Speedometer problem

The only way I know to adjust a Prius speedometer is to change the tires. The standard Prius in the US comes with Goodyear Integrity 185/65SR15 tires that are rated 855 revolutions per mile. This makes the speedometer read only slightly high. If you put on a tire that has a lower revs/mile, the speedometer will read lower by the same percentage change. You can check the revs/mile of your tires at:

Bridgestone Tires Available at Tire Rack

And see where you stand with your Bridgestone tires.

JeffD

 

If you know the TSB number and pay a short subscription to Toyota Service Information you could download it yourself.

Personally, I'm not too bothered, it means that when I don't try to compensate for the induced errors, I know I'm driving within the speed limits and I am probably getting a better MPG as a result of driving slower.

I was driving 74 on the motorway today as indicated on the speedo, so I was still below the speed limit:mod: Not so good for the MPG, but for longer trips I tend to drive at 60 so I guess that I must be zooming along at 55.

 

Based upon my Magellan GPS, the speedometer on my 2004 (with the correct size 15" tires) reads high around 2 mph at 70 mph. I am not aware of any TSB issued by TMS USA on this point; perhaps a TSB was issued by Toyota Europe?

I've heard that UK Prius speedometers read high as the OP mentioned, and that this is by intent, not error.

I am not aware of any way to "fix" the speedometer error that you noted, other than by installing larger tires so that the car will travel farther for a given number of tire revolutions.

 

Other posters have stated that Toyota uses speedos that conform to UN standards in Europe. The UN standard allows the instrument to read up to 10% higher and be in spec so I believe your dealer will say, "It's supposed to do that."

 

Ten percent high sounds about right for the European market. You have different laws over there regarding speedometer accuracy, which generally requires them to be set optimistic. I don't think there is much you can do about it other than switching tires.

Tom

 

Normally, this is a safety margin. Think about what would happen if your speedo read 6MPH slower!

My Porsche did it. My motorcycle does it.

 

Also be aware that the rated revs/mile of a tire may be fairly
inaccurate. The 855 r/m of my original integrities made the car
match the GPS almost *precisely*, when I ran the tires at a
decent pressure. Then I got the Hydroedges which were actually
rated at 854 r/m, making me think they'd be very close and only
make the car appear to be going just a teeny bit slower.
.
Guess what, the inflated HEs had a measurably *smaller* circumference
than the Integrities [yes, I checked this] and it turned out
to give me a 1.2% *faster* speed and measured distance error.
That was pretty annoying, but I just dropped a modifiable TIREFACTOR
into the scripts I use to keep track of my lmpg and periodically
compare car distance vs. GPS distance on open, straight roads
to make sure the error is still being accounted for. As the
tires wear their r/m will continue to slowly rise, so I can track
that through their lifetime.
.
Life's little surprises.
.
_H*

 

Get one of these installed
ELECTUS DISTRIBUTION - Speedo Corrector MkII Kit

 

Pat,

Thanks for that piece of electronic wizardry. Do you have one installed?

Of late, I've noticed that to keep up with a series of traffic lights I
have to exceed the posted speed limit by about 10% as indicated on
the speedometer. It seems odd that the traffic engineers would set
things up that way, having to break to law to move quickly, efficiently
along.

So, maybe my speedo is set 10% high. That would also explain why
the Prius generates a lot of negative comments and even road rage
incidents for going too slow, when the operator honestly thinks (s)he
is doing the legal maximum; doing an actual 45 MPH when the speedo
indicates 50.

As to the little gizmo, where would it be best installed?
In a place so that it corrects the speedo reading?
Or somewhere else so that it corrected the HSD inputs?

 

I don't need one, my speedo reads 1km/h under the speed my GPS reads at all speeds.
Truck drivers use these to get their speed limited trucks to go faster by fudging the speed signal. We know they do it and we know how to catch them out.

 

Hi mate, you live in the UK and have a EU Spec car.

All cars, by EU law, must have a speedo that reads at least 5% more than real.

The car knows the speed it's doing (Scanguage tells you that), the GPS calculates it in real time, but the speedo sull show the difference.

Somewhere in here (Prius at top speed) yuo will find my pictire clocking the Prius at 183 (Kph) while the GPS says 175.

Put your mind at rest: until the EU changes whis crappy homologation law, ALL speedo's will have this defect.

 

I believe the permitted range is actually -4%/+10%. It's a deliberate electronic fudge; if you go into the MFD diagnostics - press Display, tap upper-left, then lower-left of the inner frame and repeat another two times - then tap Vehicle Signal Check, it shows the real speed (in km/h). To get out, press and hold the Display button for about ten seconds.

Note that you lose your accumulated data for this trip when you go into MFD diagnostics.

As always you should not attempt to go into diagnostics mode while driving.

 

No. The UNECE Regulation No.39 does not allow minus display value.
http://www.unece.org/trans/main/wp29/wp29regs/r039r1e.pdf
0 <= (V1-V2) <= 0.1*V2+6km/h

Ken@Japan

 

The obvious downside to a speedo reading high is it will rack up mileage faster and burn through the warranty faster.

 

The speedometer and odometer are separate, the UK/EU fudge for reading high is thankfully only applied to the speedometer.

 

Yes I confirm. I tested the odometer and it loses 21 km over 1000 done.
(Car says 1000, GPS says 1021)

 

For those not wanting to download the pdf.

Tom