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15x8 4.5BS Tire Combo?


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6 replies to this topic

#1 M.schultz2311

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Posted 08 October 2017 - 06:01 AM

First post, been following the forum since I bought my first first gen.

 

I decided to make my first break from stock and installed Hotchkis 1" springs and Hotchkis/Fox adjustable struts.  Now I want to fill the wheel wells more.  I'm looking at 15x8 rally's with 4.5bs.  I would like to here some opinion on 245/60 front - 255/60 rear.  Could I go to 2" spring in the front to give it rake without any rubbing issues?  Running 225/70 on stock rally's right now on a 1971 400/400.

 

Thanks in advance.



#2 MCfan

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Posted 08 October 2017 - 09:22 AM

First post, been following the forum since I bought my first first gen.

 

I decided to make my first break from stock and installed Hotchkis 1" springs and Hotchkis/Fox adjustable struts.  Now I want to fill the wheel wells more.  I'm looking at 15x8 rally's with 4.5bs.  I would like to here some opinion on 245/60 front - 255/60 rear.  Could I go to 2" spring in the front to give it rake without any rubbing issues?  Running 225/70 on stock rally's right now on a 1971 400/400.

 

Thanks in advance.

 

 

Welcome to the Forums, Mike!  I wish more of our "silent" members would get actively involved in posting on the forums because there is great value for everyone in sharing experiences, opinions, facts and specifications related to owning/enjoying our first gen Monte Carlos.

 

Regarding your questions:

 

First, there is a wealth of information on this subject in the tire and wheel database at this LINK on our site.  Perhaps you have already reviewed it.

 

Now, from my personal experience, you can easily run 245/60-15 tires on the front and 255/60-15 on the rear. As you surmise, it is the rim width and back space dimension that will determine the body/frame clearance.  In the rear, you need to increase the back space dimension as you go to wider tires to push the extra width inboard where there is actually a lot of room and thereby keep the outer sidewall away from the wheel well lip.  My '70 SS has 245/60s all around on the stock 15X7 rally rims with no clearance issues.  My other '70 has 275/60s on 15x8 rims on the rear with a 5" BS and sufficient sidewall clearance.  I THINK you will be fine with the 255/60s on 15X8 rims and 4.5" BS but I would be more comfortable with 4.75" BS, especially if you ever want to run the taller/wider 275/60s on the rear.

 

I am running 255/60s on 15x7" rims with 4" BS on the front and can experience frame touching on full lock turns IF the suspension also has to travel as in backing out of a driveway with a drainage profile.  You don't actually need an 8" wide rim for a 245/60 but it should work fine.  I would be more concerned about increasing the BS too much as that will move the inboard sidewall closer to the frame on tight turns, especially if you are also planning to lower the front end 2".   Increasing the rim width a full inch AND going to a 4.5" BS will move the inboard sidewall considerably closer to the frame and inner fender on tight turns. Even with the width and height of a 245/60 you might experience some contact on tight turns with the 15x8 rim and 4.5" BS.  I know the BS on a stock 15x6" rim is 3.75" but I can't quote the BS on a stock 15x7" rim.  Hopefully, you already know it or you can measure your current rallys and know your starting point.

 

Hopefully someone will chime in that has tried your proposed combo and can tell you what they experienced.  Again, welcome to the forums.  Don't be bashful about sharing your experiences so we can learn from you.  Thanks.


Dennis

 

sml_gallery_567_4_470590.jpg                                     sml_gallery_567_4_3184004.jpg

1970 matching #'s 402/M20 4-speed/3.31 12-bolt Posi                1970 SS454/TH400/2.56 12-bolt Posi
Punta Gorda, FL                                                                        Crosslake, MN


#3 M.schultz2311

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Posted 08 October 2017 - 10:45 AM

Thanks MCfan. That is alot of additional insight I did not have after digging deep into the wheel and tire database. I finally had to reach out. I'm going to take some measurements using the 225/70 as a reference point once the cars put of paint. I'm essentially looking for that extra set of wheels for when I'm not looking classic, a "cool/agressive" set. It's such a survivor I can't commit to any permanent modifications. Took years just to pull the trigger on paint and that's more for preservation then looks.

