Tire size to replace 215/75R15 ?

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Tire size to replace 215/75R15 ?

Post by 76malibu on Thu Jun 25, 2015 1:37 pm

Hi everybody,
I am the new owner of a 76 Malibu Classic. I am very happy with this car however she needs new tires. What size would you recommend for 17" rims and same width on 4 tires?
Something like 2??/55R17 ... ?


I am looking for bigger tires without modifying the wheel diameter and without modifying the scrub radius (sorry if it is not the right word, i am not an expert in suspension geometry!).

It has P215/75R15 on it. It looks like it is close to the factory tires which should be GR78.

For the width:
215mm is clearly not the widest tire this car can handle. What could I put without messing the suspension setup ?
I plan to have the same width for the 4 tires to be able to do front-to-rear swaps.

For the aspect ratio:
I don't know what the original diameter was.
According to 1010tires.com/Tools/Tire-Size-Calculator
75R15 is 703.5 mm

I could then use 55R17 (-1.88%) or 60R17 (+1.46%).

Thanks for your help, it's the first time I try to fit something that is not the manufacturer recommended spec.

Alan
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Re: Tire size to replace 215/75R15 ?

Post by thatfnthing on Thu Jun 25, 2015 4:55 pm

Welcome aboard Alan!

You can stuff some pretty wide tires (275mm or more) in those wells without issues, but a lot will depend on the wheels you choose to run, particularly their backspacing.  For example, I am running 17x9 wheels (6" backspacing and 1.25"/1.5" adapters) with 275/40 up front and 275/50 in the back. As I understand it, the Chevelles can handle even wider out back -- IIRC someone on here is running 305 or 315.  Something else to keep in mind is that as the tire gets taller, the ride quality goes up and the handling goes down (and vice versa as it gets shorter), so it's all about your personal tastes and goals for the car.
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Re: Tire size to replace 215/75R15 ?

Post by dynchel on Thu Jun 25, 2015 7:09 pm

A friend of mine, and technically a member here, runs 275/60/15's on all four corners of a '73 Monte with no clearance problems. He does have a higher than stock ride height though.
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Re: Tire size to replace 215/75R15 ?

Post by pila on Thu Jun 25, 2015 11:34 pm

My Malibu has 255/60s on the back. Not very big in diameter, but nice wide tread....

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Re: Tire size to replace 215/75R15 ?

Post by Limey SE on Thu Jun 25, 2015 11:54 pm

I have 275/60/15 No issue so far

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Re: Tire size to replace 215/75R15 ?

Post by 76malibu on Fri Jun 26, 2015 12:08 am

Thanks for your answers. I'm clearly out of my field of expertise so it helps a lot.

I would like to stay close to the original offset, so I may have to do some measurements and calculations in my garage this week end Smile I keep you posted.

I managed to find a PDF of compiled data for the 1975 Chevelle line, it should apply for my 76. I got the following numbers: offset .050 (or minus .050, it's not clear). The unit may be thousands of inches ?!?!


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Re: Tire size to replace 215/75R15 ?

Post by 76malibu on Sun Jun 28, 2015 7:20 pm

The short story is I plan to upgrade to 225/70R15.
It may be BFGoodrich Radial T/A or Cooper Cobra Radial G/T. I like the big white letters Smile

After a rather "redneck style" measurement I found a 1"1/4 piece of wood can slide behind the tire without touching the frame when the wheel is cranked all the way in.
With some expert mathematics I concluded (1"1/4 x 2) + 215 mm = 221 mm is the maximum tire width before changing the offset.
225 mm is the closest tire size. An offset of 5 mm may be necessary but I'm ready to live with it.
I missed this sum, it's in fact 278.5 mm

thatfnthing you made me think again what goal I want to achieve for my car. For now I want to let her stay the smooth (dream?) cruiser she is. Then a small rim with plenty of aspect height is the way to go.
If I stay with a 15" rim the tire calculator gives 225/70R15. The current rim width seems to be 7" (not easy to measure with the tire on) so I should be able to reuse the stock rims.

As a footnote I found the original GR78-15 overall diameter in a old car magazine: 27.32" (693.9 mm) for a Firestone steel belted radial 500.


Last edited by 76malibu on Sun Jun 28, 2015 10:06 pm; edited 1 time in total
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Re: Tire size to replace 215/75R15 ?

