Speedometer error on Toyotas

[i6pwr]

If you believe that, then try this

So if I removed my 18" wheels and put on a set of 15" with a drastically smaller diameter, and the speedo was off by 15 MPH, you're saying my odometer will not change?

I will actually try to make note of the mile markers next road trip...worth the try for science.

Not exactly.

What I think @supton is saying is that the computer is calibrated to determine both speed and distance accurately, let's call it "base value," but this data is then further manipulated, let's call it "adjusted value". The adjusted value could be something simple like adding a couple of kph or mph to the base value. The car then uses base value for tracking distance, but sends adjusted value to the speedometer to be presented to the driver, for the reasons already mentioned earlier.

On my old BMW, I could access the internal computer to see both values - base and adjusted, but only the adjusted value would be shown on the main speedo.

I hear you, but my 2005 Sienna had no clue where it was....no GPS, no tracking....odometer based off speed.

Interesting about the BMW.

 

[Graham Piccinini]

So if I removed my 18" wheels and put on a set of 15" with a drastically smaller diameter, and the speedo was off by 15 MPH, you're saying my odometer will not change?

Your speedo will be off by 15MPH, while your ODO will be off by 12-13 miles.

 

[supton]

So if I removed my 18" wheels and put on a set of 15" with a drastically smaller diameter, and the speedo was off by 15 MPH, you're saying my odometer will not change?

No, if you change the tire size without telling the computer, then it will absolutely be off.

But if you do this experiment, if the speedometer was 1% high, it would still be 1% higher than what you'd expect, when the tire change was taken into account.

Your car likely has 4 ABS sensors. I don't know what tone ring is being used, how many teeth it has on it, it may be a 60 tooth target. If so, then it's capable of measuring a pulse every 6 degrees of tire rotation. 60 pulses per rotation. If your tire is 25.7" tall, it would have 785.2 revolutions per mile. Or 60*785.2 = 47,112 pulses. [Edit: it might be more likely to use transmission output speed for all I know, and that may be far few pulses per revolution? or it may be a similar number, if counting before final drive ratio is taken into account. Same concept, different numbers.]

The ECU can then log number pulses (or convert to likely km, regardless of units shown on dash) for distance traveled. But when displaying speed, it can then take that pulses per minute calculation (probably updated every 0.5 to 1 second?), convert into speed, fudge it up 1 or 2%, and display that.

 

[Passport1]

Japanese motorcycles can be as much as 10% on the high side.

 

[Graham Piccinini]

Japanese motorcycles can be as much as 10% on the high side.

Chinese is even better. Speedometer said 70MPH, while GPS confirmed 57MPH.

 

[Dave1027]

I can't think of a car or any vehicle that doesn't read high by that amount? I put ~3% larger circumference tires on my Focus and its pretty much right on now. Outback is off by 3% as well.

Agreed. I did the same on my Toyota Tacoma which removed most of the speedo error. Bonus win, the larger tires actually cost a little less than the original size. I think because it was more common.

 

[i6pwr]

@supton thanks for the detailed reply. I do understand the wheel speed data but basically my point was that the vehicle won't know it's traveling less distance if the wheel speed changes.....ie, smaller tires. Unless you reprogram the PCM obviously.

I just would like the speedo to be accurate, I guess it's better than reading low in the eyes of your driving record..lol.

I get it that there is very likely a built in error while still retaining accurate distance measurement,.unless the tire diameter changes.

 

[AZjeff]

GM, always on in my experience. Toyota and most Japanese cars I've driven read high. I speculate it's intentional to a) void the warranty before it's normal time, or b) trick the driver into thinking he or she is getting better gas mileage than they really are.

It's intentional to err on the slow side. Could you imagine the screaming and hair pulling and lawsuits if speedos were 1 or 2 mph slow and everyone would want to blame the automaker?

 

[Kestas]

SAE specifications require speedometers to be accurate -1% / +3%.

I once beat a speeding ticket with a newly purchased car when I showed paperwork that the speedometer was subsequently fixed under warranty.

 

[bullwinkle]

Agreed. I did the same on my Toyota Tacoma which removed most of the speedo error. Bonus win, the larger tires actually cost a little less than the original size. I think because it was more common.

I did the same thing on the xB in my sig-OEM was 185/60R15, switching to 185/65R15 cured it. More pothole resistance too!

 

[JohnnyG]

To the original Post. This was an old trick with Chrysler products way back in the 70's or 80's, UNTIL they got sued for it.
If you realize the benefits, you'll see exactly why it happens:
It LOOKS LIKE you get better perfomance,
It LOOKS LIKE you get better fuel economy,
It LOOKS LIKE your car lasts way more miles without problems,
It LOOKS LIKE your tires last a lot longer,
You change your oil more frequently.

 

[chris719]

Odo is a relation to speed over time, so I can't see how they would be different.

They know the "correct" speed and that is used for the odometer. They use another doctored value for display as speedometer output. Edit- @Quattro Pete beat me to it.

 

[Traction]

I've owned many Toyota cars over the years and have found the exact same issue is they all read 2-3mph faster than you are going. I figure that is one way they can inflate their fuel mileage ratings and the cars show more miles than actually travelled. Cuts down on the speeding tickets though, and the warranty ends sooner.

 

[IndyIan]

I did the same thing on the xB in my sig-OEM was 185/60R15, switching to 185/65R15 cured it. More pothole resistance too!

We should start a high profile tire club... I went from 195/60R15 to 195/65R15's and its nothing but good in the summer time. In the winter I go to 185/65's and would love to try 175/70R15's but they don't exist anymore....

 

[Rhymingmechanic]

2018 Corolla seems to read about 2-3 mph fast around 70 mph.

I bought a 2001 Silverado new, and it seemed to hold first gear for a lot longer than it needed to. I finally passed a mobile radar trailer one day and realized the speedometer was off. I already had a Hypertech programmer--it said the stock tire size was 30" but the tire measured shorter than that. Adjusting the size down corrected the speedometer and made it shift a little earlier. Now I measure the circumference of new tires and program accordingly. The speedo currently reads exactly the same as a new speed check sign nearby.

 

[HowAboutThis]

Wife's Forester matches GPS. Corolla: Google Maps GPS is 2mph less than what the speedometer is set at. So if I set the Corolla to 70 on a flat road Google Maps will show 68.

 

[alcyon]

My 1989 JDM toyota specs a 215/60 R15 tyre. Since I got the car, I installed 205/65 R15 becuase 215/60 R15 is not available in my country.
The 205/65 R15 actually gives a reading that is spot on the real speed. I suspect if I were to use the original size, it would read 3 to 5kmh higher than actual. Manufacturers over report the speeds a little, i heard that it's by law, as that would ensure customers won't get speeding tickets if they kept to the value shown on the meter as per speed limit of that road.

 

[ATex7239]

Same issue. It is super annoying. Seems like lazy engineering to me.

 

[barryh]

AE specifications require speedometers to be accurate -1% / +3%.

Those are sensible margins of error and easily achievable in modern vehicles.

By comparison in the EU, while speedos are now accurate enough the regulations are still in the dark ages. ECE-R39 states that speedometers can't show speeds less than the actual speed, and can never register more than 110 percent of the true speed, plus 4 kph.

In the UK it's worse. Same as the EU but instead of plus 4 kph it's plus 6.25 mph
That means at 30 mph a speedo can legally read 39.25 mph and at 70mph it can read 83.25 mph.

 

[Skippy722]

Can’t speak for Toyota, but all my Mopar’s have been dead on according to GPS and those mobile speed checks. I’d be driven nuts if they weren’t.