019


Seconding Larry David Syndrome above. Cars of that vintage being low by 5% is normal out of the factory, and if the problems were substantial enough to cause issues with ABS or TCS, you'd have seen a warning light at least once by now.
posted by thegears at 10:31 AM on July 21, 2019


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