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You are running wrong size of tires.
You need to keep the front and rear at the same or very close overall diameter.
To keep your speedo and odometer correct you need:
F225/30R20
R255/25/20
Where would you even find 255/25-20? I don't think anyone makes them.
Would love anyone's insight for my situation as well. The Niche wheels I'm eyeing are 8.5" front, 10" rear. I imagine I'd be fine with 225/35/20 F, 255/30/20 R? If not, or if there is a better combination, i'm all ears! My main concern with width is fender rubbing, as I'll be lowering quite a lot on coilovers.
Sorry, but no, the correct solution has been already suggested. Change the fronts to 225/35.
No that's not the correct size.
OEM 225/40R18 is overall diameter is 25.09 in
225/35R20 is 26.2".
The new size is 1" bigger and 4.44% difference.
Anything over 3% is consider wrong size.
The speedo and odo is reading wrong.
Here is a very helpful link. http://www.1010tires.com/Tools/Tire-Size-Calculator
Of course if you just want something to roll around with then anything the same overall diameter would work.
But no 225/35R20 is not the correct size.