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Discussion Starter · #1 · (Edited)
For us guys who put large tires on our trucks instead of trying to remember that when the speedometer says 70 you are actually doing 77 buy a speedometer calibrator so your wife and/or girlfriend will not get a ticket.
Rough Country sells one that is a plug n play. Made by Hypertec for Rough Country

I checked on Truck forums before buying like Ford Silverado Tundra n Taco Colorado an everybody has good things to say about it
It says it will correct shift points also n your odometer will be accurate

per several forums will not work with 37’s n larger

easy to set. Plug into laptop Just put in OEM tire n aftermarket tire size then plug into your truck
Now I don’t have to worry or check gps to see how fast I am actually going
Oh n you can change back to oem n use it on your next truck
 

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so, my sons F-150 is driving faster with the 20" rims than the factory 17"?
 

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Discussion Starter · #3 · (Edited)
so, my sons F-150 is driving faster with the 20" rims than the factory 17"?
Not the rim but the diameter of the tire Only if he put larger or smaller diameter tires on it as this will also work if he put smaller diameter tires on it

my speedometer was off 7% going from a 30.5 to a 33 inch tire
 

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Not the rim but the diameter of the tire Only if he put larger or smaller diameter tires on it as this will also work if he put smaller diameter tires on it

my speedometer was off 7% going from a 30.5 to a 33 inch tire
What if the rim is much bigger than factory and tire thickness smaller than factory? A lot if things come into play so rims and tires matter. The whole diameter of the set is what counts as it's the time of complete rotation that counts. Smaller or larger will affect speedometer.
 

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Discussion Starter · #5 ·
What if the rim is much bigger than factory and tire thickness smaller than factory? A lot if things come into play so rims and tires matter. The whole diameter of the set is what counts as it's the time of complete rotation that counts. Smaller or larger will affect speedometer.
The outside diameter of your tire is what matters no matter if you have 15 inch rims or 22’s
Easiest way is to use google n put for example
“265/65/17 specs”and it will give you diameter n width of that size tire. Yes some brands runs smaller or bigger but it gets u in the ball park
I put my oem n aftermarket tires in. Installed the unit in my truck and per 2 gps units I am off 1/10th of a mph at 70 mph
If you go to different forums some guys have to fine tune it. Some will put in what they think is the diameter and it will not be correct so they make adjustments

most tires that say 35/12.00/17are really 34.2 inches in diameter but they put 35 in the program so their speedometer is still off
 

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My experience is it's an experiment to get it right. Get a good GPS speed app and set the cruise in the vehicle. You can usually fine tune it to get the speed just right.

Overall outside diameter of the tire is what you are looking at. Bigger than original will show a faster speed and vice versa if you somehow go smaller. Bigger ordeal if you go really big is shift points and gear ratios too keep your power band in a good spot. That's why most of the big lifted trucks have big tall gear ratios (4.56:1 for example).
 

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My suburban speedometer crapped out right before a road trip. No time to solder new stepper motor on cluster. So I used this. It works very well. Down load app on you phone and sit it where the speedometer goes.
You can also use this to check how far off those big tires are making your speedometer read.
 

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Discussion Starter · #9 ·
I use the Max Speed app. To see how fast and how far I was actually going. It will also track your trip give u elevation n other stuff. Don’t need to tie up my phone anytime or have to remember how far off the speedometer is at a moment’s noyice
 
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