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My new wheels came in today now for tires I'm thinking Nitto nt01's front rims 20x10 rear 20 x 11 . the tire shop is saying 265 35 20 fronted 305 30 20. I thought I could go with wider tires 315 rear and a wider front tire with no rubbing. these tire are almost the same height as stock 2015 gt. How does the height of the tire affect gearing or does it? It seems like I'm shifting out of first and second gear so soon.
Any advice is welcome...Thanks in advance
 

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smaller od = going up numerically in the rear end (ie 3.31 to 3.55)

wording on gear ratios/smaller/taller ain't my strong point

you will accelerate faster but have a lower top end with smaller od
 

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I have nitto invo 275 front and 315 rear on my 20x10 and 20x11 forgestar rims. They fit perfectly and dont rub on stock set up, they still dont rub on my air ride set up now. The taller or shorter the overall tire and rim will change how accurate your speedo is as it is based off rotations of the stock size. With a taller tire set up the speedo will read slower than you are actually going. There should not be much of a difference with a minimal height change however. The physical gearing of the car is not changed by the tires. The change in shifting could be related to the speed you are going vs what the stock tires should be at the same rotational speed.
 

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Gearing is affected by tire height. A taller tire will knock your overall rear gearing down and vice versa. Use this calculator to figure out where you need to be.

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

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Discussion Starter · #5 · (Edited)
Grabberblue 5.0

I used the calculator I don't understand the revs/miles and the percentage cloumnand the answer is in there
 

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how big is the OD difference? You may only be changing by a few % so ignore it. When I swap out my summers/winters there is a 2-3% change. For me that is a whatever thing.
 

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My new wheels came in today now for tires I'm thinking Nitto nt01's front rims 20x10 rear 20 x 11 . the tire shop is saying 265 35 20 fronted 305 30 20. I thought I could go with wider tires 315 rear and a wider front tire with no rubbing. these tire are almost the same height as stock 2015 gt. How does the height of the tire affect gearing or does it? It seems like I'm shifting out of first and second gear so soon.
Any advice is welcome...Thanks in advance
As far as height of tires: Assuming your stock tires were 255/40R19 =27" dia.
The 265 35 20 would be about 27.37" dia or 0.37 divided by 27 = 1.4% larger dia. (Good)
The 305 30 20 wound be about 27.2" dia. = less then 1% larger (good)
So the size change impact is negligible.
The slightly larger tire dia. would result in speedometer showing a theoretical(you wouldn't see the difference) speed of 60.6 mph when traveling 60 mph. Or what is referred to as having the same effect as taller gears(so to speak).
One item which appears to be a problem is, I don't think Nitto has the 265 35 20 in their nt01 tire. Or a 305 30 20 for that matter. Appears the 315 30 20 would work from a dia. standpoint, assuming it does fit.
Nitto 01.jpg

http://www.nittotire.com/race-tires/nt01-dot-compliant-competition-road-course-tire/
 

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FYI,
less than 3% is your target
less than 2% is great
less than 1% is within the margin of variations in tires, tread remaining, etc.
 
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