How do I calibrate my speedometer?

[Anonymous]

My car came with 18"s, and I'm really worried that the speedometer is out and that I'll inadvertently get 3 points on my licence as a result.  I don't speed knowingly but if I was even 5mph out it's enough to get me in trouble!  Can the speedometer be calibrated or is this only a dealer's job?
I've searched...but to no avail.
Thanks!

[muffdan]

use a tomtom or other gps decive, it will show you your actual speed, very accurately.

Another approach is to drive down a dual carriage way or motor way at 60mph, and time the distance between mile markers, they should be 1 minute apart.

Jason

[Tem]

Can the speedometer be calibrated or is this only a dealer's job?

I'm not sure if you can call this calibration, but take the needle off and put it back in another position.

I don't think a dealer can calibrate it any better though, at least I've never heard of it being possible.

[evileye_wrx]

When I took my dials off the speedo had to be recalibrated. There should be a guide on here that Taurec did. It's a matter of using a gps to mark your real speed and see how much off the needle is, then guestimating how far it needs to be moved to match. It's a bitch to do and takes a dogs age to get it right, I know for a fact mine isn't 100% accurate any more

Phil

[Anonymous]

Or use this

Pop in the stock wheel + tyre sizes, add in what you've got on there now, and there's your difference. Given that most stock MRs that I've ever driven seem to be set 4mph faster at 70mph (i.e. it actually reads 74mph), then you should be able to work off that pretty easily.

[uktotty]

Do 80 in a 40 past a camera and you will get written proof of yoru speed though the mail  s:) s:)

[markiii]

unfortuunately it's going to be out in percentage terms not by a fixed amount you can compensate for

[roger]

Stupid question? Does the speedo work off the front or rear wheels?

[uktotty]

Front!

[markiii]

not convinced as 03 alloys change it

[uktotty]

Spin the wheels and if the speedo goes then its rear, if not then its front

[aaronjb]

Speedo takes it's feed from the ECU, so short of disassembling the ECU code I doubt anyone knows for sure..

Unless someone wants to try unplugging each ABS sensor (since that is the ultimate feed on our cars) until the speedo stops reading.. Assuming the ECU doesn't average more than one wheel speed to calculate it's speedo signal.

[Tem]

All I know is that the speedo works on a dyno, when the front wheels are still and rear wheels are rolling.

Like Aaron said, it could be more complex than that though.

[kanujunkie]

sure i read somewhere that the speedo source is from the front left wheel

[SteveJ]

(6) Vehicle speed circuit
The vehicle speed sensor, installed inside the transmission, detects the vehicle speed and inputs a control signal into
TERMINAL SPD of the engine control module.

[GSB]

Speed signal on near enough every car on the market is generated by a sensor on the final drive, i.e gearbox. So in the case of this particular car, its the rear wheels, not the front.(!):wink:

Pulling the pointer off and moving it will only make the meter read correctly at one set speed, and not actross the range. It needs to have its multipier factors changed. (As I always tell Controls & Instrument apprentices, Gauges have "pointers", not needles. Needles are what your mother uses to stitch the name labels in the back of your pants. [/pedant])

If the radius of the wheel/tyre cobmo has increased, you are basically out of luck. I've never looked, but I imagine the recalibration of the speedo is not an easy job.  Its a pulse driven affair, so you'd need to alter the circuit prior to the odometer as well as the stepper motor. No good changing the speed readout if your still putting on an additional 30 miles for every trip to the shops.

However, if I were in this postion, my over-riding concern would not be that my speedo was over by a few mph, but that I'd utterly and completely buggered the handling of my car by bolting oversized slabs of alluminium to each corner. After all, the delicacy of this cars handling is its Raison d'être. Remove that and you are left with not much more than an expensive and exceedingly impratical mid-engined Corolla.

Do yourself a favour and drive a car with stock wheels, you'll be amazed at how much of a detrimental effect the 18's have.

[Anonymous]

Speed signal on near enough every car on the market is generated by a sensor on the final drive, i.e gearbox. So in the case of this particular car, its the rear wheels, not the front.(!):wink:

Pulling the pointer off and moving it will only make the meter read correctly at one set speed, and not actross the range. It needs to have its multipier factors changed. (As I always tell Controls & Instrument apprentices, Gauges have "pointers", not needles. Needles are what your mother uses to stitch the name labels in the back of your pants. [/pedant])

If the radius of the wheel/tyre cobmo has increased, you are basically out of luck. I've never looked, but I imagine the recalibration of the speedo is not an easy job.  Its a pulse driven affair, so you'd need to alter the circuit prior to the odometer as well as the stepper motor. No good changing the speed readout if your still putting on an additional 30 miles for every trip to the shops.

However, if I were in this postion, my over-riding concern would not be that my speedo was over by a few mph, but that I'd utterly and completely buggered the handling of my car by bolting oversized slabs of alluminium to each corner. After all, the delicacy of this cars handling is its Raison d'être. Remove that and you are left with not much more than an expensive and exceedingly impratical mid-engined Corolla.

