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Discussion Starter · #1 · (Edited)
So I got a set of new wheels today, and put 275/40/20 in the back. I have a Diablo Predator where you can adjust the tire size. My two questions are;

1. Is this necessary? Will the speedometer be accurate etc?

2. In the Predator I can only put one value in on the tire size section. Since it's staggered, what should I put there then?
 

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Your 275/40-20 are the same height as a 245/45-20, so your speedo will be accurate (assuming you had 245/45-20 before)

Your cars computer will not be able to tell (nor would it matter) that your rear tires are now wider.
 
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Discussion Starter · #5 · (Edited)
If the 275's are replacing a 245 your speedo will not be accurate, just enter the change in you diablo and be done with it!

David
But in the Diablo there's only one option for changing size, so is for the front or back that is? It would have been more logical if you could change both front and back wheel settings.

I'm replacing the back wheels who were 255/45/20. But I guess the difference won't be that big on that as well then?
 

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If the 275's are replacing a 245 your speedo will not be accurate, just enter the change in you diablo and be done with it!

David
My speedo is accurate and I run 275/40/20s out back. I did not change tire size with my tuner.
 

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If I'm not mistaking I think it's how tall or short the tires are is what makes the differences to the speedo and rear end ratio...shorter tires increase the rear end ratio and speed up the speedo, visa versa the other way... width doesn't matter
 

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If you were running 255/45/20 before and now run 245/45/20 than your speedo should be correct as you are now running 275/40/20 which is the same as running stock. When you were running 255/45/20 that was when you should of changed your speedo.
 

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As shown in my previous post at 65 mph your speedo will be off by 4/100 of a mph. Your 275/40's are shorter by 2/100's of an inch compared to the 245/45's, width is irrelevant.

Check it for yourself with the Tire Size Calculator

I have the exact same set up and was pulled over last week. I'm off by 4 mph
More likely is that the radar being used has not been calibrated in years.
 

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Discussion Starter · #11 ·
Ok, so I guess I don't have to adjust it with the Predator then. But if I do, what value should I put since I can only put one value in and the wheels are staggered?
 
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