Speedometer with large tires -> fix

[haasa]

Hi all,

I found out, that the speedometer with large tires (31x10.5) is very wrong - of course It shows about 10%++ less then the actual speed (measured with GPS). I hat the same problem with my Suzuki with larger tires.
Today I fixed the problem same way I did it with the Suzuki:

It's a job of only 1-2 hrs to find a new suitable ajustment.

Follow the instructions of the repair manual to get to the speedometer...
CHANGE_1:
The speed sensor has a ajustable spring.
You'll find a very small lever with three grooves (very small). This is for pre-loading (??) the spring.
Now be careful! Just move this lever about 1mm. You will see the difference -> the pointer of the speedometer is now acting slower.
CHANGE_2:
The pointer of the speedometer has to be coreccted. Factory default the pointer shows exactly to 0km/h when there is no movement. Because of the change of the spring in step 1 you have to put the pointer a bit further downward. It is ok if the pointer shows about -5 to -10 km/h (which of course is not visible)

The whole job is "try and drive". It took me about 5 test-drives till the ajustment was done.

Table (km/h):
GPS_Speed, before_changes , after_changes
-------------------------------------------------------------------
0, 0, -7
5´,4, 0 (difficult to measure)
10, 8, 7 (difficult to measure)
30, 25, 29
50, 42, 50 (important speed)
70, 61, 71
100, 90, 103 (important speed)

I don't know so far if this changes will have affect on the speedometer only. It would be nice if the km-indicator is now also corrected - I will see and let you know.


Haasa.

Sorry for my english - but Its realy very hard to write down this technical stuff in english...
And sorry for no_pictures of the lever, spring and pointer.... (simply forgotten to take some)

[Jimmy L]

Where is the speedo sensor located, I assume it's behind the dash somewhere? Is it illegal to tamper with the speedo? In this case, since your correcting an error caused by running bigger tyres it would seem safe and acceptable.
Can someone confirm that if your speedo is out then your odometer be out also?

Cheers

Jim

[murcod]

If the speedo is reading low it stands to reason that the odometer will also be reading low.

It's only illegal to wind back the odometer reading (ie to increase the vehicle's resale value) AFAIK, there's nothing wrong with making it more accurate.

[-Scott-]

As these systems were explained to me - I make no guarantees for the accuracy, or the relevance to anything ever made by Daihatsu.

The mechanical odometer is effectively a fixed gear train - as the tyres turn the wheels inside the odometer turn too. Larger tyres make the entire drive train turn more slowly, including the wheels inside the odometer. The only way to correct that is to change the gearing somewhere in the gear train to correct the gearing change created by the taller tyres. This is normally done (I thought) at the gearbox end of the system.

The same spinning shaft which drives the odometer also spins a magnet on a tiny drum, which spins near a magnet attached to the speedo needle shaft. As the drum magnet passes the needle magnet the needle magnet attempts to follow it, but a spring holds the needle back. As the drum spins faster it pulls on the speedo needle shaft more often, causing the needle to rotate further and indicate more speed. The pulsing of the spinning magnet is effectively masked by damping in the system - primarily inertia. I'm guessing Haasa has tweaked the hold-back spring?

If it's an all electronic system the speedo is an ammeter reading a current generated by the electronics, which is counting a pulse stream sent from the gearbox. Again, tweaking a spring will affect the way the needle indicates the signal current, but have no effect on the system which is counting pulses and rotating the odometer drum.

Either way, tweaking a spring to correct the speedometer needle shouldn't affect the rate at which the odometer "measures" the distance.

If any of the above makes sense I should have opened another bottle...

Scott

[murcod]

Thanks for the explanation, I've always wondered how they worked.

The normal correction method I've heard of is to fit some sort of gear mechanism in the speedo cable to change the rotation speed. An electronic one would be harder to do....

[-Scott-]

To do it electronically you need a circuit to read the frequency of the incoming pulse train, scale it appropriately and generate a new pulse train at the corrected frequency. Could possibly be done with analogue stuff: Frequency Counter to produce analogue voltage -> amplifier to scale voltage -> Voltage Controlled Oscillator to generate new Pulse Train at the corrected frequency. It's real easy when you say it quickly and even easier when you have no intention of trying to prove it.

These days its probably easier to throw a microprocessor into it - or find another drive gear for the pulse generator...

Scott

[haasa]

Hi,

I'm guessing Haasa has tweaked the hold-back spring?

YES.

I drove to our offroad area (in Austria it is forbidden to drive in the woods, meadow, ...) today to test the offroad-abilities of my Feroza - but that's a different story. They trip to the area (~12km) gave me some time to check the operation of the modified speedometer:

- odometer (thanks for the vocabulary) works still wrong (very wrong )
- speedometer is nearly perfect. Sorry - the table I posted obove is not correct.

NEW Table (km/h):
GPS_Speed, before_changes , after_changes
-------------------------------------------------------------------
0, 0, -7
20, 12, 18
30, 25, 28
50, 42, 51
70, 61, 70
100, 90, 98

not so bad, what do you think?

But I was very suprised when I found out today that vmax = ~125km/h (GPS) with my "new" engine (new piston rings, new gaskets, new planned cylinder head, new..., new ...). Thats not very fast . Speed limit in Austria is 130km/h on highways (and billing starts at ~140-150km/h ). Shall I start playing around with pre/post igition timing? Now I'm using a bit "pre ignition" to get more torque in 1500-3000rpm for offroading.

How fast is YOUR Feroza running with 31x10.5 or 30x9.5 tires
How is YOUR ignition set (I guess factory default is +3°)

Best,
Haasa

[murcod]

Factory ignition specs are 3 degrees BTDC (+-2 degrees) with the vacuum advance blocked off.

