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MFIC
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Discussion Starter · #1 ·
(and I promised i utilized the search function)

hey since i previously had OEM 20's and factory tires on my truck, what is going to be the difference in the speedo if i went to 17" rims with the 315's? do i need to get it recalibrated since the tires are close in height (1.5")?
 

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All Around Great Guy
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I went to 17's from 20's and did not have it calibrated when I went back(winter 17's) 3 mph difference at 70. My truck used to be at about 73 mph at 2000rpms, now it is at about 70 mph 3.92 gears I have to get it in to have it recalibrated.
 

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I used a GPS to track my speedo difference. I jumped from the stock 275/70/17's to 315/70/17 BFG's and the gauge was off 4mph. I used the info from the BFG site for tire rev's and had the dealership (no charge) recalibrate the computer. GPS tells me I'm doing 79mph when the speed gauge reads 80mph. Close enough for me.
 

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MFIC
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Discussion Starter · #4 ·
i see, but that still doesnt help me. I had stock 20's & tires and went to 317/70/17's.. does the speedo (or anything) need to be adjusted?
 

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Sorry. Yes. Find out your new tires RPM from their website. Have your dealer recalibrate using the new RPM figure. Or if you have or know someone with a handheld GPS, take it for a ride. It will tell you your actual speed. If it's close enough then I would not worry about it. And at least you will know where to set you cruise control so you won't get pulled over.
 

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Russellabjr said:
i see, but that still doesnt help me. I had stock 20's & tires and went to 317/70/17's.. does the speedo (or anything) need to be adjusted?
Are you sure you don't mean 315/70/17's. Well the stock 20"s on the '04 and ups are mostly 275/60/20's. So by going with those 17"s, your jumping from a 33" tall tire to a 34.4" tire. So your speedo will be off. According to the tirecalc, when your speedo reads 60mph you are actually traveling 62.5mph since your speedo is off by 4.2%

So you can get the proper RPM listing for your tire and have the dealer recalibrate or live with it. You will rack up fewer miles on the odometer and travel a bit faster than your speedo reads.

KB
 

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MFIC
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Discussion Starter · #7 ·
I did mean 315. Typo.

Does anyone know how accurate those radar's with "YOUR SPEED" in flashing lights attached to School Zone signs are? I was the only vehicle near it and it was dead on the reading my speedo gave me..

things that make ya go hmmm..
 

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Does anyone know how accurate those radar's with "YOUR SPEED" in flashing lights attached to School Zone signs are? I was the only vehicle near it and it was dead on the reading my speedo gave me..
haha those things are fun i always see how fast i can get up to
 
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