Tire and wheel selection

When looking for tires and aftermarket wheels, how do you determine if they meet the load requirements of the car? What numbers do you look at then divide/multiply to determine it it's suitable? The last time i purchased 24" rims for my truck it came as a set so i didn't have to do too much research.

Thank you

Which car?

The first thing is to determine the GVWR (Gross Vehicle Weight Rating) of the vehicle.
Often this is placed on the vehicle, usually on a door jamb or glovebox door, and/or printed in the owner's manual. Typically, it is given for each axle, and a combined total is given.
Often, a recommended tire size is included in this info.

Now, I ask 'which car' because I bet this is for the limo, right? And, since very few were made, I bet there is no GVWR sticker, right?
Well, this makes a difference. You will have to estimate the GVWR.

1. Take the car to a weight scale. Often, places that sell sand, gravel, topsoil, asphalt, landscaping materials etc have a scale you can drive on.

Sometimes, auto wrecking yards have scales, or there may be a DOT truck inspection facility with a scale.
Ask the sale operator nicely if you could weigh your car.

2. Now, calculate the max load that car could carry. Is it a 9 passenger? Okay, allow 200 lbs per person plus 300 lbs of luggage. This gives 2100 lbs.

So, lets say the empty car weight is 6000 lbs. Add 2100 lbs and you get a GVWR of 8,100 lbs.
Given the car's length its reasonalble to assume that the weight, when loaded will be evenly distributed among the 4 tires. So you want tires with a maximum load of 2025 lbs per tire.
This is too much weight for any passenger car tire. These are prefixed with a 'P', like "P235/75R15". You will need a 'Light Truck' tire, which has the prefix 'LT'.

This max load rating is stamped on the sidewall of every tire. You must fill the tire up to the max pressure shown if you expect the tire to hold this weight. If the load is less, you don't need to add quite so much air.

Next... tire size. I am certain LT235/75R15 will fit on your car. This is an extremely common tire size. Don't get an agressive off-road-type tread pattern. Get a mild, all-season tread, for a quieter ride.

My Uniroyal Laredo tires, LT235/75R15 are rated at 1975 lbs per tire, which is typical for a 4-ply truck tire. This may be close enough for you. But, one can get 6 or 8-ply tires in this size which can hold more weight so long as you add more air pressure. But they will have a harsher ride.

Note. My passenger car tires in this size (P235/75R15) can only hold about 1600 lbs each - not enough for your car, imo.

As for the aftermarket wheels.... ask the dealer or manufacturer. They should have specs. IMO the Catalina will be too heavy for aftermarket passenger car rims and you will have to look at light truck rims.... which are made in the 5x5 (I think) bolt pattern your car probably has.

i tell you, you're pretty much dead on with with everything, except the bolt pattern. it is 5x4.75. thanks for your help. i purchased the car with 255/70/15 BF Goodrich already on it. a listing on craigslist for 235/75/15 Mastercraft tires, 4 of them @$40 total made me get them (practically new). one of mine that was on the car had already blown out so i was looking for tires. The gentleman i got them from had them on a bonneville. I don't need a silky smooth ride and the road noise won't be too much of a problem though it is annoying now. I hope to drown that out with some music.

here are the plates from the door jamb.
http://i63.photobucket.com/albums/h151/livewire1568/IMG_1318.jpg
http://i63.photobucket.com/albums/h151/livewire1568/IMG_1321.jpg
http://i63.photobucket.com/albums/h151/livewire1568/IMG_1320.jpg

Here's the curveball....... Some may hate me for this..... I'm undecided about what size rim to use. Most likely it will be a 15" or 16" rim and i plan to get a tire with a pretty wide whitewall. However, I have given thought to maybe going up a bit......... 18"-20". That is not set in stone but it has crossed my mind so i just want to make sure i do all my homework including the necessary brake modifications to do it safely. I also want to change the wheels on my Chevrolet Caprice.

Thanks for the pictures.

I guess you have noticed the ratings on the GM sticker are 800lbs less than on the coach builder's plate.

That GM sticker is for a regular GM passenger car, and not the GM commercial chassis commonly used for such conversions, as far as I know.

I wrote earlier that most GM-based limos, hearses and ambulances used the GM commercial chassis. Most of these were Cadillacs, but GM did sell some commercial Pontiac chassis.

But if it's got that GM passenger car sticker, plus the smaller 5 x 4.75 bolt pattern makes me think your limo is just a regular passenger car chassis, and not a commercial one, although the plate does say "PONTIAC HD" (heavy duty).

I just looked up the weight of a regular Catalina, at 41xx lbs... so the conversion added 800 lbs. That's lighter than I would have thought:)

I still think some LT tires are the way to go if you are renting it out. If you just use this car for your own use, I don't think it matters - unless you are loading the whiz out of the car. Just my two cents :)

As for the larger rims...... you can use those, and combine it with a lower-profile tire, so the overall tire diameter stays close to the stock diameter (235/75R15).
Imo, 16" would be perfect. 18-20's may rub unless you lift the car (which would look VERY strange) or use very low profile tires, which may not be rated for the weight, and would be harsh.

the renting out would be minimal if anything. just friend and family who may want to be driven around and family road trip stuff. i'll probably look for SUV wheels when i get to that stage. in the meantime, i have to get the mechanical stuff squared away. i wanted to plan ahead so when the funds are available, i already know what i need. thanks again

P.S. There's a guy on youtube with a 72 Catalina that has 26" rims. It doesn't look bad depending on personal preference. It's not jacked up to the point where you see all the suspension underneath.
http://i63.photobucket.com/albums/h151/livewire1568/untitled-2.jpg
Thanks to the potholes here in NY, those tires would not last me a week.

Back in the day Pontiac did make a hd chassis for funeral cars. They had larger rear axles and bigger brakes as rule. Load rated d or e tires are recommended here if any passenger loads are added. In NYS, if registered as a limo/commercial you would need these tires.

the renting out would be minimal if anything. just friend and family who may want to be driven around and family road trip stuff. i'll probably look for SUV wheels when i get to that stage. in the meantime, i have to get the mechanical stuff squared away. i wanted to plan ahead so when the funds are available, i already know what i need. thanks again

P.S. There's a guy on youtube with a 72 Catalina that has 26" rims. It doesn't look bad depending on personal preference. It's not jacked up to the point where you see all the suspension underneath.
http://i63.photobucket.com/albums/h151/livewire1568/untitled-2.jpg
Thanks to the potholes here in NY, those tires would not last me a week.

Why not just put a couple'a horses in front of it? I mean, that's what it looks like! :rofl:

Seriously though, all things =, do lower profile tires, in general, improve handling because there is less mushy sidewall between the rim and the road?

Using 205 70 r13 as an example. I know r13 is inches for the rim. 205 the width and 70 the height of the sidewall to side with treads. I think I have that right. Whats the unit of measurement for the 205 and 70? Certainly not inches.

The 205 is the tread width in millimeters and the 70 is the sidewall height as a percentage of the tire's width...long before NASA, leave it to the tire mfrs to thoroughly mix up units on anything (millimeters, percent, AND inches!)

So, the width is 205 mm, the sidewall height is 205x70%=143.5mm and the wheel diameter it fits is 13 in.

The 205 is the tread width in millimeters and the 70 is the sidewall height as a percentage of the tire's width...long before NASA, leave it to the tire mfrs to thoroughly mix up units on anything (millimeters, percent, AND inches!)

So, the width is 205 mm, the sidewall height is 205x70%=143.5mm and the wheel diameter it fits is 13 in.

Why thank you. I had kind of always wondered that. JDMCCRIGHT? Is that like JDM Importers? lol