392mm is looking good so far, and puts the speedo dead on accurate without a Check Engine Light. Here's the email I sent to Gap.
Hi Patrick & Team,
Updating on the tire radius size issue from yesterday. I have a 100mi daily commute which makes testing go quickly.
My working theory is the ECU tolerates up to an 8% variance from expectation.
Original value for 2009 US Spec Range Rover Sport with 255/55/19 wheels is 364mm radius.
My new tires are Cooper Discoverer AT3 in 275/65/18: the manufacturer publishes an "actual" diameter of 32.09" = 815mm dia = 407.5mm radius. Below are the results of my testing:
Set Value: 404mm || Result: CEL On || Variance: 110.9%
Set Value: 384mm || Result: CEL Off || Variance: 105.5%
Set Value: 394mm || Result: CEL On || Variance: 108.2%
Set Value: 390mm || Result: CEL Off || Variance: 107.1%
Set Value: 392mm || Result: CEL Off || Variance: 107.7%
In the CEL On conditions above, the CEL lit within 2-10 miles of driving. So far I have about 40 miles of city and highway driving on 392mm and the test for speed sensor is complete, and there is no pending or confirmed code. I managed to hit 90mph, GPS confirmed, in one empty stretch to confirm speedometer accuracy and see if variance at higher speeds mattered. I'm feeling pretty confident about the 8% threshold but I'll let you know if this changes.
Hope that helps.
Chad // Instagram: @AdvRovr
2009 Range Rover Sport // 2007 Mercedes GL320 Diesel // 2001 BMW 330Ci // 1996 Triumph Tiger 900
"Can you make it work?" - "Made by the English...let's not get our hopes up."