Mine has done an indicated 140km/h fully loaded with a trailer. That's on standard 225/70R15 tyres

[DeVooluff]

Mine has done an indicated 140km/h fully loaded with a trailer.

Freeowwww! Don't I push my little truck hard enough? 110kph@4000rpm is about as fast as i'd want to go in mine. Luckily thats the top legal speed here in Vic. How many revs were you cranking Dave?

[murcod]

I don't remember- I was too busy overtaking a caravan.... That was before I fitted the extractors etc, it was all standard.

I reckon they'd do over 160km/h if they could reach the redline in fifth.

[SimplyPV]

hmmm.... i've come close to the red line in 5th... got to 110 mph which i was told the conversion went to somethin like 178 km/hr, my gage maxes out at 115 mph so i got damn close... had maybe another 300 rpms befire hitting red line... was freaking sweet. boy did roza howl. she also shook pretty badly too. light as a feather she is.

[crowey]

Well, I have only been to 141kph (gps) thats running the 31x10.5's
That was fast enough, it was fairly light at that speed. tried to go faster but the old girl didnt want to put out!!

[BundyRumandCoke]

To get back to the original question, I have the same problem in my Rocky. I run a GPS on the dash at all times, and it is usually set to speedo and odometer when I am highway driving. Its always accurate, no matter what size tyres I have on.

[murcod]

To get back to the original question, I have the same problem in my Rocky. I run a GPS on the dash at all times, and it is usually set to speedo and odometer when I am highway driving. Its always accurate, no matter what size tyres I have on.

So your GPS is accurate no matter what size tyre you have on..... ... strange that!!!

What about your speedo compared to the GPS readout?

[BundyRumandCoke]

Why would that be strange???
As for the difference, with my 9R BFG A/T's on, the GPS reads 100km with the speedo reading about 92km. With the 10R Simex's on, I think its about 100km = 85km on the speedo.

[crowey]

I think that was a sly way of taking the piss out of you!!
Yeah i run the gps most of the time as well. its not too bad in the city i just drive at the same speed as everyone else but out on the freeways its good to have it running

[murcod]

I think that was a sly way of taking the piss out of you!!

Yes, I read it a few times and thought " What the hell are you trying to say?!"

I just drive at the same speed as everyone else

Here in SA that could mean you're doing 20km/h over the limit! There are lots of maniacs over here.

[Dangerous Dave]

Crowey's an adelaidian too. I went on a holiday to melbourne and my tyres got me a $125 dollar fine from the Victorian Police Force. Speedo said 100 thought i was safe, done for 105 . Since when are speed cameras mounted to camry's????

Dave

[built4thrashing]

a much easier way to correct your speed is buy using a calculator. First divide the size of you new tyres with the size of your standard tyre
( 215/75/R15 =26.9 inches) so if you run 31s its
31 divided by 26.9 = 1.152

so now you multiply your dash reading by 1.152 and this gives you your actual speed. calculate them all and stick it besides your dash and you will always know. plus when ya sell the car you just put standard size tyres on it and keep good tyres for next 4x4. complete table is below for a F300 feroza
dash speed
10 -------- 11.5
20 -------- 23.0
30 -------- 34.6
40 -------- 46.1
50 -------- 57.6
60 -------- 69.1
70 -------- 80.6
80 -------- 92.2
90 -------- 103.7
100 ------- 115.2

hope this helps

or check out this tyre calculator link http://www.miata.net/garage/tirecalc.html

[Jimmy L]

a much easier way to correct your speed is buy using a calculator. First divide the size of you new tyres with the size of your standard tyre
( 215/75/R15 =26.9 inches) so if you run 31s its
31 divided by 26.9 = 1.152

so now you multiply your dash reading by 1.152 and this gives you your actual speed. calculate them all and stick it besides your dash and you will always know. plus when ya sell the car you just put standard size tyres on it and keep good tyres for next 4x4. complete table is below for a F300 feroza
dash speed
10 -------- 11.5
20 -------- 23.0
30 -------- 34.6
40 -------- 46.1
50 -------- 57.6
60 -------- 69.1
70 -------- 80.6
80 -------- 92.2
90 -------- 103.7
100 ------- 115.2

hope this helps

or check out this tyre calculator link http://www.miata.net/garage/tirecalc.html

You da man- that's gold matey

[built4thrashing]

well after me first speeding fine after puttin bigger tyres on it i decided to do some brain rattling and actually made a new speedo sticker that just went on existing dash was pretty silly looking but stoped me gettin fines

[Big-Stu]

Well with all this discussion on speedos and bigger tyres I finally decided to have a go at correcting mine. I knew it read low after I put the 31 inch tyres on, so using the tyre calculator (thanks buddy) I reckon it was reading about 5k low at 60 k. Since me son is starting to drive it I figured I had better fix it up.

I made enquiries a couple of years ago about a ratio adjuster that goes in the side of the gearbox and the speedo cable plugs into it. It was about $140.

So after pulling apart the dash and speedo I found the diesel rocky speedo has adjustments for the spring tension and the zero position.
Good on you Daihatsu!

So I connected the speedo up to my battery drill and found I was now doing 58k while conveniently sitting at my dining room table, and not a speed camera in sight.

A few adjustments later and I was now doing 64 k. The spring adjustment is now wound back as far as it will go.
I'll have to get a hold of a GPS at some stage to confirm the accuracy, but at least its a bit closer to the real speed.

Pretty simple job to get the dash apart and adjust the speedo. Don't know why I didn't do it years ago.

Stu

[-Scott-]

So I connected the speedo up to my battery drill and found I was now doing 58k while conveniently sitting at my dining room table, and not a speed camera in sight.

A few adjustments later and I was now doing 64 k.

Luv it!

Good job!

Scott