Cold Start vs. RPMs in Neutral vs. Efficiency
 

When I am driving at speed and see a red light or traffic ahead or anything else I don't need to keep my speed for, I put the transmission in neutral. This lowers the RPMs, so I *think* I am saving fuel (still waiting on my Ultragauge to confirm/perfect). The problem is when the engine is still cold, the RPMs go up when I put it into neutral. At first I thought that I don't want to let that happen, but then I was thinking would it be beneficial for me, because the higher RPM will heat the ICE up faster, making it more efficient, sooner. I'm hoping my Ultragauge will help me determine this, but was wondering what you guys thought.

Thanks!

Your car is still on fast idle because it is not up to operating temp yet. That's why the idle RPM is higher when it's not in gear. If it is an automatic, leave it in drive. The transmission load will generate heat and take it off fast idle sooner. Either way the fuel saved will be minimal.

I used to put my car back in gear when I didn't need speed to allow for engine braking, but after I got my gauge I realised that it used more fuel to have the car in gear engine braking than in neutral using the brakes when needed. I did this testing with the car warmed up, I am not sure about the car being cold. I no longer use engine braking, but your car may be different.

Generally when slowing the engine should go into DFCO when warm, basically the injectors cut off and the car's momentum turns the engine over - 9999MPG on a gauge.

Some engines have a minimum RPM for DFCO (my Toyota Aygo does this only when the RPM is over 1400). Being in gear means you slow more so sometimes (to maintain momentum) you are better being in neutral and on the idle program - or (if brave) you select neutral and shut the engine off.