As I recall, less than ideal conditions. 100+ degrees. Faster than all their benchmark cars even in the heat.
Fastest Production Car Lap Times at Big Willow
2018 McLaren 720S 1:21.75 sec
2018 Lamborghini Huracán Performante (Euro-spec) 1:22.53 sec
2015 Porsche 918 Spyder 1:23.54 sec
2017 Ford GT 1:23.69 sec
2017 Porsche 911 Turbo S 1:24.26 sec
2016 McLaren 675LT 1:24.29 sec
2015 Chevrolet Corvette Z06 (Z07, 6M) 1:25.00 sec
2014 Lamborghini Huracán LP 610-4 1:25.17 sec
2016 Lamborghini Aventador LP 750-4 Superveloce 1:25.42 sec
2015 Nissan GT-R NISMO 1:25.70 sec
2015 Chevrolet Corvette Z06 (Z07, 8A) 1:25.76 sec
2015 McLaren 650S Spyder 1:25.88 sec
http://www.motortrend.com/cars/ford/gt/2017/2017-ford-gt-first-test-review/
Fastest Production Car Lap Times at Big Willow
2018 McLaren 720S 1:21.75 sec
2018 Lamborghini Huracán Performante (Euro-spec) 1:22.53 sec
2015 Porsche 918 Spyder 1:23.54 sec
2017 Ford GT 1:23.69 sec
2017 Porsche 911 Turbo S 1:24.26 sec
2016 McLaren 675LT 1:24.29 sec
2015 Chevrolet Corvette Z06 (Z07, 6M) 1:25.00 sec
2014 Lamborghini Huracán LP 610-4 1:25.17 sec
2016 Lamborghini Aventador LP 750-4 Superveloce 1:25.42 sec
2015 Nissan GT-R NISMO 1:25.70 sec
2015 Chevrolet Corvette Z06 (Z07, 8A) 1:25.76 sec
2015 McLaren 650S Spyder 1:25.88 sec
http://www.motortrend.com/cars/ford/gt/2017/2017-ford-gt-first-test-review/