How tire pressure impacts range - insight into impact of 245/50 R19 tires on stock 19 wheels?

stmache

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Cool video, but unless I missed it, I don't think it explains WHY the higher diameter rims reduce range. Is it just the air resistance of the higher diameters? Could that be fixed by having fins in the rims?
He addresses it down in the comments. The larger rims create more air buffering. He also mentions how closing up the rims help like Tesla does on it's SR+ Model 3 and Ford does with the California Route 1 Mach-E (both come with 18" rims, too).





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buzznwood

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Agreed, but although this is a Mustang, it's also an EV, and I think for their target market, range was, first and foremost, the priority. Especially if they wanted to compete with Tesla on paper.

So for the tests, reviews, and everything else, the car will ship with these tires to get every last mile out of its targeted range on the books.
It's up to us to revive the Mustang "soul" by putting on the performance upgrades :)
While I agree that they wanted the maximum range the mach-e tires section width is smaller than competing vehicles in its class. I could understand the 225 being the tire of choice on the 18" wheels as they could be thought of as wheel and tire combo for those wanting to maximize range, but the 19" should really come with at least a 235 tire
 

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Cool video, but unless I missed it, I don't think it explains WHY the higher diameter rims reduce range. Is it just the air resistance of the higher diameters? Could that be fixed by having fins in the rims?
Also, larger wheels tend to be heavier, overall, which contributes to rolling resistance, which reduces range.

While I agree that they wanted the maximum range the mach-e tires section width is smaller than competing vehicles in its class. I could understand the 225 being the tire of choice on the 18" wheels as they could be thought of as wheel and tire combo for those wanting to maximize range, but the 19" should really come with at least a 235 tire
I'd definitely agree with you. My Mustang had 255 width tires, and I wanted them a bit fatter. :D
For my part, the Mach-E is gonna get wider tires when it's time to replace the stocks.
 

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Cool video, but unless I missed it, I don't think it explains WHY the higher diameter rims reduce range. Is it just the air resistance of the higher diameters? Could that be fixed by having fins in the rims?
Mostly air resistance due to lower side walls and more turbulance from the rim. Also mentions wider front area of the wheels and rolling resistance.
 

phidauex

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So I know the following article is for a bike tire, but I think this analysis is applicable to car tires as well. I am not ready to jump to the conclusion that there is no range impact with wider tires, but I think this suggests maybe the impact is even less than 3%.

https://www.bicyclerollingresistance.com/specials/grand-prix-5000-comparison

You can find other articles that test wider bike tires and rolling resistance, and the results are similar. And if you look at the tires, the width difference is very significant. A 245 tire is 9% wider than a 225 tire. For the article that tested bike tires, the difference between the narrowest and widest is 30%.

30%! So with a 30% increase in width, they measured barely any difference in rolling resistance at the same tire pressure comfort level (see article for explanation). That means tire pressure impacts range much more than tire width.

"We feel the ultimate test is adjusting all tires to the same comfort level. When all tires are adjusted to the same comfort level, rolling resistance is nearly the same (0.2 watts max) for all sizes of the GP 5000. "

Conclusion: I think a 3% hit to range is really worst case, and if you keep the tire pressure at the proper level, there might not be any range hit. We need to test this!
Bicycle tires are very interesting, but there are a lot of differences that are hard to translate to a car. I do know that narrower is not always faster than wider, and higher pressure is not always faster than lower pressure. I ride 38mm Schwalbe G-Ones, run them around 50psi and have coasted past people on 25mm road racing tires at 100psi on long descents.

However, the big difference is in compliance, because the bikes have no suspension - a hard tire vibrates more and causes more losses in the system, a wider tire soaks up the micro vibrations and keeps the hub moving in more of a straight line rather than oscillating, which saves energy in the system. I'm not sure the effect would be so strong on a vehicle with a well-tuned suspension.

That said, you might still be right that the same tire at different widths may have limited impact on rolling resistance, because at the same pressure, the contact patch will be the same size, it will just go from an elongated rectangle to a flat rectangle, but equal pressure and equal vehicle weight = equal contact patch area. The same square inches of the same rubber should have the same rolling resistance.

That breaks down if you compare different pressures or different tire constructions and rubber compounds at the same time. But it is plausible that wider alone would not raise rolling resistance, though it would increase weight.
 
