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Old 03-19-2009, 02:34 PM   #1
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negative effects of changing your rolling diameter?

Hi guys,

I was wondering if someone could tell me the negatives of changing the rolling diameter on your car. I was thinking of fitting some 19''s on my c63 with some 245front 275rear rubber, and heard that it would greatly affect my rolling diameter.

What are the serious negative consequences of doing this?
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Old 03-19-2009, 02:43 PM   #2
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Reduced gas mileage, speedometer would be off
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Old 03-19-2009, 02:51 PM   #3
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Thanks for the quick response

If i moved up in size from 18'' 235/255 to 19'' 245/275 would the spedometer read lower than the vehicle's actual speed then? I guess i'd have to go to get the spedometer recalibrated or something...

I've always wondered why people are only concerned with rubbing/sidewall flex, so Draft is the mileage affect like 5-10% less effiency or is it more negligible than that.
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Old 03-19-2009, 03:15 PM   #4
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Varying the tire/rim size will mostly affect your speedometer reading.

I found this helpful web app that will most likely help you out. Plug in the sizes and read how badly it affects your speedo at the bottom.

http://www.miata.net/garage/tirecalc.html

hope that helps
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Old 03-19-2009, 03:27 PM   #5
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thanks marko just found this site a few mins earlier, I guess I'll just play it by ear renovating my kitchen right now so maybe its not the best time to buy a new set of wheels just yet.
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Old 03-19-2009, 04:02 PM   #6
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the mileage won't really change unless it's a huge difference, it'll be negligible. the weight would make more of a difference though.

gotta pay the price to look good.
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Old 03-20-2009, 06:35 AM   #7
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You should lower the profile of the 19" tires to maintain a relatively similar height to your 18" tires. If you increase the rolling diameter too much you might get rubbing issues.
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Old 03-20-2009, 06:44 AM   #8
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Overall if your going from 18" to 19" your not going to notice much at all. However when I went from stock 14" to 18" you notice a big difference. Non in the speedo (negligable difference) but in the overall slowness of the car and the increased braking distance.

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Old 03-20-2009, 06:54 AM   #9
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18'' 235/255 to 19'' 245/275
What is your stock tire size?
We need to know what the tire profile is on the stock ones, but generally speaking, tire manufacturers produce enough of a tire line that they will have a tire that is close to the stock diameter. Say for example your stock tire is 235/40/18, that is a 25.4in diameter tire. If you went with a 245/35/19 tire, that has a diameter of 25.75 so your speedo will read slightly lower than what you are actually going.
Fuel economy will depend on the total weight of each wheel and tire. Fuel consumption should not change by much when going to a taller tire, but if the wheels are heavier than stock, then the brakes will take longer to slow the car and the car will take longer to accelerate up to speed. If your wheel/tire combo is lighter than stock, then expect the opposite.
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Old 03-20-2009, 06:58 AM   #10
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However when I went from stock 14" to 18" you notice a big difference. Non in the speedo (negligable difference) but in the overall slowness of the car and the increased braking distance.
Don't forget, you are probably going from a 195/60/14 tire that has a diameter of 23.2 in to a 24.5-25 in diameter tire. Your stock steelies and crap tires probably weight 10-20 lbs less than your new 18 inch wheel combo.

That is a big difference.
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Old 03-20-2009, 06:59 AM   #11
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The size of the rim alone doesn't make a bit of difference in rolling diameter. Because the tire is the thing that actually rolls on the ground, the tire size should be changed to maintain a similar diameter.

To your specific question of moving from 18" to wider 19" rubber, you'd also need to include the aspect ratio of the tires.

Generally speaking, a wider tire will result in poorer fuel economy. A shorter tire will, all else being equal, result in poorer fuel economy. The opposite is true for a taller tire. Choose the tire size correctly though and the difference will be on the order of just 2 or 3%.
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Old 03-23-2009, 09:08 AM   #12
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Thanks for all the info guys. SumAznGuy the stock tires on the c63 I from what I remember are 235/35/18 front and 255/35/18 rears.

I dont really wanna sacrifice performance but aside from just simply fitting wider tires to the 18'' or springing 8200 for a set of dymag 19'' with wider rubber I'm not sure how to get more grip out of the car.

So far I haven't found any wheels 18 or 19'' other than dymags in 19'' which are lighter than stock. Only problem is they don't look so great. I just want a semi-OEM look and better grip, since I feel the 255's in the rear aren't enough tire.
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Old 03-23-2009, 05:55 PM   #13
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What kind of tires does your car come with right now? If you really need more grip, I would probably stick with your stock rims (unless you really want the looks of 19's) and get some stickier tires.

Something like a Bridgestone RE-01R, Falken Azenis, Kumho XS or a Dunlop Direzza Z1 would probably suit you well. If those aren't grippy enough for you, there's always streetable r-compound tires...
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Old 03-23-2009, 06:42 PM   #14
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mm the car comes stock with pirelli p-zero nero.

I was thinking maybe some michelin ps2's..
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Old 03-24-2009, 12:23 PM   #15
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i don't think you will feel much difference going from 18s to 19s.
you can definitely feel the difference when you swap in wider tires cuz you will feel every groove on the road.

i have 205/65/15 for my winters with steelies (i don't know how heavy) and the steering was fairly easy, also since my side walls are so tall and fat, i can careless about curbage

i am going to run 225/35/19 for summers, that's 4" larger and about 29lb a wheel (i know its fucking heavy). tested it and it felt like a totally different car, steering wheel was very stiff, and i felt all the grooves and bumps on the road and scared of going near the curb. needs time adjusting

stock are 215/50/17, and they are already 26lb a wheel.. =p
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Old 04-02-2009, 03:37 AM   #16
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*yawn* you worry too much.

Youre rolling diameter change from stock will be minute as youll cut tire profile to make up for diameter gain.

Plus I'm pretty sure the 6.3s don't use a tranny mounted VSS or its ABS sensors....you should be fine.

I'd be more worried about your TC and stability mgmt.
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