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Discussion Starter #1
Hey guys,So I got some 285/50r20 all terrains fitted today and I've adjusted the wheel radius using my iid tool which makes the speedo correct. All works fine until I drive a few km and the engine light comes on. The fault is for vehicle speed sensor A and prevents cruise control working and seems to stop the little blip in revs when downshifting. I should mention I have a 2007 range rover sport, 4.2l supercharged. Also everytime I clear the fault code, it's fine for a few km and then it returns to the same issue. Hopefully someone here can give some advice.
 

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Wheel radius has nothing to do with the speedo. It is the rolling circumference of the tyre that equates to distance traveled in one revolution.

If your IID tool uses any radius it would be the radius of the tyre, not the wheel. Measure from the wheel centre to the outer most height of tread for that figure.
 

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Discussion Starter #3
Yeah I didn't explain myself too well. I set it to the correct radius. I used a tyre size calculator to figure out the total diameter and halved it which was 392mm. My speedo is correct according to gps so that's all good. I might try changing it by a mm or 2 tomorrow just in case there's some kind of bug in the firmware. Very small chance it'll work but I'm all out of ideas at the moment.
 

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Discussion Starter #4
In case anyone finds this post and has the same problem, I've emailed GAP Diagnostics and it appears to be an issue with the factory ecu not being able to handle values that high. I've got mine working with the tyre radius set to 390mm which is fairly accurate.
 

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2010-2012 Range Rover Sport
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Awesome thanks for this. I've just installed Nitto Terra Grapplers (285/50R20) and still need to set the speedo cal, so this info was very timely.

In the calculation to get the rolling circumference, do you need to factor-in tyre deformation due to the weight of the car (if so how do you calculate it?), or should you just assume the tyre is a perfect circle? (i.e. just use Nitto's stated tyre circumference)?

From Nitto's website my tyre diameter is 31.26 inches (=794.004 mm) so the radius would be 794/2 = 397mm... so per trippd89's post above I'd have to use 390 right?
 

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No tire deformation need to be calculated. Use the perfect circle radius for YOUR tyres. Different makes of tyres in the same size will have different radii due to tread depth difference in design. Measure from wheel center the top of the highest tread nubs.
 

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Discussion Starter #7
It might be worth trying to use the correct input just in case not all cars have the same problem as mine. However, I have the exact same tyres as you and with it set to 390mm, when my speedo is showing 100km/h I'm doing 98km/h according to gps which I'm happy with.
 

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2010-2012 Range Rover Sport
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Thanks! Interesting how you got 392 and I got 397.... are yours original Grapplers or G2s (like mine)?
 

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2006-2009 Range Rover Sport
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I'm running 275/65R18 G2's with a 32" diameter and set to 389 using the GAP tool. It was originally 353. Now my speedo is spot on using a GPS.
 

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2006-2009 Range Rover Sport
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I just took the original setting and multiplied it by the percent change in diameter from stock tires and it worked. The 32" are a tight squeeze but a 3lb persuader inside the wheel well helped.
 

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Discussion Starter #12
I just reread my earlier post and realised I typed it wrong. It was meant to be 396mm, not 392mm. I got that figure by using a tyre size calculator based off the tyre size being 285/50r20 which i knew wouldn't be exact but should be pretty close.
 
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