Lead Foot 15G
Keep the needle above 80mph for more than ten seconds while driving.

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Most cars you see in L.A. Noire will push it to 80mph with a good enough run, but the best ones to stick with are the police cars due to the sirens. The only issue is, there is nowhere on the HUD that tells you your current speed, but if you just hold RT But down you'll reach it in no time. You can get this achievement at any point while you're driving around, so simply get in a car and give it a go.

You'll need to make sure you have a nice straight road (which there are plenty of - 2nd St for example) and a fast enough car. It'll take a short time to get to 80mph as the cars are from the 1940s, but once you think you've hit the peak, just keep driving straight and try not to crash or weave. Again, grab a police car and put on the siren as this will increase you chances of not crashing as the pedestrian cars will pull over out of your way.