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View Full Version : Tire and Rim sizing..



Deep6
08-22-2003, 02:54 PM
Supposedly I have some money available to get new tires and rims. I had a lenghly discusson a few months back on some lightweight rims that I wanted. The end result was that a tire size anything larger than 225mm on a 7" rim was too wide and it would wear the tire abnormally and not give me satisfactory handling.

Well, I've got to do something soon because the tires that I have been hobbling along with for quite some time now are down to the tread bars and I need to get new tires and I'd like to get new rims.

I was surfing www.tirerack.com and they now list the weight of each wheel that they sell. I'm not sure if that is the weight of the wheel though, or the shipping weight.

They also list recommended tire sizes. I was looking at some 17" rims on discount tire direct and they recommended 235mm rubber on some of their 17x7" wheels.

Back on tirerack.com I saw a set of 18x7" wheels that looked nice and they recommended a tire size of 245-45ZR-18. Does that seem right? Or perhaps it was a misprint or is the width of the tire different for a 17 or 18" rim respectively.

In other words would a 245/17" tire be just as wide as a 245/18" rim???

Also could someone refer me to the Tire calculator, perhaps that will spit out the answers I'm looking for. I'm pretty sure the last time I saw it, it would convert width and aspect heights to inches or did I dream it up??

At any rate, the clock is ticking and my inspection runs up this month. I saw a nice set of O.Z. Superleggera 17" rims in Road and Track on a Volvo Wagon V40. According to R+T they claimed the rims weighed 15.4lbs. But according to tirerack the rims are 17x8" and they pull a hefty 19lbs???

Were the ones for the Volvo 17x7"??? I'll look into it.

C5 Corvette
08-23-2003, 03:44 AM
245-45ZR 17 Tire rating represent the following: 245 is the track width, 45 is the height of the tire from the bead to the top of the tire, 17 represents the diameter of the rim size on the tire. 17x8" and 18x9" are possible depending on the year of your Tbird.

Deep6
08-24-2003, 01:08 AM
However, I kinda already knew that. A 245-45ZR-17 tire represents 245mm in width, 45% of the width is the tire height and the 17 represents the rim diameter.

I would be nice to hear from the SC guys though. (Sorry C5 Vette, but it just seems weird getting help from a Chevy guy on a Ford board.) LOL!

I've been having alot of errors and crap with my computer so, it's been tough to keep in the fold recently.


Once again, Anyone know about posting some links to the tire calculator (S) ???

And is the Aspect width of 245mm the same if listed in a 17" or 18" tire size.

In other words, if I compared a 245-45-17 tire to a 245-45-18 tire, would the width (in inches or millimeters) be the same?? Will I have the same contact patch?

And the last question summarizes the largest tire that could be fitted comfortably to a 7" wide rim.

I know that the stock rims are a 16x7" rim and that many folks here run a 245 or even 255mm size tire on it. Any adverse conditions from that?

fast Ed
08-24-2003, 02:53 AM
Contact patch, width, and sidewall height of a 245-45 tire will be the same for any rim diameter, doesn't matter if it's 16", 17", or 18".

Keep in mind that when you're looking at examples of people running 245 or 255 width tires on their stock 7" wheels, you're comparing apples to oranges ... those will usually be 50 or 55-series tires, more sidewall height allows the tire to curve in for a narrower wheel. I remember years ago trying to get some 245-45-16 tires on to 7" wheels for my Fox Mustang ... they barely went on, and the sidewalls were very angled inward ... most tire manufacturers recommend at least 7-1/2" or 8" wheels for mounting 245-45 dimension tires.

Just to reiterate what we discussed last time when you had posted your wheel questions a few months back, I really think that you're barking up the wrong tree trying to save 7 or 8 pounds per wheel on a 4000 lb. street car, especially when it means sacrificing the proper dimension rim width to mount your chosen tire size on.


cheers
Ed Nicholson
SCCoO

C5 Corvette
08-24-2003, 12:21 PM
I would be nice to hear from the SC guys though. (Sorry C5 Vette, but it just seems weird getting help from a Chevy guy on a Ford board.) LOL!

I am an SC guy.:rolleyes: Don't let the name fool you.:D I have a 89 black SC that I am in the process of restoring. Late week I was researching for tires and rims for my SC. I'm debating whether to put 17" or 18" rims on my SC.

Deep6
08-25-2003, 12:36 AM
Ed,

I want you to go to www.discounttiredirect.com and search for rims on a 1995 T-bird. You will get plenty of 16"-17"-18" options.

Now, I want you to click on any 17x7" rim and look at "their" recommended tire choices.

