have searched the net and am not satisfied with any answer I have found to calculate the following:- Here is an example just for reference. Suppose I can accelerate my car from 0-100km/h in 10s. That means my acceleration=2.78m/s/s. Say at 7000Rpm it hits 100km/h in the first gear itself. Now, while I am still very much in gear I lift my leg off the accelerator (no clutch is pressed). It will begin to decelerate, till it hits 1000Rpm and keeps that constant rolling pace of 14km/h. I want to find out the rate at which deceleration takes place? I want you to keep drag/aerodynamics/friction (air & road) etc etc as any normal accepted values, no preciseness required over there. Just need an approximate idea of how I could calculate the rate at which speed drops.

  • $\begingroup$ Why would you down vote a question without even a reason? $\endgroup$ – user3833732 Dec 29 '15 at 16:40
  • $\begingroup$ Just a note: if you accelerate to 100 km/h in 10 s, your acceleration would be more like 2.78 m/s/s, not 10 m/s/s. $\endgroup$ – NeutronStar Dec 29 '15 at 16:49
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    $\begingroup$ Only a measurement is going to tell you this. Too many factors like braking on engine, air drag and rolling resistance that are poorly known to make a reasonable estimate, IMHO. $\endgroup$ – Gert Dec 29 '15 at 16:50
  • $\begingroup$ Sorry @Joshua you are right.. Over sight while trying to exhibit an easy example! ;-) $\endgroup$ – user3833732 Dec 29 '15 at 18:38
  • $\begingroup$ Yes Gert you are right, but still a car rolling in neutral slows down at a lesser rate than the same car/speed when made to decelerate with still being in gear without the accelerator being pressed. In both cases the forces you have mentioned exists and act upon the same car. But I'm looking at some sort of logic or way of calculating that engine Breaking while the car is still in gear, that makes it slow down faster than the one in neutral, get what i mean? $\endgroup$ – user3833732 Dec 29 '15 at 19:06

It's easy to measure this by bringing your car to top speed, putting it into idle, and then watching the speed over time as the car decelerates. Here are some simple instructions, which will yield your deceleration rate at any given speed. Then, choose at what speeds you'll shift and how long it will take, and you'll have your answers.

  • $\begingroup$ Yes! This is definitely the best answer for drag etc. Very comprehensive. But here is where my question differs. Please see the two senarios. 1)Reaching 100km/h and then rolling in neutral and 2)Reaching 100km/h and then no more accelerator but staying in gear. In 1) this deceleration is due to drag/friction etc etc. In 2) we will slow down faster, cause besides the above forces slowing us down like in 1, there is also some kinda engine breakage. So I'm looking for a logic to approximate/calculate that. Hope I'm getting through $\endgroup$ – user3833732 Dec 29 '15 at 18:54
  • $\begingroup$ Well, measuring this is really going to be the best way to find it out. There are way too many variables to just calculate it. $\endgroup$ – Daniel Griscom Dec 29 '15 at 19:38
  • $\begingroup$ Could you measure the distance traveled in both scenarios? And then compare the two? $\endgroup$ – Žarko Tomičić Dec 29 '15 at 21:11
  • $\begingroup$ Haha, Zarko, that is exactly what I finally need, hence I'm first trying to get deceleration before I get displacement! $\endgroup$ – user3833732 Dec 29 '15 at 23:47

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