Tires and Miles Per Gallon in Vintage cars

Story and photographs copyright D. S. Brown. Tire brochure images and text copyright Goodyear tires.

Aside from low miles originals and cars using reproduction tires, most collector cars no longer ride on tires of the same configuration as the factory stock tires. Bias Ply tires have been phased out for autos for decades now. Even truck and heavy machinery tires have been switching to radial design. Advances in handling have affected typical wheel diameters and widths which have increased in tandem with diminished tire profiles.

Tires had tall sidewalls back in the 1960s: about 78 to 82% of the width. As tires moved towards 'low profile', the 78 series tires (78% as tall as wide) became the norm for passenger cars, while 70 series tires were considered performance tires, with names like 'wide oval'. 15 inch diameter rims were rarities seen only on Hemi Cars or heavy full size cars. 14 inches was pretty universal and 13 inches was still common on smaller cars. Pontiac's original compact Tempest and the tiny VW Bug were two exceptions to this rule, both running with 15 inch wheels. Rim widths of 4, 5 or 6 inches were normal back in the 1960s.

In the 1970s 15 inch rims were used more often, particularly for performance cars. 7 and 8 inch wide rims were becoming common as were the new 60 series tires. Radials gradually replaced bias ply tires as original equipment on domestic cars. The 78 size tire was phased out and the 70 series tire became the norm for passenger cars. 

By the mid 1980s most 'regular' tires were only 70% as tall as wide. Compared to modern tires, even these are tall tires, but 'low profile' by 1960s standards. 1960s bias ply tires with the same width as these radials have a larger diameter due to the different aspect ratio between width of tire to height of sidewall.

Tire diameter of the radials available for a 14 inch rim has became smaller and smaller. A low profile radial tire will revolve more times per mile than the original bias ply tires of the same width. This will force the engine to rev faster than with the factory stock tires, reducing top speed and gas mileage at steady speeds. The radial design and ability to hold higher air pressures offsets this to some degree. The 'shorter' radial tires improve off the line acceleration, but hinder the car in other areas, costing up to 10% distance covered versus the larger diameter bias ply tires used on 14 inch rims. The engine longevity will be reduced in the bargain due to higher engine revs.

Estimating an 8-10% MPG deficit caused by the smaller diameter of modern radials as a starting point, the radials will compensate for this somewhat by the fact that radials improve gas mileage by about 3-8 % (depending on what source you consult) over bias ply tires which are harder to turn due to 'tread squirm'. The diameter loss may be cancelled out by easier rolling qualities of radials.

In a direct comparison of radials vs. bias ply tires done back in 1967 using a Cutlass, the GM factory saw a 0.2 MPG gain, while the MOTOR TREND people got 0.3 MPG extra (MOTOR TREND, Feb 1967). CAR LIFE in April 1967 claimed 0.5 MPG improvement from radials. Whatever the exact amount back then we know that new radials will provide additional gains. The 1967 radial tires didn't have modern compounds to lower rolling resistance and tire pressures in 1967 were typically far lower than modern settings.

The primary gain derived from radials traces back to the angle of the reinforcing laid through the tire, which reduces 'tread squirm' and hence rolling resistance. Modern materials and engineering reduce rolling resistance further. Modern radials also enable use of higher air pressure settings. The original bias ply tires were usually set to 24-26 PSI. Those old bias ply tires could only be taken to a maximum of 32 PSI, while modern radials are safe at 35 PSI, with many able to handle over 40 PSI. The high inflation rates reduce rolling resistance, improving MPG a few more points.

Taking everything into account, the smaller diameter radial tire MPG loss is probably regained by the radial configuration and high air pressures.

This doesn't mean that throwing any low profile tire on an older car is not an issue. A big problem with the modern low profile tires is that the car will ride too close to the ground, and doesn't 'look right' with giant gaps in the wheel wells.  The best way around sizing problems on a classic car is to fit a wider tire than the original bias ply. A wide radial with only a 60 or 70 profile may still have the same sidewall height as the old bias ply, hence ensuring that you are running a similar diameter tire. The increased road gripping power and unsprung weight can create extra work for spindles and other suspension pieces so it's best not to go too crazy with oversize tires.