Question About Snow Tire Fitment on ST

[thompsdw]

I have a set of 20" snow tires on rims that used to be on my 2018 Ford Explorer Limited that I would like to install on my 2021 Ford Explorer ST. I still need to verify the offset, but my question is more related to the outside wheel diameter. The 275/45R21 tires have an outside diameter of 30.75". The 255/50R20 snow tires have a outside diameter of 30.04" and the TPMS sensors worked fine on the 18 explorer. The 21's will have 658 revs/mi and the 20's will have 673 revs/mi. I assume the speedo would be off some, but I don't know how much. Is there anything else I should be thinking of other than checking offset? I really don't want to screw anything up on the ST - just thought a post here might help me think of other things that could be wrong using old set of snow tires.

 

[zdubyadubya]

The outer diameter of the tire isn't an issue, but the wheels won't work. The bore size on the last gen explorers and this one aren't the same.

 

[Cdubya]

As above. They might work if they are aftermarket rims with center bore larger than 70.5 that you can use new hub centric rings with. The center bore of the previous Explorer was 63.4mm

 

[thompsdw]

They are aftermarket rims. http://fkwheels.com/wheels/rt-6

 

[zdubyadubya]

Time to whip out a set of calipers and start measuring.

 

[thompsdw]

Agreed. Thanks for the help! So dumb question - the diameter change doesn't matter because the explorer pulls the speedo off the transmission?

 

[TMac]

The diameter change of the tires does matter in the speedo measurement.

 

[Ford Guy]

Question:
How much difference does a 265/65/18 tire/rim make vs the stock 21" rim/tires that come with the ST?

 

[thompsdw]

The diameter change of the tires does matter in the speedo measurement.

It sure did on my F150 and Wrangler when I was playing around with tires. That's why I asked.

 

[TMac]

https://tiresize.com/speedometer-calibration/

 

[thompsdw]

I might be lucky for once. I measured the center bore with snap gauges and calipers and the bore is 72 mm. I also calculated the offset of the wheel to be 35 mm. The factory 21" wheel offset is 37.5 mm. So, adequate bore and a few mm only in offset difference. I think I am good to go as long as the TPMS talks to the car. Look good to the braintrust here?

 

[Cdubya]

Sounds good. I'd probably try to contact the manufacturer of the wheels to find out the exact center bore. I don't think 72mm is a standard. My 18" KMC wheels are 72.6mm and suspect yours might be the same. Just get 72.6-to-70.5 hub centric rings if that's the case.

 

[thompsdw]

Your probably right. To be honest I didn't play with the snap gauges for very long to make sure I was in the widest part of the bore. I was just looking for the 70.5+ number. Can someone explain where you need to use a HCR and when you do not? I know they are optional, but never really understood where you should use them and where not based on how much difference there was.

 

[TMac]

You need to make sure you have the right hub size. Otherwise, you're trusting the wheel studs to keep everything centered. It's a recipe for problems.