EV Experiments

Our new Toyota Rav4 Prime lets you manually control driving modes.

This is how I've been visualizing a single trip in our new car. The trip is 115 miles in total distance, with approximately 94 highway miles and 21 local miles. We make this trip about once a week and usually, we start our drive with full or near-full EV charge, with the objective of using up all our EV charge by the end of the trip and as little gas as possible.

On Auto EV/HV mode, the car will run the EV battery down before switching to HV mode for the rest of the trip. Some efficiency is obviously left on the table here because the car does not know the structure of our trip (Local -- Highway -- Local). With knowledge that EV is most efficient on local roads and gas engines most efficient for highway driving, at minimum, we could optimize this trip slightly by "saving" EV miles for the local stretch toward the end of our trip. And that is what is pictured above. We saved an estimated 11 miles EV range to complete the final 18 miles local journey, and unsurprisingly, we fell short about 4 miles.

The big mystery, however, is the Charge Mode feature. In HV Mode, the car sometimes uses the battery, sometimes charges the battery, with the end result that the battery charge level stays the same. In Charge Mode the gas engine runs continuously with the result that the battery is slowly charged up. The astonishing thing to us was that, less than an hour of highway driving could charge the battery up nearly halfway, something that would take us 6 hours to do on our 120 V home outlet. Could Charge Mode also be used strategically to save gas over normal HV driving?

We started off with only about 80% charge, and we ran the battery dry (including about 31 miles of highway driving), switched to Charge Mode, charged until we thought we had enough battery reserve to complete the trip. The structure of our trip was such that Charge Mode was only ever used on the highway, driving around 75 miles per hour. The data, to aid in comparison with the previous result, is from the same driver, driving the precisely same route, but we didn't control for AC usage.

In the first case, using HV mode for most of the highway driving, we totaled 47.5 miles of EV only driving, for a total MPG (dashboard end-of-trip report) of 64.7, covering 115 miles. This calculates out to a gas usage of 1.8 gallons. Assuming that no gas was used during the EV portions, then the total distance traveled in HV mode was 67.5 miles, implying a HV Mode Fuel Efficiency of 38 MPG.

In the second case, the only instance where gas was used was in Charge Mode, so we could compute from the reported 55 MPG (2 gallons of gas), and 56 miles driven in Charge mode, a Charge Mode Fuel Efficiency of 28 MPG. This appears much lower than the HV Mode Fuel Efficiency above, however, if we add in the 20 EV miles we charged up on the battery, the Effective Charge Mode Fuel Efficiency is actually 38 MPG, same as above.

This would suggest the possibility that Charge Mode could be as efficient, if not more efficient, than HV mode when used thoughtfully.

But the comparison, aside from being just a single trip in each mode, is limited in other ways. We hadn't charged up the car fully before starting the second trip, so we couldn't make a direct gallons-to-gallons comparison. We didn't actually complete our trip before the EV range ran out (fell short 5 miles), so chose to pull over and record the data on only a 110 mile trip. The last 5 miles are typically the most fuel-efficient miles in our journey because of rolling country roads and 30 MPH speed limit, so there's another inconsistency here. We don't have a gas station near our place in Maine, and there is some inconsistency in the fill level at which gas stations "click", so we didn't try to directly "measure" the gas spent, choosing, instead to go off the dash MPG display at the end of a trip. Some suggest that these displays are bias a little higher MPG, but assuming that the bias is generally consistent from trip to trip, we can still get comparative results.

Hoping to do an update at some future time once we have a lot of data under our belt to be able to say something statistical.