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Discussion Starter #1
It's finally time for tires for our 2014 Samrt ED. We currently have the factory 155/60-15s (22.3" OD) up front and the 175/55-15s (22.6" OD)on the back.

1) Can I go to a more commonly available size like 185/55-15 on all four?
2) If I need to maintain something close to a factory front/back diamter ratio, what sizes can be used if I want to go bigger?

Like maybe 185/55-15 (23" OD) up front and 185/60-15 (23.7" OD) in the back....or maybe 175/55-15 (22.6" OD) in front and 185/55-15 (23" OD) in back.

Anybody thinking about this?
 

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My intention is to run 175/55-15 up front and 185/55-15 in back, on four OEM rear wheels. And as far as I know, there is only one tire model that has both of these sizes, the Vredestein Quatrac5.

You need to get wider front wheels. Most tire shops will refuse, from a liability perspective, to install anything wider than a 165/60 on the stock front wheel.
 

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Discussion Starter #3
My intention is to run 175/55-15 up front and 185/55-15 in back, on four OEM rear wheels. And as far as I know, there is only one tire model that has both of these sizes, the Vredestein Quatrac5.

You need to get wider front wheels. Most tire shops will refuse, from a liability perspective, to install anything wider than a 165/60 on the stock front wheel.
I have also come to the conclusion that the rear rim size at all four corners is a good solution. Do you have confidence that the 175/55s will clear the front wheel well?
Have you seen this done?
 

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I have also come to the conclusion that the rear rim size at all four corners is a good solution. Do you have confidence that the 175/55s will clear the front wheel well?
Have you seen this done?
This is one of the most desired upgrades, which is why stock rear rims are in such short supply and command so much money. My understanding that the original plans for the smart included four equal sized wheels, but an emergency swerve test as part of some European regulation revealed that there was too much grip at the front wheels and that the smart could tip over. So, Mercedes deliberately reduced front grip by using smaller tires, and built specifications into the suspension that created lots of understeer. Using four rear rims just brings it back to the original design intent.

Those that have used four rear rims report minimal or no rubbing with 195/50-15 tires, and just some light rubbing with 185/60-15's. So anything narrower or smaller in circumference should work fine. Owners also report less wind sensitivity and just a lot more grip with more equally sized wheels and tires. It's also been confirmed that despite removing the tire size stagger - the front wheels are 1.1% smaller in circumference than the rears - that the stability control system does not seem to complain with the resulting zero stagger.
 

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The Moose test failure is not unique to smart: https://www.autoblog.com/tag/moose+test/

If a company wants to sell cars in Scandinavia they need to be able to pass the Moose test. Not only did the early smart fail, but so did the early A-Class. Mercedes responded accordingly, by increasing the smart's rear grip through larger rear tires and by adding a full time Electronic Stability Program (ESP) - that's why the smart's ESP cannot be disabled with a switch (although there's a work around.) :)
 

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Is there a reason not to use the same tire size on all four (rear) wheels? That way you could rotate them...
No reason not to run four equal sized tires if the only change is swapping the front rims for rear rims.

For me, I'm doing a couple of other suspension mods that may make retaining a tire size stagger a good idea. First, I'm currently running 20 mm spacers behind my stock front wheels. This made a significant difference in reducing understeer and reducing tracking/wind sensitivity issues. The front wheels are 4" closer to each other than the rears, and the 20 mm spacers reduce this amount by almost 40%. The increase in track width dynamically softens the front suspension and creates a better turning arc. What I mean by that is that with the front wheels pinched together, it becomes more like a tricycle - you get this weird lean because of location of the outside front tire.

I'm also planning on stiffening the roll resistance of the rear suspension by changing the center bushing for a harder compound. There's also an outside possibility I may install a rear anti-roll bar. Both of these changes will reduce grip balance at the rear wheels.

I found out last winter that despite a whole lot time spent on a race track, that I'm not good enough of a driver to reliably catch the smart if it starts to spin, and that was in a slow speed, low grip condition of being on snow and ice. Once it starts to pirouette, it required an inordinate amount of correction, way more than I've been used to, and combined with the slow steering made it very difficult for me.

With all that, I feel like it'd be better to retain a small stagger, and live with a small lack of front grip rather than having something a bit tail happy. I feel relatively confident I can get the handling and grip balance with small changes in tire pressures. For everyone else, just going four equal sized tires is probably the best bet.
 

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Discussion Starter #8
All being said, this electric car is driven by my wife 90% of the time a s grocery getter. The reason I want to go with the same size (rear) rims at all four corners is to allow tire rotation.
I'm thinking that for our purposes, 175/55-15 for all four will be fine. The reaon to consider the 185's is more tire choices, but I don't want top del with rubbing the inner fender.
No snow around here, so if the thing is wanting to swap ends, she's driving too fast!
 

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Then the 175/55-15 is a great choice. The only other reason for going 185 would be if you want to correct for the optomistic spedometer that reads about 6% too fast. Some owners here have said that the speedometer shows the average of the four wheels, so putting rear tires on the front without if that is the case. Still, the change would be too small to make up for the entire 6%. If this isn't a consideration, then 175 width front tires are probably the best bet as that will minimize your rolling resistance.

The ED is a **great** grocery getter!
 

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Discussion Starter #10
The speedo already reads about 6-7% fast. Or is it mph? Either way, if the car calculates from all four, then the sightly bigger 175/55s up front should bring that down a tiny bit. She actually likes the optimistic speedo. Other than electricity, the wheels and tires will literally be the first dime we have spent on the car since new. (2014). OK, bug wash- that's it!
 

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2009 451 Rally Red, Pennzoil Platinum Euro 5W-40, Wix XP 51356, 185/55R15 tires, 44 psl
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185/55R15 on four rear rims. General RT43's in "H" speed rating.


Very Happy....


As these are 51 PSI max tires, I can run 37 front, 44 rear PSI in them, and get much better mileage.
 

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I am running 145 snows on both smarts 2009 and 2013 ed
and 155 all around summer tires cars handle great
I was giving some serious consideration to running 155's all around on four front wheels at one point, but never had the opportunity to give it a try. I found that running 20 mm spacers behind the OEM front rims helped driving behavior significantly, and would probably run some 10 mm spacers behind front wheels on the rear if I were going to do this.

So are you running four front wheels, and are you using any wheel spacers or any other modification to the OEM suspension?
 

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Just check original size against the new size you want to try, I picked sizes you can get from tire dealers, and diameter was changed very little, that is what affects the speedo.


https://tiresize.com/calculator/
I'm aware of the ramifications of altering the tire sizes, both as it relates to the speedometer/odometer and the stability/traction control systems.

Are you running 155/60-15 tires on four front rims for your summer use?
 

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I use front and rear rims , the diff in width is not enough to matter, all 15 inch with same tires all corners.
Handles great, I think even better that it did with stagger.
 

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Thanks! The stock size 155/60-15 front tire will fit on both front and rear rims so that's even better, as the larger rear rim will give a slightly wider contact patch for the same tire. I don't know why I didn't look more closely before at using this size - I think I was fixated in running 165/60-15 all around but not many places will mount that tire on a front rim, and the only tire model in that size is the OEM Continental th at I don't really care for.
 

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The same specs for hitting a Moose or an Elk !



Here's another reason your Smart might tip over regardless of tire size....lol



Or even this reason too....


Running the same size tires on the front and back makes a tremendous improvement in ride and handling....DCO
 

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If ir were me, i would run 195/50/15 on all 4 wheels. You will have a great selection of tires available to you in that size.
 
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