I'm thinking of upgrading to either 245/45/18 or 245/40/18. Right now my sedan is lowered approximately 1" all around with D-Specs and 05 350z springs. Which size tire would you guys recommend? Thanks.

Travis

 

If you have the OEM 18" Sport wheels, then the 245/40/18's just don't really fit. You'll have too much "bulge" in the sidewalls which will hinder cornering/turning performance. The 245/45/18's will fit but, keep in mind, they'll add 2-3 lbs of weight over a comparable tire in the stock size (235/45/18). So there's a trade-off.

I went with ContiSportContact 2's in the stock size. They're much lighter than both the stock Turanzas and just about any 245/45/18 out there. Only problem is that you have VERY few choices in the stock size. But so far, the Conti's are performing very well. A huge improvement over the Horrenzas.

-T

 

Yes, I have OEM 18" Sport wheels. Why would 245/40/18 cause more bulge vs. 245/45/18? Thanks.

 

I have 245/40/18 S0-3's on my sport sedan wheels and they fit perfect. you are saying that the 245/40 would bulge... then that means the 245/45 would bulge because they are both the same width. In my experience the tires dont bulge and there is no hindering in cornering or turning performace. It also depends on what kind of tire you purchase. It could be a tire with a sqaure sidewall or one that is more round and will "stretch" more across the rim. You would be fine going either way with this. 245/40 or 245/45, it all depends on how much sidewall you want. 245/40 is the closest you can get to the stock diameter without throwing off the speedo too much.
-GP-

 

The calculation is as follows. The tires width (245mm) is measured at the side wall. THe height of the tire is expressed as a percentage of the width. Hence, 245/40 = a height of 98mm. A 245/45 = a height of 110.25mm. Therefore, the 245/40 should bulge LESS than the 245/45 and allow for more tread to actually touch the pavement. Now, there are no exact standards from manufacturer to manufacturer. So these calculations are based on theories only. I would stay with the tire that has the closest height to stock to keep the speedo and odo as close to accurate as possible. Remember that the ride should be a smidge harsher with the tire that has the shorter side wall.

 

if you go up in width of tire you need to go down in profile

245 40 18 will be perfect

 

heres the specs

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
235/45-18 4.2in 13.2in 26.3in 82.7in 766 0.0%
245/40-18 3.9in 12.9in 25.7in 80.8in 784 -2.3%

you should just go with 255/40-18

235/45-18 4.2in 13.2in 26.3in 82.7in 766 0.0%
255/40-18 4.0in 13.0in 26.0in 81.8in 775 -1.1%

 

Show me one tire mfg who recommends a 245/40/18 to fit a 7.5" wide rim.

PS There aren't any! And you know why? Because performance will suffer even if you can fit them on the rim. 245/45/18 works because the sidewall is deeper and therefore doesn't bulge as severely as the shorter 245/40/18. There's simply a point of diminishing returns when you keep going wider and wider.

You can tell yourself whatever you want, but why wouldn't they recommend 245/40/18's if they'd work fine as you say they do? Do you think the tire mfg's are trying to trick you and you've somehow outsmarted them? Call me crazy, but I'm inclined to get with the experts on this one.

 

The tire mfg doesnt have to trick me, i work for the tire mfg, so get off your high horse, you can run a 245/40 on a wheel with the width anywhere from 7.5-9 inches. If you were getting with the experts you'd be with me becuase i am a tire expert... you saying that 245/45 works better makes no sense as the width is in the number (245) and its the same number for 245/40 or 245/45, there is no difference in width. So you can keep going on and telling yourself that 245/40 is going to bulge more than 245/45. There is a difference in sidewall height, overall diameter and circumference, i dont know if you are getting it all mixed up or what, but the section width is the same, so once again get off your high horse. Also teeter, it all depends on what tire mfg you are using as there are some tires that have more of a square sidewall and somewith a rounded sidewall. Bridgestone recommends a 245 width for 7.5-9 inch rims so there, i told you a mfg that recommends it.
-GP-

 

First of all, I can appreciate what you're saying. I'm not looking for a fight. I'm just trying to give some decent advice.

However, I'm going to back up my comments with specs provided by Bridgestone (below). Obviously, there is no difference in section width between 245/40 and 245/45 tires. However, with a short sidewall (40), the angle from the rim to the outer edge of the tread is more severe than it is when the sidewall is deeper. For extreme example, on a 245/70 any bulge would be almost imperceptible and, basically, negligible. But on a 245/20 - a tire of the exact same width - the bulge would be not only quite noticeable but would prove a great hindrance to sidewall performance under load.

Bridgestone and other tire manufacturers are aware of this. If you check their specs, you'll find that none of them recommend 245/40's for 7.5" rims. Yet they ALL recommend that 245/45's are within acceptable limits for 7.5" rims. So obviously, there's more to it than just section width. The depth of the sidewall clearly plays a role in the way a tire performs on a given rim. On the chart below, you'll see that Bridgestone shows "Approved Rim Widths" to be 8.0"-9.5" for 245/45/18's and 7.5"-9.0" for 245/45/18's.

Again, not looking for a war here. Just going for accurate info. Peace!

-T

 

I'm not trying to start a war either. Just trying to get some friendly advice from fellow G35 enthusiasts. Thanks guys.

 

I wouldn't do a 245/40R18 on the 7.5" wide rim for a handful of reasons.

1) no mfg. recommends it.
2) 245/40 are too short. Depending on the make of the tires, you're looking at a 25.6-25.8 tall tires. That's .5" to .7" too short.
3) The shorter tire can introduce a slightly rougher ride and even worse, larger wheel gaps.

245/45s are a better fit, but they're actually a little too tall, but not by much (~.3").

Depending on mfg, a "245" section widths can vary largely as does the true tread width. A 245 section width can be anywhere from 9.5" to 9.9" wide and thread width anywhere from 7.9" to 8.9". You really need to research this stuff before buying tires. Go to the www.tirerack.com and you can look up all the specs for basically any tire you want.

 

here you go, just check the differences yourself

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

 

Having worked in the performance indsutry, I need to back up Teeter on this one - he's right.

As the sidewall aspect ratio gets smaller and smaller (the size/height of the sidewall) then the difference between rim width and tire width becomes more and more important. Make no mistake - sidewall performance is very important to the handling of the vehicle, and sidewalls are designed to be at a certain deflection to provide the best performance possible. Go outside those parameters, and handling, ride comfort and others aspects suffer.

 

Atta boy!!