Honda were aiming to set an official record for an F1 car above 400 kph. They achieved 400.454 kph on one pass of the measured mile at Bonneville but weren't able to repeat it on the return leg. The official records they set were 397.360kph (246.908mph) for the flying mile and 397.481kph (246.983mph) for the flying kilometer.
Honda actually weren't too keen on the project and did not provide their tip top engine for fear of the embarassment of having it fail. The cars were developed to run with almost closed off side pods & many of the aero downforce elements removed in order to reduce drag. They also developed a hydraulically operated vertical fin to assist the stability at high speed, but I'm not sure if they used it. (Not strictly to F1 regs.) I believe it had to run on intermediate tyres because of lack of downforce & grip on the salt surface. The South African van de Merwe was the driver.