Post by 76malibu on Sun Jun 28, 2015 10:03 pm

Damn, I failed at this simple sum!
(1"1/4 x 2) + 215 mm = 278.5 mm

So I can fit 275/60R15 as you suggest without having to change the offset.

For the overall diameter here are the variations I have calculated:
275/60R15 diameter is +1.07% over 215/75R15 | +2.15% over GR78
255/60R15 diameter is -2.35% over 215/75R15 | -1% over GR78

The 255 tire can fit in the 7" rim. For 275 tire I need new rims.
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Re: Tire size to replace 215/75R15 ?

Post by alowerlevel on Mon Jun 29, 2015 2:01 am

Play with this may help you visualize the differences in tires https://www.rimsntires.com/specspro.jsp
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Re: Tire size to replace 215/75R15 ?

Post by Dorianl on Mon Jun 29, 2015 4:22 am

I run 295/50R15 in the rear with no rubbing at all.  I have even had 325s MTs on there with minimal rubbing.

They were on 15 x 10 Torq Thrusts with an appropriate offset.

It fits.

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Re: Tire size to replace 215/75R15 ?

Post by thatfnthing on Mon Jun 29, 2015 9:48 am

76malibu wrote:thatfnthing you made me think again what goal I want to achieve for my car. For now I want to let her stay the smooth (dream?) cruiser she is. Then a small rim with plenty of aspect height is the way to go.
If I stay with a 15" rim the tire calculator gives 225/70R15. The current rim width seems to be 7" (not easy to measure with the tire on) so I should be able to reuse the stock rims

Sounds about right. You should have no trouble going up only 10mm in width. Back in the 80's I actually had 255/70/15 on the stock rims -- another plus to taller tires is that they're more flexible in the sidewall with regard to rim width.
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Re: Tire size to replace 215/75R15 ?

Post by dynchel on Mon Jun 29, 2015 9:50 am

Taller tire also act like a overdrive, lowering RPM's.
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Re: Tire size to replace 215/75R15 ?

Post by thatfnthing on Mon Jun 29, 2015 10:45 am

John brings up an excellent point -- if your tire changes overall height, you can expect to have to recalibrate your speedo also.
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Re: Tire size to replace 215/75R15 ?

Post by 76malibu on Wed Jul 01, 2015 1:30 am

Thank you all.
I am not used to my (new) car yet, and I have never tried wide tires before.
I don't know what to expect so I consider starting with 225/70R15 then switch later to bigger rims and 275mm tires (that sounds crazy wide to me!) Smile This way I will be able to feel the difference.

My spare tire is an original GR78. A true time capsule!
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Wheel and Tire Comparision Tool

Post by CraigH on Fri Jul 03, 2015 6:54 pm

I found this webpage a few years ago:

https://www.rimsntires.com/specspro.jsp

It is very useful to compare tire and wheel combinations. It allows bs changes and makes nominal comments on probable issues. There is a lot of room in the G3GM wheel houses - the tools of course cannot factor such things in, but it is very useful.

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Re: Tire size to replace 215/75R15 ?

Post by alowerlevel on Fri Jul 03, 2015 7:45 pm

CraigH wrote:I found this webpage a few years ago:

https://www.rimsntires.com/specspro.jsp

It is very useful to compare tire and wheel combinations.  It allows bs changes and makes nominal comments on probable issues.  There is a lot of room in the G3GM wheel houses - the tools of course cannot factor such things in, but it is very useful.

I posted that link a few days ago

alowerlevel wrote:Play with this may help you visualize the differences in tires https://www.rimsntires.com/specspro.jsp
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Re: Tire size to replace 215/75R15 ?

Post by Dinomyte on Fri Jul 03, 2015 9:30 pm

I've got 255/60/15s on the front BF Goodrich Radial T/As and 255/70/15s BFG T/As on the back, I like the stance that combination gives. i don't have stock rims but the mid 80's Camaro rims I've got have the same back spacing as stock rally's do. No rubbing issues, big enough contact patch too. Some days I wish I'd gone with 60s in the back like the front.
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Re: Tire size to replace 215/75R15 ?

