I just heard Tom and Ray say that using the AC to slow the car probably won’t work due to the way car engine idle speeds are regulated today. I use it in my 1993 Civic and my experience says otherwise.
There are a number of reasons to use this practice YEAR ROUND. Read on for more info…
I recall my father using this trick on our 1971 Ford wagon as we descended Pike’s Peak in CO and, of course, it worked well at that time. I started doing it in my first car with working AC (that had not lost it’s charge by the time I bought it!) which is my present car, a 1993 Honda Civic VX hatchback that I purchased in 2001.
I can report that it works well to slow the car - slightly but noticeably deceleration occurs .
The other benefits of this practice: 1) less wear and tear on brakes; 2) when done in summer, you get free AC (I call this “poor man’s regenerative braking”); 3) when done in winter, this exercises the AC system, which is VERY IMPORTANT if you want the AC coolant NOT to leak out of the system over time (see below - I will explain how this works), and; 4) this practice can keep the car from getting away from you and lessen the likelihood of brakes overheating on steep hills.
I have heard one reason why this may not be a good idea - all that on/off cycling of the AC clutch might wear it out sooner. This does not seem to be an issue: the AC clutch will cycle every 20 to 30 seconds anyway so I am not giving it that much more of a workout than it would have had under normal usage. In my case, the car has 266K on it and the AC has never had a repair - not even a recharge after 18 years!
Which gets me back to the point above about exercising the AC system regularly to keep it working properly: your owner’s manual will tell you that you should run the AC for at least 10 minutes a week to keep it working- specifically to keep the coolant from leaking out. What happens is that the shaft seal (where the mechanical drive is attached to the flywheel pulley) will dry out if you do not circulate the coolant (once a week or so for 10 minutes), which is mixed with lubricating oil to keep the shaft seal flexible. It’s really nice to have to your AC working in the spring and not have an expensive repair bill.
(An aside, you will never have this problem with an old window AC, refrigerator, freezer, etc because their refrigerant systems are completely sealed with no moving parts - so you can store them for 20 years and then plug them in and they still work fine).
Since I am also a geeky physicist who hates to waste energy (my occupation), I will simply run my AC when descending hills (there are several on my regular commutes) during BOTH summer and winter: it is free all the time, I get AC in summer and I preserve the life of the system in winter.
One more thing: the engine in my Civic is small - so it really is slow to accelerate when the AC is on. Therefore, in summer, after slowing the car with the AC and often sitting at the light or stop sign with it on, I will turn it off to accelerate again and then, when I reach speed, if it is hot enough, I will turn the AC back on again as needed. This practice not only makes the car’s acceleration much better but it also increases gas mileage (slightly) while hardly affecting AC effectiveness.
So try this in your car and see if it slows your car down at all.
And even if it does not slow the car, you are still extending your AC life.