DenisC

Wheels size 1975 Buick century

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Good morning Gentlemen, I am writing to request a piece of information, I write from Italy and I just bought a 1975 buick century to be restored, the My question is this: I'm looking for a way to have a document certifying the various measures of wheels that can mount the car as the car that I bought features a 15 "and I would put the largest  wheels

could you kindly tell me where I could write to get a document? 

Thanks in advance 

best regards

Denis Cattin

italy

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Greetings Dennis,  I'm not sure what you are looking for exactly.  Are you just looking for what would have been offered from the factory for your car?  Or are you considering custom rims and tires? 

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I try to explain. I do car license plates to put it on the street and I need a factory document or other document that says I can also mounted bigger tires and wheels or custom rims and wheels.

thanks for r answer sir

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I don't think you are going to find any factory literature that advises you can install something bigger than was offered from the factory.  In 1975, the biggest offered on your car would have been a 15-inch rim (15x6).  Here's the specification page from the brochure  http://www.lov2xlr8.no/brochures/buick/75_31.html

Edited by dmfconsult (see edit history)
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Thanks a lot for the answer, if I can make one last question, how do you do in the US, and you can fit larger wheels?

if I can make one last question, how do you do in the US, and you can fit larger wheels?

by fitting larger wheels do not require some a piece of paper certified to be able to flow with different rims than the factory?

thank you very much

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Well, I'm in Canada, but I think things work similarly in the US... I don't think you need any documentation to replace your wheels with something bigger.  There may have to be suspension modifications and/or body modifications to make the bigger wheels fit but that's just the cost of adding larger wheels.  Of course, in some jurisdictions where safety inspections are required, those modifications may not pass an inspection, but based on the number of old cars with aftermarket, larger rims, I would think it's not that difficult. 

 

Once you own the car, you can do what you like as long as it's safe for the road, you do not need a piece of paper from the manufacturer approving a larger wheel.

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No documentation would be needed in the US. In 1975 the only common wheel diameters for cars were 13" , 14" , and 15". Anything larger in diameter would have been for a very old car or a truck. Wider 15" (and 14" and 13") tires and wheels were readily available. You just bought them and put them on.

 

No one had to approve a different size. It is still like this, but today the common sizes are 16" and larger.

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Howdy --

 

On the driver's side middle door post should be a decal with the minimum tire size on it plus the recommended inflation pressure.  In 1975, GM standardized radial tires on their vehicles as a part of the then-new efficiency system (retuned engines, catalytic converters, radial tires).  For a car such as pictured, it probably has P215/75R-15 tires on 15x6 steel wheels.  Possibly originally GR78-15 was the original size tire, which would later equate to the P215/75R-15 size.  There is also a P225/70R-15 tire that is very close in overall diameter but with a wider section width and tread . . . which would need a 15x7 wheel to work and look correct.  Those two sizes are basically interchangeable on the same vehicle.

 

What is your desired "end goal" for the wheel change?  Better handling?  Cosmetics?

 

In that general time frame at GM, there was NO passenger car wheel larger than 15" diameter.  The 16" wheels came in the 1990s, after the downsized intermediates went to 14" and then to 15" in the 1980s performance models.

 

Something tells me that you're looking for justification to change the wheels to something in the 20" diameter range?  There ARE some issues with such a change!!  It is possible to find a 17" tire of the same basic dimensions as the P225/70R-15 tire (I've looked about that on my own) AND some aftermarket wheels in the 17x8 size range (although a 7" rim width would be better).  BUT with the larger diameter wheels and tires, the WEIGHT of each assembly increases a good bit beyond what's now on the car.  That weight has inertia which the BRAKES will have to overcome to STOP the car quickly!  Until GM upgraded the brakes on their pickup trucks to handle the additional wheel weight, they did NOT recommend 20" wheels on those vehicles at all.  A model year or two later, no problem.

 

It has become quite popular to put 24" wheels on Ford Crown Victorias and Mercury Grand Marquis vehicles.  I'm not sure how they modify the suspension to do that, but it also raises the ride height of the vehicle to nearly pickup truck 4 wheel drive heights!

 

In general, it's best to keep the car at "production" ride height for the best and safest ride and handling.  There are a few states in the USA (as Hawaii) where altered ride heights are legislatively-regulated.  There are also federal regulations as to bumper height (as a part of the vehicle's design), too, but it appears those only affect cars and not pickup trucks.

 

In addition to the weight/braking issue, generally the BEST ride happens when the lightest wheels/tires are on the vehicle.  Less inertia for the shocks/springs to overcome to keep the wheels planted on the pavement.  A bouncing wheel/tire is dangerous as vehicle control is compromised!

 

GM used a lot of European influence in the design of the intermediate-size cars in the middle 1970s.  Clean and uncluttered lines prevailed.  The wheel covers on the car now might look "plain", but when buffed and shined some, they can look pretty nice.  It CAN be tricky to find different wheels that will look good on that car and compliment the styling.  THAT is the tricky part!  Unfortunately, many wheels are near or past $200.00 USD each, just for the wheel and a good performance tire can run that tab up to about $350.00 USD per wheel.  Nothing's really "cheap" any more.

 

Just some thoughts,

NTX5467

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 I would suggest that if you see someone there sporting the big wheels, flag them down and ask them how they got around the original equipment requirements. 

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