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Hi all. Taking in all I can at this point with my new (to me) Jeep. So, as I have been going over details of my Jeep - I stumbled across getting the build sheet for it.

One thing I did notice is that the build sheet says my Jeep came with 255/70R17 but I have the wheels I typically see on the Sahara edition at 255/70R18.

I know the difference is minimal (only 3mph less at 65mph on Speedo) - but is there an easy way to tell?
 

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Or maybe test it with another car you are sure it's speedo is accurate?

In any case, would the wheel size really matter? The tire size is the same.
 

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Sahara or Sport wheels will make no difference on your speedometer. Wheel size doesn't matter. Tire size is what changes it.

Ways to check your speedometer:

Download a speedometer/GPS app on your phone and compare at different speeds.

Drive past one of those speedometer signs on the side of the road and compare.
 

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I'll check using the GPS method - I did not even think about that at all... I had seen this website that said the tires are not the same size - slightly bigger so that's why I asked.
 

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I'll check using the GPS method - I did not even think about that at all... I had seen this website that said the tires are not the same size - slightly bigger so that's why I asked.
It's showing bigger because the sport size tire entered is incorrect.

Sport 255x75r17
Sahara 255x70r18

Both are 32x10.
 

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>snip>
In any case, would the wheel size really matter? The tire size is the same.
>snip<
Wheel size doesn't matter. Tire size is what changes it.
>snip<
Wheel size does matter. The tire size tells you the profile height. 255-70R will give the same sidewall height whether on 17" or 18" wheels. But mounted on different size wheels will alter the diameter of the circle.
 

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>snip>
In any case, would the wheel size really matter? The tire size is the same.
>snip<
Wheel size doesn't matter. Tire size is what changes it.
>snip<
Wheel size does matter. The tire size tells you the profile height. 255-70R will give the same sidewall height whether on 17" or 18" wheels. But mounted on different size wheels will alter the diameter of the circle.
Stock Sport wheels are 255x75r17
Stock Sahara wheels are 255x70r18.

Both are 32x10. Wheel size does not matter when comparing stock Sport vs Sahara because they are the same size just a different wheel size.
 

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Stock Sport wheels are 255x75r17
Stock Sahara wheels are 255x70r18.

Both are 32x10. Wheel size does not matter when comparing stock Sport vs Sahara because they are the same size just a different wheel size.
I responded to the OP that stated both tyres were 255-70R and he was questioning between 17" or 18" wheels. Whether he was correct or not about OEM sizes was not in my response, just the fact that the same profile tyres on different sized OEM wheels would result in different circumferences. Mebbe we are talking about different things but the wheels make a difference when considering tyre size.
 

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I am not sure about the stock tires, but the same size tires on different size rims even the same model from the same manufacturer often have slightly different diameters/ circumferences.
 

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Get on an interstate. Set the cruise control for exactly 60mph. When you pass the first mile marker look at the second hand on your watch. If your speedometer is accurate, you should pass the next mile marker in 60 seconds. For more accuracy, check if you pass 10 mile markers in 10 minutes. If you're early to the marker, your speedometer is reading low. If you're late to the marker, your speedometer is high.

Whatever any differential is in seconds between your watch and your theoretical OTA in seconds, divide the error by 60 (for 1 mile) or 600 (for 10 miles) and you will get the error %. Multiply the error % times 60 and you will see how many mph you are off.
 

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Get on an interstate. Set the cruise control for exactly 60mph. When you pass the first mile marker look at the second hand on your watch. If your speedometer is accurate, you should pass the next mile marker in 60 seconds. For more accuracy, check if you pass 10 mile markers in 10 minutes. If you're early to the marker, your speedometer is reading low. If you're late to the marker, your speedometer is high.

Whatever any differential is in seconds between your watch and your theoretical OTA in seconds, divide the error by 60 (for 1 mile) or 600 (for 10 miles) and you will get the error %. Multiply the error % times 60 and you will see how many mph you are off.
Ahh physics who would've thought...

Sent from my Nexus 6P using Tapatalk
 

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That's what I did in the days before GPS (but at 120kph with km markers).
Now in LA most of the time I'd just be happy to travel at 60 mph for one mile.

For the OP: GPS app on your smart phone. Those speed sign things are usually very accurate too.
 
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