Contents

HP definition

Measurement

Usability

The Small Tractor FAQ
Frequently Asked Questions

"What's the story with tractor horsepower?"

It seems that in life there are liars, damn liars and tractor salesmen -- if you can get the truth out of any of them, it's a miracle. The upcoming answer to this question will attempt to sort out the wheat from the chaff, the curd from the whey, the flotsam from the jetsam, Shinola from the other stuff and the facts from the myths about tractor horsepower ratings.

Stay tuned.

Calculating horsepower is simple. Horsepower is defined scientifically as "550 ft-lbs/second," or the amount of energy required to lift 550 pounds of weight 1 foot in 1 second. It is a standardized measure of work performed over a period of time.

Measuring tractor horsepower is not so simple. Like any other measurement where more is better -- such as audio amplifier watts or car miles-per-gallon -- manufacturers have devised ways to one-up the competition in the game of "specmanship." Many tractor manufacturers simply rate their equipment with the horsepower rating of the engine, ignoring the fact that this energy must be transferred through the tractor's drivetrain to the wheels or PTO in order to be useful. Others contrive unrealistic test conditions that encourage one brief measurement of high horsepower, which is then used to rate the tractor, even though the machine is unlikely to ever perform that powerfully in actual use. In fact, misrepresenting tractor horsepower is a pretty old game.

Tractor manufacturers began overstating their products' horsepower in the very early days of the industry. Figuring out the truth quickly became almost impossible. In 1919, the Governor of Nebraska commissioned the state university at Lincoln to begin a series of standardized tests on agricultural tractor horsepower and to publish the results, so that at least the farmers in Nebraska would have some objective data on which to base their purchase decisions. For nearly 80 years the "Nebraska Tractor Tests" have been the standard source of independent information on agricultural tractor performance. There is an excellent article about the Nebraska tractor testing facility in the Nov/Dec 1997 issue of "Implement & Tractor" magazine. To this day it is illegal to sell an agricultural tractor in the state of Nebraska without a certificate from the testing lab. Unfortunately, these tests do not include lawn and garden tractors or any tractors less than 40 HP.

Lawn and garden tractor manufacturers love to use non-standardized ratings, such as "rated," "developed," "maximum" and the ever-popular "peak" horsepower in order to make their equipment seem more powerful to potential buyers. Although they are a credit to manufacturers' creativity, these terms are useless in determining the sustained power output capability of a particular machine.

A more useful measure of how much work a tractor can perform over a long period of time is sustained or continuous horsepower, measured by a dynamometer. To test a tractor's pulling ability, its wheels are rested on the dynamometer platform and the tractor is operated at varying speeds in all gear ranges against standardized resistances. The performance is then averaged and the resulting pulling capacity is known its drawbar horsepower, or how much power the tractor can continuously deliver to the drive wheels. In another test, the dynamometer is connected to the tractor PTO and the engine is operated at various speeds against standard resistances. The resulting averaged value is known as the PTO horsepower, or how much power the tractor can continuously deliver to an auxiliary piece of equipment.