tl;dr: My Prius unpredictably idles when it shouldn't, this is a relatively new behavior, and I have mild autism. To be clear: this thread is not about the EV mode button, but rather the ICE engaging or dis-engaging as shown in the HV indicator. Any mention of EV mode refers to ICE stopped while in motion. First, I'm going to define normal behavior. Around 73mph, the ICE will kick on to protect MG1. It won't necessarily consume fuel to do so, if for example we have gravity available to avoid high current draw from the traction battery. For some reason, cruise control alters this, forcing ICE to spin at speeds above 63mph, more often with fuel consumption. I find this weird, but since purchasing my '17 Two Eco, it has been consistent. I am measuring fuel consumption using a ScanGauge, and verifying that it's reading actual fuel consumption by taking measurements at the injectors themselves. What has NOT been consistent, however, is the behavior I have experienced on and off for the past 5k miles or so. (I'm at 51k as I type this) At some point the system decided that consistent behavior was over-rated, and EV mode went from 73 down to 62 without CC active, and with CC active any speed over 35 required the ICE to spin, ALWAYS with fuel consumption. It did this for 2, perhaps 3 weeks, then returned to normal. While it was doing this, it was still switching to battery power when appropriate, and the ICE would spin ~990 rpms, and consume around 0.2 GPH, effectively idling. I've seen no degradation of battery power, based upon voltage readings, amperage draw readings, etc, and the engine is always hot (>80c) when this even becomes an issue, so i doubt that I'm not in at least stage 3 or 4. Things that I thought might have an impact: I had an unrepairable hole in one tire, and due to budgetary constraints, replaced it with a used tire of a different size (all that was available at the time). This tire is a 195/60-15, whereas the stock are 195/65-15, and it was rotated to the rear to avoid ratio grind in the differential. I would think that if perhaps THIS was the problem, then I would get some kind of ABS error, or some kind of code at least, but no codes. Before anyone asks: I'm getting new tires as soon as the budget allows. I skipped the 45k service, as the dealership stated they only change the oil at 10k intervals anyway, and I'm not keen on paying for just a tire rotation when I don't need service otherwise. I reset the service gauge myself, and thought that might have something to do with it, but the problem persisted after the 50k service. Around the time this started, I began towing a trailer on a regular basis. I thought that perhaps the added load was causing the computer to go into some different mode, but this too has outlasted the behavior, and the last time I towed, I had the CC set at 60, and while the load causes it to go into EV mode less, it does still go into EV mode on downhill areas. As an aside, this next bit HAS been consistent as long as I've owned my Prius: When decelerating, and the traction battery reaches a high state of charge while still decelerating, the ICE will kick on. Depending, I think, on pack temperature, it may do this around 60, or 70 SoC, but it definitely does it above 80 SoC. When it does it at lower SoC, it merely idles the ICE, using fuel, instead of putting it into B mode automatically. I really do not see the purpose in this. I CAN see a purpose behind using the ICE in B mode to reduce battery stress, but this is not being done if the ICE is spinning at idle on fuel. And why not one more gripe for good measure: B mode is extremely difficult to control. It almost always has too much braking, and controlling it via the accelerator pedal IS possible, as in it was an actual intended feature, but the sensitivity is JACKED, even with the drive mode in ECO.