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mkhuffman

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"What contributes to rolling resistance? Experts say that 1% to 5% of it stems from aerodynamic drag. Another 9% to 10% comes from road friction. And the remaining 85% to 95% is due to internal friction or hysteresis of materials. Engineers cannot do much about aerodynamic losses, and some degree of road friction is needed to keep cars and trucks on the road and not sliding off of them. That’s why they concentrate on hysteresis losses, those caused chiefly by the flexing or the tire materials as the vehicle moves, to cut rolling resistance. "

"The most influential factor on rolling resistance is tire pressure. With greater pressure, the tire becomes rounder, approaching a perfect circle, and this reduces resistance. The tire also becomes stiffer so there is less flexing. However, higher inflation pressures also influence handling and ride, so tires should not just be overinflated. "

https://www.machinedesign.com/marke...understanding-rolling-resistance-in-car-tires

The article does not mention wheel weight, but I think that has been reported other places with Tesla testing that indicates a heavier wheel reduces city range due to F=m*a.

In the end, I think putting 245s on the MME will have more of an impact on city range than highway range, and I am comfortable with a worst case 3% hit to highway range. It will probably be less if the tires are inflated to the high side of recommended, and with the use of "eco" tires designed for low rolling resistance.
 

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I have watched the video from EE. What he is saying is true however his testing he did is far from proof. he needs to compare as identical of items as possible. IE get the same model wheel in two different diameters because the aerodynamics of wheels is a large factor. 2nd use two aftermarket tires of the same make. OEM tires are purpose built just looking at the OE tesla tire you can see it was designed for a wider contact patch ie more rolling resistance.
 

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How about 255/50 R19s? These appear to fit the rims. Note the huge increase in weight. That will definitely impact city range. Probably steady state highway range not so much, with proper tire pressure. (Specs from Tire Rack.)

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Those should fit, but keep in mind when you put a larger tire on a vehicle it has a larger load rating. The RMA (rubber manufacturers association) makes what's called a load and inflation table. The load and inflation table show how much weight the tire will support at a particular PSI of inflation pressure. In the case of the Mach E a 225/55/19 Load rating 99 will support 1704lbs per tire at 39psi the 255/50/19 load rating 107 will support the same weight at 32psi and should be run at the lower pressure to support the load properly. Essentially if you inflated a 255/50/19 to 39psi it would be overinflated for the weight needs and you would prematurely wear the center of the tire out. Ford chose a skinny tire because it can run a high PSI and that will help efficiency. I think the 225/55/19 is too little tire for this car and will wear out quickly anyway so maybe it won't be a big deal.
 

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Larger OD on the tire (regardless of wheel size, since it's the OD we're talking about here) reduces the mechanical advantage that the motors have on the road, which increases the amount of torque needed to accelerate the car. The propulsion system just sees the vehicle as being heavier.

Section width can also alter fuel economy, but there are a few variables that interact with it in order to either make it worse or better. Rim width also affects it, as the relationship between rim width and section width and vehicle weight play a part in how a tire will need to be inflated. The lower the ratio of rim width to section width, the lower the pressure needs to be in the tire to maintain a proper contact patch, given the same vehicle weight. So, a wider tire without changing the rim width is going to require a lower tire pressure, which will then increase rolling resistance.
 

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Whenever I think about whether I can optimize something better than the $50B car company, I ask myself if I'm a $50B car company, THEN I ask myself if I'm an engineer. (No to the former, yes to the latter.) Point is, I'm absolutely certain that Ford chose the narrow 225s on the Mach-e for a reason.

With that said, the GT will come on 245s. Same battery, bigger motors, 250 mile range. That's a fair number of variables changing, but only an 8% hit on range. So, that 8% is probably the ceiling on the hit you would take by switching to 245s.

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This guy maths.
 

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For several years I have made hypermiling my on-road sport. Keeping tires properly set to OEM spec is critical for matching EPA numbers. To go for max range during however I push the limits of safety with increased tire pressure. High pressures are first of all a safety issue, reducing traction, especially in cold or wet conditions. They also make the ride much more harsh and may increase the risk of a blowout on hitting debris or a pothole. Since I seldom go as high as the speed limit and never go onto a freeway when pushing for max range that has not been a problem.

My recent Mach E test drive told me right away that Ford took this route when choosing tires and pressures. The bouncy MME ride would almost certainly be cured by using normal tires and pressures. The range reduction would be noticable.
 

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