1970 Plymouth: Tire sizing made sexy. Sort of.

Putting twice the rubber on the road is a beautiful thing. When it works. The trick? Actually making it work.

 

*Part 6 of Jeff Melnychuk’s series on rebuilding a 1970 Plymouth. Read Part 1 here.

 

Despite my attempts to make tire sizing and selection sound sexy and sizzling hot, I will likely fail miserably. Unless you like a lot of numbers ’n’ stuff.

I get all jazzed up about tires. Compounds. Styles. Aspect ratio and section width. I could go on for days and days.

How much do I love tires? I have been known to pay more for a tire just to get a better looking tread pattern or sidewall.

Clearly I have issues that need professional attention, but I’m going to attempt to relay the art of sizing, here and now, without you nodding off and plowing face-first into your bowl of breakfast flakes.

Pro tip: Get coffee now.

Since tires are the only part of the car touching the road, and picking the right size is as much about creating the right proportions and stance as it is about not dying, well maybe I can, at the very least, hold your attention for the duration of this story.

First, forget the wheels for a minute. They are metal, which means if they can’t be purchased in the right size, they can often be made (and remade) to do the job.

There’s only one tire measurement that’s close to the original: the height. Begin with that and the desired wheel diameter. In back there are 20-inchers. In front, 19s.

 

Tires, you’re stuck with, in whatever sizes are available, so it makes sense to begin with them and then buy wheels that fit.

The exception is if you already have wheels that you know will fit. Perhaps it’s a set of factory wheels, in which case there’s plenty of information out there as to the sizes of tires you need. For restoration purposes, Coker Tire (www.cokertire.com) is a great place to begin a tire search.

But in the case of the Project Plymouth, selection begins with a blank sheet of paper — a rumpled bar napkin, actually — with the goal of getting as much rubber on the road as possible for max grip.

While width might be the first thing that comes to mind, the best and most important place to begin is with the height. From there you can work out the width. You probably already have a good idea of the wheel diameter you want to run, so the rest is pretty simple once you have the height dialed in.

 

Charts such as this from www.tirerack.com help you quickly scan tire heights and widths to dial in the right size for your vehicle.

Many muscle cars came with tires in the 26-27-inch-tall range, so I began with that as a starting point.

Only small increments — like half an inch — are needed to make a big difference, not just to the look, but to stay within whatever clearance limitations exist.

To do this, you have to simulate the correct ride height and take into account two to three inches for suspension compression, which is tougher to gauge at the front because you have to test it through the full steering range.

To get a rough idea, you can install a factory wheel and tire and measure how far the tire is from any contact points at full lock in either direction.

It’s more critical when the leading edge of the wheel is turned in because that side of the car will lean into the tire, further reducing clearances.

 

How much clearance is necessary depends on factors like the type of suspension, the height of the tire

But how much clearance do you really need?

If you’re running low-profile tires with short sidewalls with little flex, you need very little room for them, perhaps one eighth to one quarter of an inch on each side. You’ll need more clearance for tall tires with big, bulging sidewalls that move around while driving.

It also depends on the suspension. Rear leaf springs allow the body to move left and right over the tires, randomly reducing clearances. Leaf springs almost never center the rear differential in the car, which means one tire will have all kinds of clearance and the other side might be rubbing.

With the suspension in the Project Plymouth, there is no lateral motion of the body relative to the tires and it’s fully adjustable to center the rear differential to keep tire spacing the same on the both sides of the car.

Careful measuring before purchasing the wheels puts the tires dead centre in the wheelwells. The suspension can be adjusted to move the rear differential left or right to create exactly equal spacing for the left and right tires.

 

The great part about having a robust platform like the Plymouth is that you have to go absurdly tall and wide to run into clearance problems.

I chose to have the wheel wells widened by two inches to provide more clearance, although it wasn’t really needed to squeeze in the 335/30-20 Michelins, even with a two-inch drop in ride height. The tires are about 13 inches wide and a seemingly low-profile aspect ratio of 30, still yields a healthy sidewall height of about four inches.

 

Calculations for 335/30-20 tires

335 = the section width in millimetres. Divide by 25.4 mm to convert to inches: (335 ÷  25.4 = 13.2 inches)

30 = the aspect ratio, or percentage sidewall height to the section width: (13.2 inches x 30% = 3.95 inches sidewall height)

Tire height = 2 x the sidewall height + wheel diameter: (2 x 3.95 = 7.90 inches, + 20 inches = 27.9 inches)

 

Still awake?

At the front are 275/40-19 tires, which is a still-wide 11 inches (275 mm  ÷  25.4 = 10.8 inches). The sidewall height is 10.8 inches x 40% aspect ratio, which is 4.33 inches, or 0.35 of an inch greater than that of the rear tires.

The tires are close in height (2 x 4.33 inches, + 19 inches = 27.66 inches).

This is OK, but because I wanted a tall tire to fill out the large front fender openings (and the front wheel has one-inch less diameter), I really needed a rear tire that was closer to 29 inches in height so the sidewall heights matched front to back.

Such a rear tire does not exist in the required width of 335 millimetres.

Changing tire sizes when the entire front suspension is different than stock presents an extra challenge. In older cars, there’s usually plenty of room, however. There’s a lot of trial fitting and measuring clearances while turning the wheels.

 

If you’re reading all this math and sweating in your slippers right now — or asleep in them — there’s no need to worry. As usual, research triumphs over all and you can generally begin by investigating similar cars to see what they’re getting away with.

That still doesn’t mean you’ll get off free and clear as you’ll find out next time over an ordering foul-up with the first wheel company.

That’s right, there’s a first wheel company. And a second.

 

Read Part 1 of the 1970 Plymouth series here.

Read Part 2 of the 1970 Plymouth series here.

Read Part 3 of the 1970 Plymouth series here.

Read Part 4 of the 1970 Plymouth series here.

Read Part 5 of the 1970 Plymouth series here.

Read Part 7 of the 1970 Plymouth series here.

 

*Next week: Part 6 of Jeff’s series on rebuilding a 1963 Corvette Sting Ray.

 

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