What a Differential is and How it Works [Video]

An incredibly well-done instructional video produced for Chevrolet by Jam Handy in 1937.

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Andrew is the founder and editor of Primer. He's a graduate of American University and currently lives in Los Angeles. Read more about Primer on our About page. On Instagram: @andrewsnavely and @primermagazine.


  • Reply June 10, 2015

    Jessy Mina

    Wow! This is awesome.

  • Reply June 11, 2015

    Jeremy David

    That’s pretty cool. I’ve wondered for a while how a differential worked but never took the time to find out. Nice post Andrew.

  • Reply June 11, 2015


    That was great! I’ve been wanting to learn about cars but everything I’ve found was too advanced for a beginner. Thanks!

  • Reply June 11, 2015

    Rob Dc

    I knew it had to end with crazy trumpet music. Great video. Can the rear differential fluid affect gas milage at all?

    • Reply June 11, 2015


      Three part answer. First part addresses two wheel drive, second part addresses additional information for all wheel drive and third part addresses four wheel drive.
      First part:
      Very minimally, if at all.
      Comparing diff oil to motor oil, think of molasses compared to table syrup.
      Diff oil has to have a higher beginning viscosity compared to motor oil for a few reasons.
      1. Smaller volume. Diffs generally hold 2-4 quarts of oil compared to 5-8 quarts for motor oil.
      2. Diff oil is captured, i.e. not recirculated/filtered. This means minimal cooling compared to motor oil and all contaminants stay in the oil. Diff cooling is accomplished primarily via air flow across the diff housing.
      3. Due to being captured, diff oil is pretty much (considering gear loading and bearing loading) under constant mechanical load compared to motor oil.
      The only real way for the diff to have an adverse impact on mileage is when something is mechanically wrong (parts worn out causing misalignment of the gears/bearings) and causing drag on the system. Once mechanical problems begin and continue without being addressed the oil will be burned and will lose its viscosity and lubricating properties, further compounding the mechanical problems.
      Part two:
      Current systems generally use a constant front/rear ratio for power (torque) input while some are dynamic, depending on road conditions. This ratio generally ranges from 30/70 to 50/50. What this means is that both diffs are constantly being driven and the same basic principals above apply here. One aspect of all wheel drive that does have an adverse impact on mileage is that with more mechanical components there is more (even when working properly) drag on the entire system and more weight in the entire system.
      Part three:
      The main difference between four wheel drive and all wheel drive (for purpose of this discussion) is that the secondary diff is not being driven, unless it is actively engaged, and is instead being pushed. By this I mean that there is a mechanical load on the system that is not contributing to the movement of the vehicle, it’s basically dead weight. This adds up to having a larger, average adverse impact on mileage over all wheel drive.
      I hope this answers your question and wasn’t simply information overload.

  • Reply June 14, 2015


    Is there any way to save articles from the site for future reference aside from bookmarking them?

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