Do yourself a favour and drive a car with stock wheels, you'll be amazed at how much of a detrimental effect the 18's have.

I take your point.  My problem is that the car CAME with the aftermarket alloys; I have no option of putting the stocks back on.  And regarding the speed factor of having 18s I'm becoming less and less convinced- the alloys are surprisingly lightweight and the tyre profile almost accommodates for the size gain of the rim.  It's twitchy in its handling, I'll give you that, but since adjusting the tyre pressures and a considerable learning curve I've got the car pretty much tamed lol...well in the dry anyways lol

[aaronjb]

(6) Vehicle speed circuit
The vehicle speed sensor, installed inside the transmission, detects the vehicle speed and inputs a control signal into
TERMINAL SPD of the engine control module.

Speed signal on near enough every car on the market is generated by a sensor on the final drive, i.e gearbox. So in the case of this particular car, its the rear wheels, not the front.(!):wink:

While you both have valid points, I bet neither of you can point to the actual speed sensor on the gearbox, can you?

That's because there's just a blanking plate there on our cars.

Oh, Steve - since you copied & pasted from the wiring diagrams.. can you find the input from a speed sensor on them?  I certainly can't... (because it comes from the ABS ECU)

[GSB]

Oh, Steve - since you copied & pasted from the wiring diagrams.. can you find the input from a speed sensor on them?  I certainly can't... (because it comes from the ABS ECU)

Well, how about that... I thought there was a hall effect sensor plugged into the diff casing. There is in every other car I've looked at recently, including a fairly recent Peugeot belonging to a colleague at work. It was in need of a new clutch, so one day, while the boss was away...  s:wink: :wink: s:wink:  

I always wondered why manufacturers didn't make use of the ABS system to provide a speed signal. After all, it would save the cost of a further sensor in the gearbox, and it's asscociated costs. Both for provision and installation, and for warranty. (After all, as a manufacturer you have to assume that a certian % of parts you fit will go wrong, so if you can get away without fitting them, theres more profit available.) It seems Toyota have seen the same benefit. Sadly, this information doesn't help resolve the problem posted above. We still have no way to modify the way that the BCM and the instrument cluster interpret the pulses generated by the ABS system and make the speedo read more accuratley...

Still, Well done Aaron Holmes...  s:wink: :wink: s:wink:

[kentsmudger]

To go back to the OP question, one way to achieve accurate calibration would be to re-draw the scale marked on the dial. I did this on a beetle years ago.

Drive at a few known speeds (30, 50, 70 etc), with GPS or similar reference,  and note the indicated speed on the dial as it is, then mark a new scale.

The problem with this on this car is that the markings are not just on the face of the dial, but cut through the face for back-lighting. I have seen a few people on here with replacement dial faces, so maybe a manufacturer of these could make up a custom one.

This is all looking a bit involved.

[GSB]

If the Original poster is using 18's with correctly sized tyres, its all immaterial anyway. The speedo will only be a few percent out, and if its that great a concern, it will be easier / simpler / cheaper to install either a GPS device to rely on instead, or buy a cheap set of OEM wheels and enjoy the car as intended. One solution could be something like this:

[Tem]

If you really want to calibrate the speedo to be exact at positions...well, we were kinda just talking about this on SC. We were talking about modifying the speed signal to affect how the power steering works. The same could be applied to speedo calibration.

What would you do to make the pulses faster?

Being a software guy, I'd use a simple chip for it.  s:D s:D

Maybe PIC12HV609/PIC12HV615, which work with 2-15V DC and cost $1 or so:
 m http://www.microchip.com/ParamChartSear ... &pageId=74 m

Wouldn't you have to count the input pulses during a period of time (or count time intervals while the input is one state)?

Yeah. I was thinking about just counting the time between last two pulses and then outputting X times that. Rough, but I'm guessing it should work.

Then again, to make the speedo accurate, you'd have to calibrate it all the time. For example the difference between new and worn tyres is equal to new 205/50/15 vs new 205/55/15.

[philster_d]

You also have to remember the gauge isnt linear.

[Anonymous]

This is all very valuable, and many thanks to posters.
I'm a little concerned as to the accuracy of my satnav...yes i realise not only is it digital but they work on highly accurate time measurements from satellites...but what about the processing time?  Mine's a tad slow in processing and sometimes I'll even miss a turn as it is 40 yards or so out at certain speeds.

[Tem]

I'm a little concerned as to the accuracy of my satnav...

I once had a chance to try out three different satnavs. They never had the same speed on any two of them.  s:lol: :lol: s:lol:  Two were off by 1kmh and the third additional 2kmh, making it differ 3kmh from the other.

The problem with them is that they average the speed. Some average it more, some less. They should be quite accurate, if you're going at a steady speed, but apparently they aren't even then.  s:? :? s:?

Go figure, but I wouldn't count on them. Not if you need 1-2kmh accuracy anyway.