Less Regen Miles with 18-inch Tires?

[hybridbear]

In the first few hundred miles on the new FFH I have found that on common trips that I drove many times in the old car and have now driven multiple times in the new car I am seeing a big variation in regen miles. The trip to & from my parents' house always previously saw 1.2 or 1.3 regen miles. Now I'm seeing only .8 regen miles on that trip. Another common trip that before always had .8 regen miles now shows only .5 regen miles. That's a 30-40% drop approximately. A 30-40% drop in regen miles would have a big impact on fuel economy. Is the rolling resistance of the 18-inch tires really that much greater than the 17-inch tires??

[ptjones]

Just a thought, I was wondering if anyone was connected with the Prius Site and see if they had any info on tire size and brand that got the best MPG's? :)

 

Paul

[kuzzi]

Contact patch may be slightly larger at the tangent apex where the wheel meets the road, but it shouldn't be wider than the 17's should it?

 

Larger wheel means more spinning inertia which means more force to both stop and start rolling, meaning you should get more energy per unit distance at a given speed when regen braking with the larger wheels. It also means you'll use more energy getting them up to speed, but they should allow for coasting to require less input from the power supplies (thinking flywheel).

 

Are you coasting more and regenning less but still achieving same brake score?

[hybridbear]

Contact patch may be slightly larger at the tangent apex where the wheel meets the road, but it shouldn't be wider than the 17's should it?

 

Larger wheel means more spinning inertia which means more force to both stop and start rolling, meaning you should get more energy per unit distance at a given speed when regen braking with the larger wheels. It also means you'll use more energy getting them up to speed, but they should allow for coasting to require less input from the power supplies (thinking flywheel).

 

Are you coasting more and regenning less but still achieving same brake score?

I suppose I could be. I feel like this car doesn't roll as far when coasting, it seems to slow down faster. It also seems to require more power to maintain speed. All those things indicate higher rolling resistance, which is why I asked about that.

 

Thanks for the physics lesson.

[GrySql]

I suppose I could be. I feel like this car doesn't roll as far when coasting, it seems to slow down faster. It also seems to require more power to maintain speed. All those things indicate higher rolling resistance, which is why I asked about that.

 

Thanks for the physics lesson.

What brand of 18" tires does your car have?

[hybridbear]

What brand of 18" tires does your car have?

The Goodyear ones, same as your car with the 18-inch luxury wheels.

[dalesky]

Try mounting 4 'spare tires' and see it that changes anything!

Seriously tho, since Ford has both 17 and 18 inch tires, I think it would make little to no difference with size the car uses. Great question tho.

[Waldo]

Try mounting 4 'spare tires' and see it that changes anything!

Seriously tho, since Ford has both 17 and 18 inch tires, I think it would make little to no difference with size the car uses. Great question tho.

 

That's not true at all. Ford very closely manages the volumes of the 17 and 18in tires on the different models so that the 17in tire is the one that can legally be used for all EPA fuel economy testing. That means the 18in tire does not factor into the fuel economy number and thus the rolling resistance can be very different.

 

Having said that though, there is no way that anyone could ever "feel" the difference in rolling resistance between two tires. Driveline and aero drag is still a much larger component of the overall drag than tire rolling resistance. There's obviously something else going on if you're feeling like it takes more power to keep moving.

[ptjones]

What PSI are you using now? Try 50psi and see if you notice any difference.

 

Paul

[hybridbear]

What PSI are you using now? Try 50psi and see if you notice any difference.

 

Paul

Exact same PSI as the last car, measured with the same tire gauge. This is not the difference.

Edited December 2, 2013 by hybridbear

[ptjones]

Exact same PSI as the last car, measured with the same tire gauge. This is not the difference.

I was thinking about whether you could tell the difference in rolling resistance with change of tire pressure. :)

 

Paul

[acdii]

Actually there is quite a difference between the Goodyear and Michelin tires when it comes to rolling resistance. Some may remember me bringing this question up last year when I was having so many problems. I kept going back to the 18" tires as the problem, however I never did get to try out a set of 17" tires to see if there is a difference, but from what HB is saying, I do believe there is.

 

I also have to add, the Goodyears are very much affected by cold weather, their RR changes when cold and the car is slowing down much much sooner than when it was warm out. Where I would normally be applying brake, I now just lift my foot, or I will be stopped long before I reach the stop sign.

 

With that being said, and after having driven on snow covered roads with the Goodyears, I will take the higher RR as these tires handle most snowfalls very well. The only time they didn't do so good, NO ONE did well! You will also notice no complaints about getting 43 MPG in mine, I factored in the 18" tires when I figured what I expected out of this car.

[hybridbear]

I now have stats to back up my observation about higher rolling resistance. See this post: http://fordfusionhybridforum.com/topic/8737-mpg-difference-17-inch-tires-versus-18-inch-tires/

[acdii]

LOL I read my post above yours, boy was I wrong about the snow traction after that post. :P

[hybridbear]

LOL I read my post above yours, boy was I wrong about the snow traction after that post. :P

What do you mean?

[acdii]

What do you mean?

The crap years that are on my HyTi have HORRIBLE snow traction! Really odd since the ones on the BD gripped snow quite well.

[hybridbear]

What I have been trying to do recently, based on larryh's tests, is put the car in neutral and coast (to not consume energy from the HVB to maintain speed) and then wait longer to brake. I've been learning how to judge based on my speed and the incline of the road how long I can wait before braking to get near the max charge limit while not losing potential energy to heating the brake pads. In the city I'm not getting near the 35 kW max charge limit, but I do try to brake hard enough to exceed 20 kW. I've found that at the low speeds the traction motor can't put out 35 kW of regen braking because the RPM is too low. I've had some stops with less than a 100% brake score even though my highest power level going to the HVB was around 22 kW.

 

I have seen a statistically significant increase in Regen miles from doing this. I don't have enough data to know if there's a corresponding increase in MPG or not since MPG is so much more variable. But, I've been able to increase my Regen miles by about 25% in city driving by braking more efficiently.

 

There is definitely a difference between the 17-inch tires & the 18-inch tires. I can wait much longer to brake in our FFH with the 18-inch tires and still get 100% brake score than I could in our previous FFH with 17-inch tires. I also see the difference when I have driven my parents' C-Max Energi. The rolling resistance difference is noticeable when coasting in Neutral as well.

[Texasota]

There is definitely a difference between the 17-inch tires & the 18-inch tires. I can wait much longer to brake in our FFH with the 18-inch tires and still get 100% brake score than I could in our previous FFH with 17-inch tires. I also see the difference when I have driven my parents' C-Max Energi. The rolling resistance difference is noticeable when coasting in Neutral as well.

HB, do you think this is the reason they have removed 18-inch wheels as an option on the 2015 FFHs?

[hybridbear]

HB, do you think this is the reason they have removed 18-inch wheels as an option on the 2015 FFHs?

Absolutely.

[corncobs]

Absolutely.

I agree that's the only reason and as Waldo explained its likely that too many people choose the HyTi or SE w/ the 18" option.

[GrySql]

So what, they look great. I'm willing to pay the difference.

[acdii]

It threw off the bell curve LOL. Ford probably said, Oh Crap, if we dont offer them on the FFH then we wont have to re-issue the EPA! :) Whatever, mesotiredandramblin