Classic Car Tire Sizes: Converting the Old Codes

American cars used three sizing systems in thirty years. A 1957 Chevrolet wears 6.70-15, a 1970 Chevelle wears F70-14, and anything built after about 1980 wears a metric size such as P215/70R14. The codes do not translate by eye: the letter in F70-14 is a load class, not a width, and the 6.70 in 6.70-15 is inches while the 215 in 215/70R14 is millimeters.

Below, all three are converted into sizes you can buy, cross-checked against the industry conversion chart and the dimensions the vintage tire makers publish. Overall diameter decides fitment, so every row lists it.

The three eras of American tire sizing

The letter ladder runs light to heavy: A and B economy cars, E and F intermediates, G the classic muscle-car class, H and J full-size sedans, L Cadillac and big-block territory. It does not fix the height: a G78-15, a G70-15 and a G60-15 carry similar loads but stand about 28.0, 27.4 and 26.6 in tall. For the metric code, see how to read a tire size.

Alphanumeric to metric: the conversion table

Convert inside the same aspect ratio: a 78-series code takes a 75-series metric tire, a 70-series code a 70-series tire. Widths are the original tire's.

The 60- and 70-series rows are where charts disagree. F70-14 is also given as 205/70R14 and G60-15 as 235/60R15, each about half an inch shorter. A genuine F70-14 measures 26.2 by 8.3 in and a G60-15 26.6 by 9.2 in, so the wider pick matches diameter better; the chart's J78-15 entry, 225/75R15, is 0.6 in short.

Numeric sizes: 6.70-15 and the rest

A numeric size names the section width in inches and says nothing about the sidewall, so the era matters: pre-1965 sizes ran about 90 percent profile, 1965 to 1970 sizes about 78 percent.

Alternates: 5.60-15 is often replaced with a narrower 155/80R15 and 6.45-14 with 175/70R14. They survive today mostly as van and light-truck tires with S speed ratings, for example 215/75R15.

Bias-ply, bias-belted and radial

• Bias-ply: plies cross at about 35 degrees, so tread and sidewall act as one stiff unit. The car follows the road crown and needs constant correction, the tread squirms in corners, wet grip is modest, 25,000 miles is a long life, and it flat-spots when parked.
• Bias-belted: a bias body with two fiberglass belts under the tread, the Polyglas and Wide Oval formula. Sharper steering and longer wear than plain bias, a firmer ride, and the period-correct choice for most muscle cars.
• Radial: plies run straight across and steel belts hold the tread flat, so the sidewall flexes while the contact patch stays put: straight tracking, far better wet grip, two to three times the life.

Never mix constructions on one axle: a radial and a bias tire build cornering force at different slip angles, so the axle gives up unevenly and without warning. If a car must run mixed, radials go on the rear.

Overall diameter is the number that matters

Diameter sets the speedometer reading, the fender clearance and the stance: diameter = rim + 2 x (width in mm x aspect ratio / 100) / 25.4.

For 215/70R14: 215 x 0.70 = 150.5 mm, or 5.93 in of sidewall; doubled and added to the 14 in rim, that is 25.9 in. The F70-14 it replaces measures about 26.2 in, so the new tire is 1.2 percent smaller. The speedometer then reads about 0.7 mph high at an indicated 60, and the fender gap opens by 0.15 in, half the diameter change.

• Stay within 3 percent of the original diameter, which is 0.8 in on a 27 in tire.
• Check width too: half of any extra goes inward, toward the spring or control arm. Turn the wheel lock to lock on the ground.
• Revolutions per mile is 63,360 / (3.1416 x diameter).

Load rating, speed rating and that 24 psi placard

An old placard was written for a four-ply bias tire and usually asks for 24 psi front and 26 psi rear. Copy that onto radials and they run hot, wear their shoulders and feel vague.

• Pressure: most collector cars on 75-series radials run 30 to 32 psi cold. Reproduction bias-ply tires differ: run what the tire maker specifies, often 24 to 32 psi.
• Load index: a 215/75R15 at index 100 carries 1,764 lbs, so four cover 7,056 lbs, more than any 1960s intermediate weighs. Capacity is rarely the constraint.
• Speed rating: reproduction bias-ply tires are usually good for about 85 mph, and some carry no speed symbol. The tire sets the limit, not the car.
• Alignment: bias-era specs assumed a squirming tire; with radials, cut toe-in to about 1/16 in total and add positive caster if the car allows.

Wheel width: what narrow classic rims can take

Many classics run 5.0, 5.5 or 6.0 in rims, which caps how wide a tire makes sense. Half an inch of rim width moves the mounted section width about 0.2 in (5 mm); too narrow a rim crowns the tread and rolls the shoulder under in corners.

Wire wheels and most rims built before the mid-1950s have no safety bead and need tubes, and a pitted bead seat leaks whatever you mount. A wide tire on a stock rim buys a rounded contact patch, not a wide one.

Whitewalls, redlines and the date on the sidewall

• Coker Tire holds the Firestone and BFGoodrich Silvertown licenses, in genuine bias-ply and in radials with period sidewalls; a bias Silvertown whitewall in G78-14 is about $310.
• Kelsey Tire sells the Goodyear Collector Series, including the raised-white-letter Polyglas GT: about $495 in G60-15, $520 in L60-15.
• Firestone Wide Oval reproductions in redline or raised white letters, about $374 in F70-14.
• Diamond Back Classics vulcanizes whitewalls, redlines and goldlines onto modern radials from BFGoodrich, Nexen, Milestar and Falken.

Whatever you buy, read the DOT date: the last four digits are the week and year of manufacture, and a three-digit code means a pre-2000 tire that belongs in the scrap pile. Vintage sizes are molded in small batches, so a new reproduction can already be two years old. Ten years from the build date is the limit, and a car that sits gets there with full tread: rubber ages by time and ozone, not mileage. See the DOT date code and choosing tires for a classic.

Frequently asked questions

What is F70-14 in modern tire sizes?

215/70R14, which stands 25.9 in tall against 26.2 in for a genuine F70-14. Some charts list 205/70R14; that is 25.3 in tall and 0.4 in narrower, so it fits but sits lower.

What does the letter in an old tire size mean?

It is a load and size class, not a width: A is an economy-car tire, G covers most muscle cars, L is Cadillac and big-block rears. It does not set the height, so G78, G70 and G60 are three diameters.

Can I put radial tires on a car built for bias-ply?

Yes, and most people who drive their cars do. Match the original diameter within about 3 percent, check clearance at full lock, reset the alignment with less toe-in, and never mix radial and bias on one axle.

What modern size replaces a G78-15?

215/75R15, at 27.7 in tall against roughly 28.0 in for the original, inside 1 percent. It is still a common van and light-truck size, so budget and premium tires exist in it.

Updated September 2026.