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Assuming you're not foot to the floor on the motorway, I would presume the stop/start is worse.
Oh, except you said M25, which is a fucking carpark and will be stop/start anyway :)
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The simple answer is whichever one uses more petrol. Probably the 50 miles in my experience.
Edit: found some random ballpark numbers. Assuming idling consumes 0.6l/hr and your efficiency is 0.1l/km (~28 mpg) on the short route, and 0.05l/km (~56 mpg) on the long route, and you're idling 50% of the time on the short route, you're consuming 3.2l + 0.6l per 1 hour journey on the short route and 4l on the long route. So that's about the same in each case. I don't know how efficient your car is though.
If you've got a little computer in the car you could do both journeys at typical traffic conditions and try to record the fuel use in each case and report back.
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hippy
frankenbike
@abr
What's worse for the environment: driving 20 miles on the South Circular (low speed, lots of stopping), or 50 miles on the M25 to reach the same destination?
M25 better just to keep the fumes out of the city?