43 vs. 49 mpg synchronized mpg testing with another Prius

31 miles of data is not enough to form a conclusion. There are still many variables in play, despite the make and model of the car being the same.

 

Wheels/tires - Pressure, tread depth, rubber compound, wheel weight, overall diameter
Driving style - Even the same person in 2 cars would have trouble being consistent
Car condition - It's more than just a battery. Compression, filters, oil, gas are a few variables. The O2 sensor is a big one that can effect mileage. Despite having similar miles the drive-trains can be in very different states. There are also things like wheel bearings and brakes that could potentially create different resistances between the cars.

And ideal test would be you driving each car on a pre-determined route on the same day. You would need to use cruise control and do several passes to come up with an average MPG (tank and computer calculated) for each car.

 

The larger wheels with lower profile tyres has a considerable affect on mpg because the tyres have a higher rolling resistance. Even if your tyres are the same make and type the rolling resistance will not be the same. Tyre pressures will also have a bearing as the wider tyres will have a greater footprint area. This means even at the same pressure the wider tyres will carry less weight per square inch of contact than the narrow tyres.
Your 215/45/17 tyres are larger in diameter than the stock gen2 touring tyres sized at 195/55/16. This means your wheels will go round the same number of turns to register 1 mile, but the car will have traveled further than this using more fuel for the extra distance. The MFD mpg calculation is done for a set number of turns of the wheels for 1 mile and the fuel used for that number of turns.
Barber0015 is correct you have no where near enough data in 31 miles to compare considering the variables. Your car may well be using more fuel, but you do not have 2 "identical" cars and drivers to compare.

John

 

Just swap out the tires from one car to another and retest. See what the 'cause and effect' scenario will bring

 

I feel like the MFD can be wrong sometimes...you can be going 55 MPG, stop for gas, and now it shows 45 MPG.

 

Imagine a flat road with cruise control set at 65 mph in car 1. Then you drive car 2 to match. The car with CC will likely use less fuel since you will speed up and down unnecessarily.

For example, I will always get better fuel economy than my wife averaging the same speed. This is because I keep the throttle above the regen line on the HSD and rarely use brakes. Or in hypermiling terms Driving Without Brakes (DWB) Watching my wife drive, she does a pump and release on the freeway. Its imperceptible by feel but with the display, I can see her wasting momentum.

 

absolutely. but you can't get warranty until your dash lights up.

 

A bit too short of a test and really to be exact on short distance you would have to have an elaborate setup such as the ability to weigh the car before and after with the weights exact down the gram to calculate fuel use.

If the HV was charged fully on one car and low on the other, this might have effected the ability of the engine to shut off going down the highway on one car.

 

BTW, it may be the other battery.

Some failure modes of the 12 volt battery short a cell, yielding about 10 volts, still enough to power the computers.

The inverter will work constantly to 'recharge' that shorted cell which is electric power wasted, which is gas power wasted. Since all power comes from the engine eventually, that is lower MPG over time.

You can test the 12 battery in the morning before starting and ought to get about 12.5 volts. If it is closer to 10 volts, consider a new 12 volt battery.