Question Model 3 - 18 inch Outofspec mod

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cramey3321

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This question is primarily for The Mad Hungarian. On a recent “Out of Spec Motoring” video, on YouTube, he changes to some light weight Martian wheels (width 7.5) and goes with 225 45 18 for the tire size. The lighter and thinner size certainly seems like it would give better efficiency. But going from 235 to 225 will also result in a smaller diameter. How much do you think this would throw off the Wh per mile calculation for the cars computer?
 

garsh

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If you tag somebody using an @-sign followed by their username, then they'll be notified.
@Mad Hungarian

I assume you're talking about this video?

 
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GDN

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I haven't watched the video and will let @Mad Hungarian reply to what is discussed there.

Typically, however, if the tire width changes, the second number, the aspect ratio may change based on the tire chosen. If you choose a different tire width, choosing an appropriate aspect ratio will keep the overall diameter of the tire the same so the rolling distance of the tire (speedometer) doesn't change.

Tires are a crazy thing - the first number, tire width, is in metric - mm specifically. The second number is a ratio/percentage, which makes the tire "height" in also mm and then the 3rd number, the wheel size, well, it is in inches.

Google "tire size explained" and you'll find several good articles. This is just one - https://www.goodyearautoservice.com/en-US/tire-basics/tire-size
 

GDN

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I'll share a spreadsheet I put together about 20 years ago. I put some oversize tires on a pickup. It wasn't easy to change the speedometer, so my goal was to compare tire sizes and also to know how far off my speedometer was going to be. I also found the gear ratios for the transmission. Knowing the different tire size, the rolling distance of that tire and the transmission gear ratios I calculated the RPM the engine would be running at for a given speed limit. I knew my speed based on the tach.

You can see how you can change the width and then adjust the ratio and wheel size appropriately to keep the rolling distance almost identical on different tires.
 

garsh

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But going from 235 to 225 will also result in a smaller diameter. How much do you think this would throw off the Wh per mile calculation for the cars computer?
Not enough to matter.

Comparing the OEM 235/45R18 to 225/45R18, it's only a 1.2% difference in diameter and circumference. So at 80 mph, you'll see a 1 mph difference. But remember, tires wear down, so it's not like a tire's diameter is a precise measurement. It keeps changing as the tire gets used and worn down.

1585052452978.png
 
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cramey3321

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So, I guess the main thing I was trying to get at was, how much stock can one put into something like this. I am trying to figure out if it'd be even slightly worth it to me to do this upgrade when I replace my tires? Is it going to increase the range that one can get, enough to justify the rims and different tires?
 

garsh

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So, I guess the main thing I was trying to get at was, how much stock can one put into something like this. I am trying to figure out if it'd be even slightly worth it to me to do this upgrade when I replace my tires? Is it going to increase the range that one can get, enough to justify the rims and different tires?
From a money point of view? No, definitely not. The amount of money that you save on electricity won't come anywhere close to paying for the new wheels.

But there are other ways of justifying such a purchase. Maybe the 5% increase in efficiency will allow your car to go 5% farther on long road trips, which could be nice.

If you're really interested in efficiency, then I think you'd be better off getting either the Tesla Aero wheel, or the FastWheels EV01+. , The face of these wheels (with inserts/caps installed) has been shown to be more aerodynamic. Aerodynamics are more important than weight at highway speeds (weight is more important at low speeds & stop & go traffic). You can get the EV01+ in both 8" and 8.5" wide versions as well.

https://www.fastwheels.ca/productinfo.aspx?wn=ev01(+)&fn=titanium
https://teslaownersonline.com/threads/new-ev01-wheels-revealed.13818/
 

cramey3321

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I currently already have the aero wheels that came with the car. I’m not really looking at it from a finance perspective. I’m mostly wondering if they’re more efficient than the Aero wheels.
 

garsh

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I’m mostly wondering if they’re more efficient than the Aero wheels.
Not at highway speeds. Aerodynamics dictate efficiency then.

Rotational mass (your wheels) makes a difference when accelerating or decelerating. So lighter wheels will make your car quicker.
This video will show you how a lighter wheel & tire can make a difference when it comes to acceleration.


Weights:
I have the FastWheels FC04 (19 lbs) for my winter wheels. :)