If you break it down by mm, I come up with 110 for the front and 118 for the rear.
110 is 93.22% of 118
What I can't remember is if the 3% number is diameter or sidewall height.
I do know that seriously uneven treadwear has cooked some local LEO PTUs, so I'd say there's not a ton of wiggle room. Apparently this is pretty common...
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The PTU is not a viscous coupling. Its basically a ring and pinion bolted to the transaxle. The viscous coupling is attached to the rear axle.
To go along with fordtech1's uneven tire wear comment I have seen uneven tire wear (front to rear) prematurely break down the lubricant on utility interceptors in as low as 30,000 miles.
I just wish they had a higher lube capacity. Its only .56 quarts! I'm no engineer, but I feel a higher capacity may help. It won't help if the rear tires are bald and the fronts are new though."http://www.blueovalforums.com/forums/index.php?/topic/59381-ptu-issues-possible-long-term-solutions/You didn't by chance exceed 100 MPH when it happened did you? Both SHO's liked to shutdown AWD when I cross 100 at the track.
The easiest solution if it continues is put the stockers back on and see if it stops, I'm afraid that id the only definite answer you will get.
If you take it in I'd suggest having them call the hotline regardless of what the advisor says, maybe you can get an official answer from them.