1st one is 235-55-17
2nd one is 245-45-17
3rd one is 255-45-17

I'm certianly not doubting your advice, both from a personal and professional point of view. Your experience with road racing pretty much drives home the validity of your responses. But, to play devil's advocate, why, then would discount tire direct list those larger sizes as a "recommended" tire choice?

Don't they know that they are recommneding something bad?

But since you listed your concerns a few months ago, I've been keeping my eyes peeled for 17x7.5 and 17x8" rim sizes, which of course are far and few between while still being able to keep price and weight costs down.

Also, I appreciate you mentioning that most folks who run 245 and 255 series rubber on their 16x7" stock wheels are running an aspect height of 50 or 55.

What, then would happen if I wanted to run a 255-50 or 55 -17" rim on a 17x7" rim, would there then be a little more sidewall to stretch over? Of course though, one could assume a tire that would be slightly bigger in overall diameter and cause speedometer readings to be further off.

And I also think that I've seen some guys here who have successfully run 245-45-16" rubber on their stock rims. But I don't know if they've had abnormal wear and quirky handling.

[...I really think that you're barking up the wrong tree trying to save 7 or 8 pounds per wheel on a 4000 lb. street car...]

I know, I know. But a 4000lbs street car is on the hefty side. There have got to be lots of little ways of saving weight which can add up to much more weight lost and I'd like to investigate each option. As you know Ed, reducing weight will increase a car's performance potential in just about every measured dimension, Acceleration, Handling, Braking...even fuel economy. So I don't think that it is unwise to toss aside the idea of saving a few pounds at each corner of the car.

C5 Corvette,

You're on the same track as me. When I'm about 20 years older than I am now, I'll be buying my next dream car... a Corvette!

Some adivice that I read about a few years ago in a performance magazine. 17" vs 18". The 17" rims will be cheaper and still look good. they'll also be lighter in weight. The 18" rims will be a little more expensive, weigh a little more but the advantage is the look of filling in the wheel well even more. Of course to get the same rolling diameter, you'll need to run a lower aspect ratio like a 40 series tire. This can lead to a rougher ride, that and the fact that the unsprung weight will be higher too. But the lack of sidewall flex in a lower profile tire will pay dividends in more communicative
steering and turn in as well as sidewall flex.

The magazine article came to a conclusion that neither rim/tire combo was able to out handle the other. There is no performance "advantage" to going to the 18" vs the 17". Although, if you have some crazy brake mods in mind, the 18" will give you more clearance.

C5 Corvette
08-25-2003, 01:23 AM
I am considering 17" inch rims. 18" inches look better but the pot holes in Brooklyn NY will destroy the 18" inch rims. I can drive slower but what the since?:D

By the way, I not 20 years older and still I do not have my Corvette.:( (YET)! But I did get this SC given to me for free, my second love.:D :D I just could not say no.:D

HSKR
08-25-2003, 04:16 AM
It's a Dodge Dakota website, but is one of the best tire/wheel calculators I've found yet.
http://www.dakota-truck.net/TIRECALC/tirecalc.html
As far as the size of tire that will fit on the rim width, when you look at the tire specs, each tire will have recomended rim width, and also the range of widths they recomend as safe. Follow those recomendations from the tire manufacturer to be safe, and also in case anything happens. Running a wide tire on too small a rim, or vice versa can cause you problems should the tire fail and you want to do something about it.

fast Ed
08-25-2003, 10:57 AM
Originally posted by Deep6
Ed,

I want you to go to www.discounttiredirect.com and search for rims on a 1995 T-bird. You will get plenty of 16"-17"-18" options.

Now, I want you to click on any 17x7" rim and look at "their" recommended tire choices.

1st one is 235-55-17
2nd one is 245-45-17
3rd one is 255-45-17

I'm certianly not doubting your advice, both from a personal and professional point of view. Your experience with road racing pretty much drives home the validity of your responses. But, to play devil's advocate, why, then would discount tire direct list those larger sizes as a "recommended" tire choice?

Don't they know that they are recommneding something bad?


When I look at several tire manufacturers' catalogues I have at work here, and the Tire Rack website, 245-45-17 shows as 7.5 to 9.0" recommended width, and 255-45-17 shows as 8.0 to 9.5". Only 235-55-17 would go properly on the rim width you're looking at, 6.5 to 8.5". They usually give a "measured rim width" as well, this is the size of rim the tire is mounted on to give all the dimensions they are showing for section width and tread width, since section width will change by about 0.2" for every 0.5" change in rim width. So just picking a Kuhmo Ectsa MX as an example, it shows a section width of 9.7" mounted on an 8" wheel. If you were to jam that on a 7" wheel, it would now only be approx. 9.3" wide. So that's another reason why you really shouldn't go small on the wheels, you lose section width on the tires.

