105 Percent Rule Confusing

How many of you out there found the INDY Car 105% rule confusing?   Am I the only one?

We were all taught in grade school that 100%  is all there is.  You can have a glass of milk that is anywhere from 1% to 99% full and still have room for more. But once it reaches 100% full there is no more you can add.  I eat 100% of a raspberry pie, guess what you aren’t getting any.  There is none left.

Because the 105% mostly applies to the slower cars in the race, one would have to assume the slowest cars on the track would have to run 105%, of the lead car.  Example say the leader has a lap speed of 220 mph.  This would mean the slowest car, in order to maintain the 105% rule would have to be traveling a speed of 231 mph. (220=100%)  (22o x .05 [ 5%]  = 231.) That would be 5% faster than the leader.  WAIT A MINUTE !!!! If the slowest car is running 11 mph faster than the leader then the leader can not be leading, at least not for long.  Now I realize we aren’t all engineers, Thus the 105% rule was not meant to be understood by us mere mortals.

Now, let me put it this way.  And I think this really makes more sense, as the apparent reason behind the rule is to keep the slower cars in a safer operating range with the average race speed.   So would it not make more sense to say…call it a 95%  RULE.

Again let us use the 220 mph model as before.  220 x .95 (95%) = 209.  An 11 mph  difference.  Slower still but you must admit more obtainable for the LOTUS than 231 mph.

Like I have said before I am no mathematician.  But 105% can’t make sense in any ones mind when it comes to racing speed (except an engineer).  Especially when you are talking of slower cars.  But a 95% rule holds water.

Is there anyone out there who can explain a car going 105% faster that the fastest car (let alone the average speed) can be the slowest car.

I can see the government taxing us 105% by the time all taxes are applied to my dollar.  Perhaps this is how INDY Racing came up with that percentage.

That Is How I See It


  1. Hi Wayne,

    I think your confusion comes from the fact that you’re applying the 105% to the wrong driver. You shouldn’t be applying it to the leader, but to the follower. In other words, what that rule says is that if you’re not the leader of the race, then the LEADER cannot be 105% faster than YOU. Do you see the difference? the 105% should be applied to your speed, not the leader’s – because, obviously, the leader is the LEADER – if you’re going at 105% speed of the leader, then you’re faster than the leader, right?

    Now, mathematically, applying 105% to your speed to calculate the threshold is NOT the same as taking 95% of the leader’s speed. The difference is tiny, but it’s there. Here is the math: in your example, if the leader’s speed is 220mph, then by taking 95% of it, you get 209 mph (11 mph difference just as you correctly calculated). Now, according to the rule, if the leader is going at 220 mph, you need to calculate the speed of the follower, which if increased by 5% (i.e. taking the 105% of it) would equal the speed of the leader. Meaning:

    speed of follower * 1.05 (105%) = speed of leader (220 mph). Therefore,
    speed of follower = 220 / 1.05 = 209.52

    So, what this means is that the follower’s speed has to be at least 209.52 mph, not 209 mph. That 0.52 may not seem a lot, but it does add up when you consider the fact that the race can go on for hours (i.e. the slowest car will be lapped by the faster cars and may even cause potentially hazardous situation on the track).

    Hope this helps.


    1. So,here is an example of two different views,which result in the same end. We come to the same conclusion. I still feel more people would find my 95% rule more easily understood. But I am just a common sense guy and not an engineer or mathematician. Thank you for your explanation. I can see your point. HOWEVER. Only the slow cars are affected. Agreed?

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