Today I bought another Prius (a white 2007 touring package with 16" tires) for my gf and my coworker (professional test driver) and I drove it to her place - 25 miles away. Both of us were driving at the same speed (not close to each other to avoid drafting) 55-65 mph and we were using the same accessories. The Prius that I got for her had more than $3k in recent receipts (all services at the dealership) and recent battery for the motor replacement. It rolls on stock (for touring package) 16" wheels and it's a 2007 model with 140k miles. (Notice the beautiful long spoiler) My car is 2008 with 130k miles and had a new 12v battery - since it's a California car the batteries should be still under warranty. I have 17x7.5 wheels with (oem spec Michelin 215 tires - same as the forge package of the 3rd gen). I typically don't care about my mpg but this time I wanted to check my mpg against the new vehicle. After 29 min /25 miles of mostly fwy driving my car returned 43.1 mpg while her car was at 49.1 mpg- again same driving style and synchronized accessory use. My question is if you think that my car needs new batteries or if you think that the decreased mpg was due to the wider tires and heavier wheel/tire combo. I'm obviously going to investigate this further / will drive for a week the car with the old tires vs the new tires and see what's the difference but I just wanted to hear some opinions. I would also like to add that her car was clearly pulling away during a "drag racing" that we did with my friend/coworker - I guess that was due to the lighter wheels and tires. iPhone ?
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.
A little bit more info: Touring model wheel weight 36.2 lbs (17.2 wheel - 19 for the tire) My wheels weight 42.8 lbs The difference is 6.6 lbs The difference in diameter between 195/55-16 and 215/45-17 is just 0.6% Both of us were traveling at the same speed without cruise control / if one car was driving a bit slower that was the car which got the worst mpg. Ken who drove that car is a Prius aficionado for the last 10 years / has two of them and constantly getting above 50 mpg when he drives his cars. Question: is there any way for me to test if there are any bad cells in my battery? My car is a California car and is covered by 150k miles or 10 years warranty that hasn't expired yet. iPhone ?
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.
