Flash2k2

so what's the proper tire size, 235/40 or 245/40, for 18" rims???

y2kse

iTrader: (1)

All right, Flash2k2. Let's use your inquiry as the basis for a tutorial on tire upsizing.

First, I'm only going to deal with four aspects of upsizing: overall wheel/tire diameter, rim width, speed rating and load rating. These are the most critical parameters in terms of safety.

1. OVERALL WHEEL/TIRE DIAMETER: You should maintain an overall wheel/tire diameter that is no more than 3% greater or less than the OEM wheel/tire diameter. Otherwise you'll incur an unacceptable speedometer error. An excellent tool for comparing overall wheel/tire diameters is located at:

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

The OEM wheel/tire diameter is based on a tire size of 225/50R17.

2. RIM WIDTH: The acceptable rim width ranges for various tire sizes are posted on the manufacturers' web sites. NEVER EXCEED THE MINIMUM OR MAXIMUM ACCEPTABLE RIM WIDTH FOR WHATEVER TIRE YOU SELECT. And never accept anyone's advice on what constitutes acceptable rim widths OTHER than the manufacturer's. The word "anyone" specifically includes tire DEALERS and people on this forum. Both are notorious for providing dangerous and irresponsible advice in this area. If you're interested in knowing why you should not exceed acceptable rim widths, take a look at the discussion under the topic of RIM WIDTH in the following article from Dunlop:

http://www.dunloptire.com/tiretech/?...tire_width.txt

Pay particular attention to the words, " . . . could result in failure."

3. SPEED RATING: NEVER PURCHASE A TIRE WITH A SPEED RATING LESS THAN THE SPEED RATING OF YOUR OEM TIRE. Speed ratings are designated by letter (i.e., H, V, Z, etc.) and are imprinted on the tire's sidewall. The speed rating for a 5th Gen Nissan Maxima SE with OEM 17" tires is a "V".

4. LOAD RATING: NEVER PURCHASE A TIRE WITH A LOAD RATING LESS THAN THE LOAD RATING OF THE OEM TIRE. Load ratings are designated by number (i.e, 89, 91, 93, etc.) and are imprinted on the tire's sidewall. The load rating for a 5th Gen Nissan Maxima SE with OEM 17" tires is a "93". In general, the lower the aspect ratio, the lower the load rating. (The Dunlop article referenced above underscores this point. It says, "[c]hanging tires on a vehicle from one aspect ratio to another also influences section width, which relates directly to the load carrying capacity of the tire. The load carrying capacity of the original equipment tire must always be maintained or increased.")

For a complete definition of all major tire sidewall markings and their locations, take a look here:

http://www.dunloptire.com/sidewall.html

Now that you know what to look for, here are a few questions to test your knowledge.

1) Is it OK to mount 235/45R17 tires on OEM 5th Gen Maxima SE rims? The answer is NO! The reason is that the 5th Gen SE rim is only 7" wide. The minimum acceptable rim width for 235/45R17 tires is 7.5". There are no exceptions to this regardless of the tire's make or manufacturer. Once again, check the manufacturer's web site for confirmation.

2) Is it OK to mount 235/40R18 Kumho ECSTA 712 tires with 18" X 8" rims on a 5th Gen Maxima SE? The answer is MAYBE! Why is that? Well let's break it down. The overall wheel/tire diameter only produces a 1.8% speedometer error when compared to the OEM tire size of 225/50R17, so you're OK there. The minimum acceptable rim width is 8", so you're OK there. The speed rating is a "W", so you're OK there. But the load rating is only a 91 as compared to the load rating of 93 for your OEM 17" tires. So on the surface, the tire is not acceptable for use on a 5th Gen Nissan Maxima SE. However, the standard OEM tire on the SE is a 215/55R16 with a 91 load rating. So in this case, you can probably get away with it.

So here's the final exam question. Is it OK to mount 245/40R18 Kumho ECSTA 712 tires with 18" X 8" rims on a 5th Gen Maxima SE? Do the work yourself, Flash2k2. Then let us know what you find out. (And please don't anyone else help him.) If it turns out that your answer is yes, then tell us which tire is the better fit . . . the 235/40R18 or the 245/40R18?

A final comment: NEVER PURCHASE RIMS UNTIL YOU KNOW WHAT SIZE TIRES YOU'RE GOING TO RUN ON THOSE RIMS. Make certain that the tires you select meet all of the above criteria. Then purchase rims that are wide enough to properly support those tires.

Hope that helps.

Flash2k2

Uhhhhh... the 245/40R18 technically is a better fit for 18x8" rims using AVS db's tire specs

y2kse

iTrader: (1)

That's right! And if you did your homework, you also know that Kumho ECSTA 712s in size 245/40R18 meet all the other criteria I specified. So from now on, you don't need me or anybody else to answer your tire size questions. You can do it all by yourself.

Consider yourself empowered, Flash2k2. And enjoy the ride!

MaxRPM

iTrader: (1)

That was well written, how ever I disagree with your last statement. It should be the other way around, figure out what size wheel you want first then what tire size based.

Your car dictates what wheel size and your wheel dictates what tire size.

You are certainly right about listening to "other" people...I bought wheels and tires and they order the wrong size wheels. 18x7.5 and mounted 245s on them, minimum recommended is 8", nuts!, so I cancelled the order just as they where taking my wheels off, luckly I had 8.5 on the bill of sale.

Anyhow, now I am picking a different set of wheels, 18x8.5 with 245/40s.

freeze2k2

it seems this question will never go away

20 MAXIMA 02

so whats the wides tire i could fit safely on 18x8.5 rims?

y2kse

iTrader: (1)

Point well taken, MaxRPM. There certainly are limits to the wheel size the car will accept, and that's an important factor to take into consideration as well. I suspect, however, that many people purchase rims without really considering the importance of rim width. For example, I've seen more than one person "upgrade" from their OEM 17" by 7" rims to aftermarket 17" by 7" rims and then run 235/45R17 tires on the new rims. What kind of sense does that make?

So both rim width and tire size should be taken into account when arriving at a decision.

y2kse

iTrader: (1)

No, freeze2k2. What it shows is that there are a lot of factors to consider when deciding what tires and rims to buy. And everybody has an opinion on this topic, especially around here. What's important to realize is that a lot of the opinions you hear are dangerously misinformed, and it's your responsibility to discern between useful information and bullsh!t. In other words, you have to DO YOUR OWN HOMEWORK before making your decision, particularly when your life might end up hanging in the balance.