Percival Guy Tunmer was born in Fricksburg in the Transvaal. He started racing in saloon cars before entering the Springbok series in 1973. Driving with Martin Birrane in a Porsche RSR, they came 10th in the 9 hour race at Kyalami and 6th at Killarney, Cape Town. However the series ended after those two races due to the Middle East oil crisis and never was restarted. He won a non championship round of the World Sports Car Championship, the 2 hour race at Luanda in Angola, driving a Chevron. At the same time Guy was running the an ex-Peterson March 712 F2 machine in the B class of the local F1 series.
In 1974, he acquired a Chevron and entered the domestic Formula Atlantic championship and proceeded to dominate the proceedings, taking 9 poles and 9 wins from his 9 starts.
In 1975 Tunmer was signed up by South Africa's premier team, Gunston, to partner Keizan in its two-car Lotus 72E team contesting the last South African championship to be run to F1 regulations.
Team Gunston also entered the South African GP, with Keizan qualifying 22nd and Tunmer 25th. In the race, Tunmer passed both Keizan and works Lotus driver Ickx, eventually finishing 11th, just behind Ronnie Peterson.
Later that year at the Spanish Grand Prix at Barcelona when the GPDA announced a strike due to poor safety standards at Montjuich Park, Bernie Ecclestone offered Guy a drive in the Brabham BT44B. He rushed back to his hotel to get his helmet and overalls but by the time he returned to the paddock the strike was over as was Guy's F1 career. In July he did win an F1 race, the False Bay 100 at Killarney, driving the aging Lotus 72.
For 1976 Tunmer raced his Formula Atlantic Chevron B43 again. After starting the season well his form dropped off and he retired from racing shortly after to concentrate on his business involved in training and development for motor industry clients.
He was killed in a motorcycle accident in his native South Africa in 1999.