Actually, if you consider that all that`s pulling it down at the apex is its weight, the speed in which it needs to be to complete the loop is the square root of (radius of loop times gravity). Not considering the reaction of the lane is a practical way to find the lower limit speed.

If that loop is 10 meters across and gravity is 10 meters per second squared, it only needs to go at 10 meters per second, which is, roughly, 20 mph.

And I`m actually rounding up the math with those figures...

Revising my math here: turns out that, if the driver turns the car off right as it starts climbing, it needs to be at a tad slower than 50 mph to go all the way around, not considering speed loss due to friction.

As such, it seems far too easy to do these loops in any half-decent car. Get a big enough track, set any number of loops in it and a Civic could probably up that record by a big margin.

- Top Gear Stunt Team has set a new world record by becoming the first to complete a double loop-the-loop on four wheels!!
If that loop is 10 meters across and gravity is 10 meters per second squared, it only needs to go at 10 meters per second, which is, roughly, 20 mph.

And I`m actually rounding up the math with those figures...

As such, it seems far too easy to do these loops in any half-decent car. Get a big enough track, set any number of loops in it and a Civic could probably up that record by a big margin.