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Old 25th November 2011, 12:30   #1
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Question How to Caliberate the Speedo after tyre upsize?

My MM540 recently got a massive tyre upsize from stock which gives out a speedo error of about ~25%. Now, I'm completely OK with the error, and don't bother much, there are certain concerning things associated with this.

1. The vehicle will be driven by many (used in farm), and down here 60kmph is a physiological safe speed to travel, without losing much time. I have seen many put effort to take my Jeep till its 60 mark, which well.. is ~80kmph in reality, and is a very unsafe speed in a 540.

2. I need to compute more to find the FE. Also, trip et al becomes a bit confusing.


So.. I was wondering if there is a way by which one can calibrate the speedo to match the upsize'd tyres?. DIY will be appreciated.

Thanks,

Dhanush
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Old 25th November 2011, 12:55   #2
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Default Re: How to Caliberate the Speedo after tyre upsize?

Calculate speeds (original speed vs speed with new tyres)
Open it up
Stickering job on the dial

Cheap, quick way to do it. Else find the dial from another speedo that closely shows the actual current speeds
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Old 25th November 2011, 17:52   #3
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Default Re: How to Caliberate the Speedo after tyre upsize?

There are two methods.

. Calculate the speed corresponding to each 10km/h mark on the meter (use a GPS on an open straight stretch of road), and design an overlay in a graphics program, with new speed markings. Print it on plain paper and check its accuracy (both speed and lay out). Once it is validated, print it on a CD/Lable gummed sticker and paste it on the speedometer dial.

. Find a speedometer which will give the correct reading with your configuration, and install it. It may require quite some trial and error, but once you get the correct meter, then both your speed and distance will be correct.

. If none of the above suit you, calculate the ratios needed, and design an adapter which will connect to your speedometer cable and increase the number of revolutions in the ratio required. Now both your speedometer and odometer will be correct.

Last edited by Aroy : 25th November 2011 at 17:53.
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Old 25th November 2011, 17:54   #4
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Default Re: How to Caliberate the Speedo after tyre upsize?

BTW What was your original Tyre size and what is it now? How did you end up with such a huge difference?
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Old 25th November 2011, 18:03   #5
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Default Re: How to Caliberate the Speedo after tyre upsize?

Quote:
Originally Posted by DWind View Post
BTW What was your original Tyre size and what is it now? How did you end up with such a huge difference?
It is easy, upsize from normal 16" to 33" Maxis wide tyres and the total diameter, hence circumference increases dramatically. He must have installed a lift of at least 4" in the Jeep.
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Old 28th November 2011, 09:59   #6
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Default Re: How to Caliberate the Speedo after tyre upsize?

Quote:
Originally Posted by condor View Post
Stickering job on the dial
Quote:
Originally Posted by Aroy View Post
sticker and paste it on the speedometer dial.
It still leaves the odo error unsolved!
Quote:
Originally Posted by Aroy View Post
Find a speedometer which will give the correct reading with your configuration, and install it.
Quote:
calculate the ratios needed, and design an adapter which will connect to your speedometer cable and increase the number of revolutions in the ratio required
I guess I have some work to do on it..
Quote:
Originally Posted by DWind View Post
BTW What was your original Tyre size and what is it now? How did you end up with such a huge difference?
The stock tyres were 155/165(?)/70 - R16
Upgrade: 265/75 - R16.
So error ~20%
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Old 28th November 2011, 10:51   #7
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Default Re: How to Caliberate the Speedo after tyre upsize?

What is the speedo ratio running in your Jeep currently? 0.65? Think along these lines.

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Old 28th November 2011, 13:41   #8
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Default Re: How to Caliberate the Speedo after tyre upsize?

Dhanush,
Use a tyre size calculator to find the error.
Ex: Tire size calculator
Enter the sizes of the stock & the new tyre. The ratio correction required will be directly available. This application says that if the vehicle is travelling at 74.8kmph with the 265/70/16, the speedo shows only 60kmph (assuming that the stock is 155/70/13).
Hence the required ratio change will be 74.8/60 = 1.247. Which means you need a converter that will output 1.247 revolution for 1 rotation of the speedo cable. I guess, people who work with autorickshaw meters should be able to help you on this. And this conversion setup has to be inside a housing where you can connect the speedo cable on 1 side and to the speedo meter on the other side.

The second and lengthier option is to do this correction at the speedo output at he gear box end. But I wouldnt prefer this as it would require opening up the gear box.
Off Topic: How about a tacho on the Jeep? Does it already have one?
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Old 28th November 2011, 13:48   #9
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Default Re: How to Caliberate the Speedo after tyre upsize?

Quote:
Originally Posted by dhanushs View Post
It still leaves the odo error unsolved!
I guess I have some work to do on it..

The stock tyres were 155/165(?)/70 - R16
Upgrade: 265/75 - R16.
So error ~20%
165/70 R16 = 25.09 Dia
265/75 R16 = 31.65 Dia
Difference = 6.55" ~ 26%

if original speedometer ratio = X, then only solution is to find a speedometer which runs at the speedometer ratio of 0.74X. May be some truck speedometer will match the ratios.

Some articles on how it was done by others
Mini Spares - Article - Engine transplants - Changing FDs and speedo drive ratios.

Just the thing needed
Speedometer Gear, Speedometer Gears, Ford, GM, Dodge 700R4
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