Just to put all previous numbers in the shade: Chris Craft, de Cadenet-Lola T380, M4, 1976, approximately 209mph. The car finished third at Le Mans.
Wow. Which part of the M4?
I've always remembered it as being the de Cadenet-Lola in 1976, as Peter wrote, but as Chris Craft told the scurrilous tale in 'Lunch with... Keith Greene and Chris Craft' in Motor Sport it was 1973, in which case it would have been the Duckhams LM, revised with a long tail since its heroic first run at Le Mans in '72 (maybe they did it more than once!). Either way it's a good story...
"We couldn't get the straight-line speed out of the car and I said to Alain, 'We're wasting our time. I'm not going back if we can't get more than 205mph.' So various mods were made, which we needed to try. We decided to run it down the M4 at night, but the headlights weren't fitted at this point, so we had to wait until dawn was breaking. We unloaded the car on the roundabout over Junction 16, and sent the van 13 miles down to Junction 17. Then off I went. I pulled 7200rpm, which I knew was 213mph, so that was all right. I peeled off at Junction 17, the van was waiting on the roundabout, and we stuffed the car in quick and went back to London. Unfortunately at full chat I'd had to go between two lorries, one was pulling out to pass the other and I couldn't lift off. So it got reported to the police. But we never heard anything more."
http://www.motorspor...ene-chris-craft