Calculating MPG. Tires and speedometers in Vintage cars Print
Written by Double Dragon
Friday, 22 October 2010 20:52


Article copyright D. S. Brown, except for the gas calculator image which is copyright Rand McNally 1975. Tire brochure images and text copyright Goodyear tires.



If you have a modern car chances are it has a trip computer which tracks your MPG. For those of you who lack a trip computer, or who wish to directly check the mileage for yourself here is the procedure.

Completely fill up your tank each time you get gas. Write the mileage from your odometer on the receipt that comes out of the pump. Some prefer to record the mileage and amount of gas in a log book. Some readers simply use their cell phone to take a photo of the odometer and the gas pump reading after fill-up. The cell phone automatically date stamps the image so the two numbers can be easily matched up.

Take the current mileage; subtract the previous mileage at the prior fill up to determine how far you went between fills. Divide the distance by the number of gallons printed on each pump receipt to get the miles per gallon. Canadian and European readers can convert this figure to L/100k by googling 'conversion' and finding an online site to convert MPG to metric.

True fanatics wanting more precision should use the same pump at the same gas station to ensure that the car sits on the same angle each fill up. This ensures that the 'shut off' happens at the same point. For exactness you need the same load distribution in the car, too. If you have a passenger and items in the trunk, and next time you don't, the angle of the gas tank changes which can alter the tank 'full' point. Obviously, some modern luxury cars have solved this issue with load leveling air suspension, but for most cars variable load distribution changes car angle. Use the same fill speed on the pump gun so that the automatic shut-off happens at the same level.

Below is a nifty little MPG 'computer' printed on the back of a Rand McNally 1975 San Francisco, CA map.


Tires, speedometers and MPG in Vintage cars

Aside from low mileage 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 on autos for decades now. Even truck and heavy machinery tires have switched to radial design. Advances in handling technology have led the change in affect standard wheel diameters and widths on modern cars. New cars have wheels that are gigantic compared to the 1960s. These large wheels 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. The 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% 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 modern 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 because radials improve gas mileage by about 3-8 % (depending on what source you consult) over bias ply tires. Bias ply tires are harder to turn due to 'tread squirm' which forces the engine to work harder for each wheel revolution. The diameter loss found in modern replacement tires may be cancelled out by the easier rolling qualities of radials.

In a direct comparison of radials vs. bias ply tires done back in 1967 using an Oldsmobile Cutlass, the General Motors factory crew observed 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 on old classics.

-------------SPEEDOMETERS IN OLD CARS----------------------------------------------------------------

Measuring miles per gallon requires an accurate odometer and speedometer. Most speedometers don't tell real distance or speed.

Car manufacturers usually engineer more accuracy into the odometer than the speedometer because the service intervals are tied to this number. The speedometer frequently gives high readings intentionally, especially for cars made back in the 1960s. Even modern electronic speedometers which are capable of great accuracy usually have a built in 'exaggeration factor'. A bit of 'extra' is put in to insure the manufacturer is blameless if someone is nailed for speeding.

Some speedometers are even further out than the usual 5% or so. One of the craziest examples of this happened in the 1950s and 1960s when a high performance variant of a pedestrian sedan received a 140 or 160 mph speedometer. The face of the unit was changed while retaining the 120 MPH passenger car internals. They didn't even bother to recalibrate!

The speedometer of older cars used to run off the front wheel. By the sixties most cars ran the cable off a gear in the transmission. Newer speedometers are advanced electronic units capable of great accuracy. In older cars, the speedometer is a mechanical device. A wheel or transmission turning at a specific speed will produce different road speeds depending on variations in tire diameters and rear axles. Putting different diameter tires on a car will allow the car to run slower or faster at a set engine RPM. The mileage and speedometer indications remain tied into the factory issued tire and wheel size. The speedometer merely counts wheel revolutions, oblivious to the actual speed or distance covered.

The same variables occur with optional high or low geared rear axles. Sometimes the car is not fitted with the correct size speedo gear in the transmission when an optional axle ratio is installed. The genuine speed will differ from indicated speed in the same ratio that the axle differs from the standard issue one accounted for by the number of teeth on the transmission speedo gear.

Speedometer problems crop up when outfitting a classic car with modern radials. Current tires have lower profile and hence smaller diameter for the same width as a comparable old bias ply tire. This will cause the speedometer to indicate 65 mph when you are really only going 61 mph. Your odometer says that you traveled 100 miles when you only covered 95 miles.

One way around this is to fit a wider tire, so that the lower profile radial has the same sidewall height as the original bias ply, creating approximately the same diameter as the original tire. Caution is needed here because the extra gripping power and unsprung weight of wider tires creates strain on spindles, bearings and other suspension components.

If you want to know exact, true MPG you need to correct the speedometer and odometer. Companies can do this for a fee, but you can do it yourself.

A simple and free way to do this is to use modern satellite technology. Smart phones now have GPS programs that take readings of your changing position via satellite and can tell you your current speed. If you drive a few miles at a steady speed you can compare your speedometer reading to the GPS result.

The old school way to do this if you don't have a cel phone is to check the car's readings against highway mileposts. Interstates frequently make this task even easier with 'odometer check signs' sporadically posted in open stretches. Any stretch of regular mile posts will accomplish the same goal. Comparison of the odometer to the measured mile markings will tell you how accurate your odometer is.

The speedometer is frequently calibrated differently than the odometer. You can check the speedometer using the second hand in the car clock, or a stopwatch while driving a steady speed past mileposts. Time how long it takes to travel one mile or the full five miles of the check posts. Easiest is to go a steady indicated 60 MPH which should be exactly one mile per minute. If you hold 60 for all five mileposts your stopwatch should show five minutes exactly. Covering the distance quicker means the speedometer is reading lower than true speed. Taking longer than five minutes indicates speedometer exaggeration.


Last Updated ( Friday, 19 March 2021 22:54 )