As for Discount Tire recommending those rims for those tire sizes, all I can think is that maybe they are simply trying to sell what they have available ... as you have found out, there aren't many wide wheels available for the MN-12 bolt pattern. I'd sooner go with the tire companies' recommendations ... they're the ones building the tires.

And if your tires aren't gripping properly because they're jammed on too narrow wheels, I doubt you'll feel a handling difference from lighter rims. Saving 20 - 30 pounds on a 4000 lb. car would make negligible difference to fuel economy ... you're starting with the wrong car if you think shaving a few pounds here and there will make much difference !! ;)


cheers
Ed N.

Deep6
08-25-2003, 01:58 PM
OK, I definatly understand what you are trying to tell me. Took several posts and a couple of months, but I do understand now! :) LOL!

I guess then my final question is what is the largest size (width wise) that can be fitted to the front and rear?

There is a rim that I have selected that I think will look good and save me a couple of lbs (though not as much as those centerline rotary forged ones) and it's a 17x8" rim.

Isn't a 255mm roughly the largest up front? I know some guys here are also running like 275mm in the back, but I want to keep the size of tires the same size for rotation purposes.


[...Saving 20 - 30 pounds on a 4000 lb. car would make negligible difference to fuel economy ... you're starting with the wrong car if you think shaving a few pounds here and there will make much difference !! ...]


Ed, This is a discussion for another time. Your right 20-30 lbs won't make a measurable difference on it's own, but several 20-30 lbs mods WILL eventually add up. My goal is to loose as much weight as possible without making some of the other sacrifices, like gutting the interior etc. One day you'll see!

HSKR
08-25-2003, 02:43 PM
Originally posted by fast Ed N
Saving 20 - 30 pounds on a 4000 lb. car would make negligible difference to fuel economy ... you're starting with the wrong car if you think shaving a few pounds here and there will make much difference !! ;)


cheers
Ed N.

Saving 20-30lbs of dead weight won't make much of a difference, but saving 20lbs off a wheel/tire combo equals more than just 20-30lbs of svaings since rotational weight makes such a large difference. Just lookat the 10+hp gains from puttin on thelighter jackshaft pulley. Really, not that much much weight difference as far as whatis measured on a scale, but it makes a nice differnce when you start spinning it.

fast Ed
08-25-2003, 07:24 PM
I understand about rotational weight and inertia, plus we're looking at reducing unsprung weight with lighter wheels, which isn't a bad thing. However, as I said before, I think that on a big street car like an SC, the average driver isn't going to notice any difference in the seat of the pants, other than maybe a bit more feel because his / her as$ is closer to the seat, due to the reduced wallet thickness. ;)

For tire sizes, 255-45-17 is about the biggest that will fit comfortably on an 8" rim at all four corners of the car so that you can rotate front to back. I notice in your sig as well, you have the "aggressive alignment specs", off TCCoA I presume. From my experience, they are too much for track events, let alone street driving. All you'll end up doing is wearing out the insides of your tires, especially when you put on wider boots with a lower profile that have less flex in the sidewalls. Even with the grip I can generate with Hoosier road-race radials (basically DOT slicks for the track, like M/T E-T Streets for drag racing), my tire wear was excellent with about 0.7 deg. neg. camber on the front, and 0.3 neg. on the rear ... so there's a whole nother can of worms opened up !! :p

Here's my recommendation for better handling on any car you drive ... spend $300 or so on a performance driving school ... usually the biggest improvement that can be made is with the big spacer between the seat and the steering wheel. :D


cheers
Ed N.

rivlee
08-25-2003, 08:49 PM
The tire/wheel and speedometer gear calculators are in our FAQ's section.

The designation 255/45/17 means:

255 = the tread width in mm, 45 means the sidewall heighth is 45% of the tread width (255mm x .45 = 114.75mm) and 17 is the rim diameter in inches.

My .02 on max width is based on safety for emergency cornering - not having the tire come off the rim if you hit a pothole when cornering hard.

7" = 245 (will fit in front wheelwell)
9" = 275 (will not fit in front wheelwell)

Part of the problem with going up in width, means you've got to compensate with the profile (sidewall heighth) to keep the overall diameter close. In other words, going from a 225/60/16 to a 245 tread width requires going to a 50% profile - 245/50/16.

I'm using 255/45/17 on 9" Cobra's. There's only a slight bulge past the rim and the ride is not overly harsh and the handling is precise.