wescherry

Ahh, I didn't realize the tire would be that much larger. I may have to pass up on this deal then. Don't want my Mustang turning into a 4x4 Thanks for the info ShaneM.

ShaneM

NP, this is a great calculator for looking at sizing:

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

Whiskey11

Should be the other way around. Taller tires reduce the number of rotations per mile and cause the speedo to read lower than you are actually going. Smaller tires have more revs per mile and cause the speedo to read higher than you are actually traveling.

ShaneM

that's not what the calculator says. as much as that thing is referenced, id be very surprised if its wrong.

DPE

Calculator is wrong then; smaller overall diameter than OEM = speedo reads too high, larger overall diameter than OEM = speedo reads too low. My 2010 came with the all season 19s, which measure 703.1mm in diameter. Running 275/40/18s all around now, with an overall diameter of about 677mm, and my speedo reads about 4% faster than actual (as verified by GPS). Need to get a tuner so I can address that, in fact .

Also, a 275/45 will ride about a HALF inch higher than a 275/40. The difference in overall diameter is indeed an inch (just a bit over), but the increase in radius is the important thing here as far as ride-height and that's only about a half inch increase.

FWIW, I run Sumitomo HTRZ IIIs. Not a fantastic tire, but not a bad tire and they are cheap in 275/40. I don't feel cheated on grip, and they have progressive breakaway and a decent overall feel to them. They also hold up on track better than most non-track-specific summer tires; they wear like crazy when tracked, but they handle heat reasonably well and they don't chunk out or do anything too untoward. If I can ever afford proper track rubber I'll go with NT-01s, but until then the Sumis are treating me fine.

Whiskey11

Which tire calculator? I use this guy:

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

Says that from the stock 235/50/18 tires to 245/45/18 (what I have on now) I should be reading 2.1% faster than I'm actually going. Common sense also supports my statement. Less revolutions per mile = reading slow, more revolutions per mile = reading fast.

Norm Peterson

The "difference" calculated is the difference in diameter/circumference referenced to the first tire, and that is what the algebraic sign refers to. Expect your speed to be above the speedometer reading if the sign is '+' (the risky direction), below it if '-'.

Speedometer-wise, unless you've already corrected for tire size through a reflash, you really need to compare the 275/45-18's against an OE size. The difference between 235/55-17 and 235/50-18 is meaningless, so use either one. With 275/40-18's, you will be traveling at about 59 mph at a speedometer indication of 60. Moving from there to 275/45-18's puts your road speed at about 61 against an indicated 60.

Obsessing over a 1 mph error on one side of 60 vs a 1 mph error on the other side seems rather pointless if you ask me.


Edit and FWIW, the Miata.net calculator estimates the revs per mile about 3% to 3.5% lower than is actually the case. Seems the developer of that little application was not aware that a tire tread compresses circumferentially by roughly that much as each segment passes through the contact patch. Fortunately, the % difference results in the extreme right hand column are not affected by ignoring this effect.


Norm

Hamidar05

Tire rack says they are 1/2 inch taller.

I have the 235/50/18 BFG T/A KDWS as OEM with diameter of 27.2"
http://www.tirerack.com/tires/tires....=yes&tab=Specs

Says the 275/45/18 Conti Extreme DW is 27.7"
http://www.tirerack.com/tires/tires....=yes&tab=Specs

The side walls do look a little tall but I think it will fill out the wheel wells a bit better. I have not got rims yet or tried to mount them up, so unless someone else fit them... not sure yet. At a minimum I can use them for rears and get a bit skinner fronts if I have to.

As far as the speedo.. that's why they make tuners. I'll post how it works out if/when I get wheels.

Hamidar05

Oh yeah.. and 275/45/18s are the same diameter as 275 /40/19s another popular tire wheel/combo. For what it is worth

Greg06GT

So if the OEM tire 235/50-18 is say 27 inches in diameter and a 275/40-18 is slightly less diameter. Does this mean there will be more wheel well gap? Or the car will sit lower.

I'd like to minimize wheel well gap or keep it the same when I get new wheels and tires