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changing tires size & how to correct?????

1755 Views 12 Replies 6 Participants Last post by  WhonosWho
i am lifting my truck 8" and putting 38" tires on it. How can i correct the speedo. Ithink it will be off alot. and don't want a ticket for speeding. Any sugestions???????

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Assuming you have 31.5 stock tires right now, the error is equal to

38/31.5 = 1.2

Or, 20%. So if your speedo shows 50, you'll really be doing 60. You have a few options:

1) Get one of the tuners that allows speedo correction
2) Get a cheaper speed corrector, or visit the Ford dealer, I heard they will do it also.
3) Do nothing and just use the rule "take mph on speed, divide by 10 and double it" then add that to what the speedo shows.

For example, 60 on speed: 60/10 = 6...double that is 12....add that to 60, you get 72. So your real speed is 72 mph

Why would you want option 3? Your odometer will show less miles. If you correct it, then the miles your tires see is what is registered on the odometer, while your engine actually only did 20% less rpms/etc. Helps resale value, etc. The rule is very easy, you'll get accostomed to it FAST.

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For 38" tires you're going to need a True Speed to correct your speedo.

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If he wants to.......and some of the tuners can do it too.

I heard the SCT can do it. Can someone please confirm?

Ok, I dug around, look here:

Scroll down, center column. You can play with axle ratio (2.73 to 5.14) and tire size (205/65/15 to 315/35/19). Me thinks the SCT CAN do tires......maybe whoever you spoke with at Innovative was misinformed (or SCT's website is misleading).....

So, can anyone confirm whether the new SCT can do tire sizes???

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