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Discussion Starter #1
There is probably a thread on this already but I can't find one. Looking at 20 inch deep dish wheels that clear the brakes. With the tire I want to use, total diameter will increase by 1.5 inches over the stock combo. Is there any difficulty to reprograming the computer to keep the speedo accurate and not screw up any other settings? Can the local Ford dealer do this? I could go with a combo that would keep the diameter the same, but I am an old geezer and the current super low profile tires just don't do anything for me. Thanks in advance for replies.
 

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There is probably a thread on this already but I can't find one. Looking at 20 inch deep dish wheels that clear the brakes. With the tire I want to use, total diameter will increase by 1.5 inches over the stock combo. Is there any difficulty to reprograming the computer to keep the speedo accurate and not screw up any other settings? Can the local Ford dealer do this? I could go with a combo that would keep the diameter the same, but I am an old geezer and the current super low profile tires just don't do anything for me. Thanks in advance for replies.
I can answer part of your question. I know if you buy an SCT tuner you can do it easily. I don't know whether Ford can...but I would think so given that FRPP sells non-stock rear gears.

Keep in mind that if you're going with that much taller of a tire, you'll lose "effective" gear ratio. In other words, your stock 3.31 rear gear will seem more like a 3.00 now. If you go to a 3.73 gear, you'll get back to where you were.

I've tossed around the idea you mention...and if I did that i'd probably go to a 4.11 gear.

Dave
 

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Unless you invest in a tuner, you should stay within 3% of your overall tire diameter when upgrading. In other words, the overall tire diameter of the 20s should not vary more than 3% from the overall tire diameter of the stock tires. Failure to do so can cause issues with the speedo, TCS, and ABS. Here's a calculator to do the math for you:

http://www.1010tires.com/TireSizeCalculator.asp
 

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Yep...as stated ideally you want to stay the same diameter. Plus 1 or plus 2 setups means you increase wheel size diameter by 1" or 2" but keep the same overall diameter of the stock tire/wheel setup. You do this by going with a lower profile tire. In your case it would be a step two. I am doing the same thing...going to 20" wheels and by going with a 305-30/20 and 255-35/20 this will keep me within range of the near 27" stock diameter. I was tossing around the nitto dr's and they are 1.5" more in diameter and didn't want that.
 

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Yep...as stated ideally you want to stay the same diameter. Plus 1 or plus 2 setups means you increase wheel size diameter by 1" or 2" but keep the same overall diameter of the stock tire/wheel setup. You do this by going with a lower profile tire. In your case it would be a step two. I am doing the same thing...going to 20" wheels and by going with a 305-30/20 and 255-35/20 this will keep me within range of the near 27" stock diameter. I was tossing around the nitto dr's and they are 1.5" more in diameter and didn't want that.
I know a few people who have gone 1.8" or so taller on rear tires with no issues, although they did have a tuner.
 

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If you are running the stock tune, I'm pretty sure Ford can adjust the speedo. If you are running an SCT tune, I've been told you can adjust the speedo using the handheld. I have a XCal2 and I cannot make that adjustment. You may need an XCal3.
 

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I know a few people who have gone 1.8" or so taller on rear tires with no issues, although they did have a tuner.
I have MT 305/45-18's which are 28.8 inches tall. They are 1.8 inches taller and I have put about 350 miles on the car with out adjusting anything. My speedo reads a little slow than my actual speed. Nice thing about that is my car is registering less mile then I am actually driving!

I think it is aobut 5-7mph differance at highway speeds.(just guessing here)

JJ
 

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Discussion Starter #9
I already have 4:10 gears. I stopped by the local Ford dealer today (not where I bought the car) and the service manager was good enough ot hook up their laptop to the car. He went thru several progressions and found where he could change the tire size. Shouldn't be any problem, hopefully. When I find out, I will post how things went. Thanks for the replys.
 
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