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#4 Rock

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Posted 08 October 2017 - 11:31 AM

I'm running 15x8 with 4 1/2" BS with 255 tires and I have plenty of clearance in the rear.Attached File  IMG_2027.JPG   181.87KB   0 downloads
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Nothing special about this Monte. No SS, no rare options just a super sweet car to drive and take me back to the good old days!

#5 MCfan

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Posted 08 October 2017 - 01:14 PM

Mike,

 

Thanks for sharing a photo of your sharp '71.  I can understand keeping her mostly stock.  My '70 SS is about 99% stock but my '70 4-speed had such a modified engine when I bought it that I have no problem making the changes I like.

 

As you probably know, there are some nifty tire size calculators online.  One that I like (because it also calculates and compares speed, gear ratios and wheel offset) is tiresize.com.  You can first compare all of the key tire dimensions between you 225/70s and any of the 60 series tires.  Then, if you scroll down a ways and look on the left side of your screen you will see a list of the other calculators.  Click on the Wheel Offset Calculator and it will give you a visual comparison of two different tires mounted on either the same rim (offset = 0) or a rim with a different "offset" or back space

 

The dimensional results of your comparison are shown to the right of the visual comparison.  Given those changes in tire height, width and position when mounted with no offset, you can then add whatever positive or negative offset (difference between original and new back space dimensions) and it will show you the positional comparison visually.  If you are increasing the back space dimension from stock to a new rim, enter the offset amount (difference in back space dimensions) as a positive number.  It is really fun to play around with different tire sizes and rim offsets and see how it will affect the new tire position relative to the car's structure around it.  Note that you can use this feature for both front and rear tires.

 

Increasing the rim width from 7" to 8" should not change the sidewall dimension on a 60 series tire but increasing the back space dimension (relative to the  back space of the stock rally rims will move the tire sidewall inboard by that amount.  It is the combination of a significantly wider tire and increased back space that can create clearance problems on the front.  For example, compared to your 225/70 on a stock rim, a 245/60 on a rim with .4" (10mm) greater back space will put the inboard sidewall .74" closer to the inner fender/frame while the outer sidewall remains in almost exactly the same position. 

 

You can see why going with any greater than a 10mm (.4") backspace increase compared to your current rim would be completely unnecessary and could cause rubbing on turns.  In fact, you could somewhat "split the difference" of the extra tire width by going with only a 6mm (1/4") positive offset (greater back space) which would only move the outboard sidewall .11" further out than your current tire and the inboard sidewall .59" further inboard giving more clearance for turns.

 

On the rear, a 255/60 will be .55" further outboard with zero offset so a rim with a backspace at least .5" greater than your current stock rim will be necessary to keep the outboard sidewall in about the same position as your current 225/70 tire.

 

Net: While greater back space is definitely the way to go for bigger tires on the rear, too much back space increase can cause problems on the front.


Dennis

 

sml_gallery_567_4_470590.jpg                                     sml_gallery_567_4_3184004.jpg

1970 matching #'s 402/M20 4-speed/3.31 12-bolt Posi                1970 SS454/TH400/2.56 12-bolt Posi
Punta Gorda, FL                                                                        Crosslake, MN


#6 M.schultz2311

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Posted 09 October 2017 - 09:46 AM

MCfan.  It ironic your jumped right in because constantly ran into your replies and pictures while I was researching 255 and 245 60 series.  Im going to stop creeping and start contributing.  Soon ill post pics of my last 1971 monte carlo that started it all for me.  It was a 350 with buckets and console but I finally got my low mile big block i always wanted.  

 

Thanks again



#7 monte70car

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Posted 09 October 2017 - 04:32 PM

I have nothing to add but welcome Michael. Wheels and tires have alot of options and its the owners choice on what they like. Personal I have 15x8's on the rear of Monte with 4.5" backspacing for 26x10 Mickey Thompson sportsman tires. Then my other Monte runs 15x15 rim 5" backspacing and a 33x22 sportsman tire.


robert

 





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