Post by sportster_restorer on Sat Aug 15, 2015 9:28 am

FYI

I've got 275x45-17s all the way around on my 74 Laguna on 17 x 8 rims. These size tires were common on late 90s and 00 Camaros, Firebirds, and Vettes so I've found decent inexpensive used ones. No rubbing anywhere so far. I've taken my hand around the rear tires and there's room for more back there. Almost guarantee a 285 would fit, possibly a 305 or even 315. One thing to consider when going that wide is rubbing against the exhaust pipe.

275x45s do have a short profile, and I'm not sure I like that. I've been thinking about 275x50 or even 55.

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Re: Tire size to replace 215/75R15 ?

Post by 77cruiser on Sun Aug 16, 2015 9:10 am

275/60/15 rear & 255/6015 front both on 8" Vette rallies
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Re: Tire size to replace 215/75R15 ?

Post by 76malibu on Thu Aug 27, 2015 1:38 pm

What tire pressure do you use?
I guess the original specs for GR-78 is 24 psi.
BF Goodrich Radial T/A 225/70R15 is rated 35 psi MAX.

Should I put around 32-35 psi or stick with 24 psi?

Note that even at 35 psi the sidewalls have a round balloon shape on the stock wheels.

Thanks
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Re: Tire size to replace 215/75R15 ?

Post by thatfnthing on Thu Aug 27, 2015 5:47 pm

Pressure will depend on what balance you want to strike between ride quality and fuel economy.  Higher pressure makes for better mileage, but a stiffer ride.  Vice versa for lower pressure.  I tend to run mine near the max rated pressure for the tire (40 psi).  The factory typically picks a number closer to the middle as a compromise. Obviously if you go too high or low, though, your tires will wear incorrectly.

As for the balloon shape, that'll be a function of the wider tire.  As the tire gets wider, the sidewall has to bend in more to fit the same stock rim, resulting in the 'balloon' shape.  To keep the wide tires and lose the puffiness, you should consider a wider rim.  Luckily a 70 series has a lot of sidewall and can conform to a narrower rim, but the 'balloon' is the trade-off.


Last edited by thatfnthing on Thu Aug 27, 2015 5:48 pm; edited 1 time in total
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Re: Tire size to replace 215/75R15 ?

Post by 77cruiser on Thu Aug 27, 2015 5:48 pm

I run around 26.
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Re: Tire size to replace 215/75R15 ?

Post by dynchel on Thu Aug 27, 2015 5:49 pm

I never really thought about it, but I run mine at 32 psi.
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Re: Tire size to replace 215/75R15 ?

Post by chevellelaguna on Thu Aug 27, 2015 6:51 pm

Tires manufactured by today's standards are an entirely new breed in comparison to the tires manufactured years ago. You want to run whatever psi the tire says to run, minus 1-2 psi is acceptable. When brought up to temperature, especially during highway use, they will typically gain the 1-2 psi from molecule expansion. You can run straight Nitrogen as well. Any pressure lower than what the max pressure stated will reduce the tread life of the tire. It is designed for optimum tread life at the tires specified pressure. All too often we have customers come in and have the pressure set at what the sticker in their door says.... They don't have those brand tires any longer, the ones they have recommend 44 psi and they run 32, that's a 27percent reduction in pressure... That's huge!
Tire compounds give you that cushy soft ride... Not lower tire pressure. If you have a rough hard ride, lowering your pressure will only worsen the problem and wear your tires down quicker. Tire manufactures also have tread wear ratings if 32,000-60,000 miles, those are achieved under ideal controlled conditions. You almost never get the actual mileage rating out of the tire anymore.... Unless you are religious about cross rotation and maintaining correct tire pressure, not speeding and taking sharp turns on windy roads.
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Re: Tire size to replace 215/75R15 ?

Post by thatfnthing on Fri Aug 28, 2015 9:32 am

I didn't mean to imply that there was a gigantic difference in ride quality with lower air pressure in the tires.  It's actually a small difference, but it is enough to be noticeable.

The brand new tires on my truck say '44 psi max press', which implies that there are acceptable pressure levels below that.  I have also run tires below the max (~5-10 pounds) without any adverse tire wear.

And nitrogen has one enormous drawback that outweighs everything else: you have no way to fill it yourself.  It's ridiculous to have to drive miles out of the way to the tire shop (and then wait an hour when you get there) to top off a couple psi.
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