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Gearing Formulas

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    • Gearing Formulas

      Authored by Wes Vann, April 3, 1999​

      • Opening comments;

        On this page, I'm going to go into the different formulas that youmay want to use in order to set up the gearing on your car.

        It's not really a "stand-alone" page, in that you shouldhave the "gearing" data from the transmission pages (I have toadd an "automatic transmissions" page). I'd recommend that youprint them out and have them handy. The April Car Craft (1999) has a reallygood listing of what rear-end ratios are currently available!

        This is a long "text" page and in it's final format, therewill be "jumps" to the desired sections.

        Personal thoughts;

        1) What I want out of a car is probably different than what you want.Although some of my opinions may be present, don't let it bug you or thinkit's a personal slam!

        2) We are talking about cars with V8 engines here and not some highrpm, narrow power band 4 cylinder. As such, I really don't see any reasonfor a 6 speed transmission.

        3) Any transmission with an overdrive will give you better fuel economyand lower the rpm while on the freeway. That lower rpm means that the enginewill last longer! I don't see the need for two overdrives.

        4) Currently, this page is geared (sorry) towards manual transmissions.The formulas are still valid for automatics and I may revise or add moreinfo latter.

        Things you need know!

        1) The formulas don't take into account things like aerodynamicsor frictional loss at speed. As such, even if a formula says you can do200 mph, the engine (or common sense) may say no!

        2) The formulas also assume that the tire diameter stays the same atdifferent speeds. If you watch a dragster at launch, you may notice thatthe tire expands in diameter and that changes the total over-all gear ratio!(My wagon sure doesn't do this!)

      • Directory; (andfuture jump point)
        • 1. Terms Used
        • 2. the "Speed" formula
        • 3. the "RPM" formula
        • 4. the "Launch" or "Top End"formula
        • 5. the myth of "I've got to have 4:11's"
        • 6. "Viper" top end got you down?

        1. Terms Used;

        Terms I'm going to use;

        "circ", this is the circumference of the rear tire measuredin inches.
        "diam", diameter of your tire measured in inches.
        "RPM", revolutions per minute of the engine (engine speed)
        "trans", transmission gear ratio, "0.68" would be anoverdrive ratio.
        "rear", rear end ratio, as in "3.31" to one.
        "60", this is a correction factor converting hours to 60 minutes.
        "63371.4", this is a correction factor converting inches to miles.
        "speed", this is the miles per hour you are going

        To get the circumference of the rear tire, you could put a mark on yourtire toward the bottom, and a mark on the ground adjacent to the tire mark.Then roll the car until the mark is down in the same position. Make anothermark on the ground and measure from it to the original mark on the ground.This is the circumference. Or you could use the following formula;

        "circ" = 3.14159 x diam

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        2. the "Speed" formula;

        Here is the formula for computing the speed of the car if you know theRPM;

        "speed" = RPM x circ x60
        trans x rear x 63371.4

        As an example, if you had a transmission with a 0.68 overdrive, 3.31rear end gears, tires with a 75 inch circumference, and (finally) the engineis turning at 3000 RPM;

        "speed" = 3000 x 75 x 60= 95 miles per hour
        0.68 x 3.31 x 63371.4

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        3. the "RPM" formula;

        Here is the formula for the RPM if you know the cars speed;

        "RPM"= speed x trans xrear x 63371.4
        circ x 60

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        4. the "Launch" or "TopEnd" formula;

        This formula serves two different purposes, and as such could be called"launch" or "top end" depending on what youare after. For simplicity, I'm just going to call it "launch".

        If using "first gear" values, the formula will give a "launch"value. The higher the number (larger), the stronger the launch.

        If using the transmission "final gear" (fourth, fifth, orsixth?) values, the formula will give a "top end" value. Thelower the number (smaller), the better top end speed (for any given enginespeed).

        This formula does not take into account the tire diameter andjust assumes that you are not going to be changing to a different diametertire. It's not the "over-all" gear ratio, just a number thatyou can use for comparison purposes!

        "Launch"= rear x trans

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        5. the myth of "I've gotto have 4:11's";

        One of the things that really bugs me is hearing somebody saythat he just has to run 4.11's! I personally think that it's justone of those things that get into peoples minds because they hear aboutsome 60's muscle car that ran them.

        We are not in the 60's and a car with 4.11's is not something youwould want to drive on the freeway!

        The wording should really be that the individual wants "over-all"low-end gearing similar to what an early car with 4.11's had. You haveto take into account the transmission gearing!

        There are options! You can have your cake and eat it to! (butyour wallet will pay for the cake)

        Let's get real and use the formulas.

        As a "base-line" example, assume a Muncie M22 rock crusherand 4.11 rear gearing (also assume a tire circumference of 75 inches).The M22 had a low gear of 2.20 and a final drive of 1.00. We are goingto compute the "launch" value and use it for comparison. We alsowant to check the engine speed at a legal 65 mph. For fun, we will alsocheck the rpm at an illegal 120. (I'm not telling you it's ok todo 120!)

        Launch = 4.11 x 2.20 = 9.04 (it's just a number for comparison,but keep the number in mind!)

        RPM (@ 65mph) = (65 x 1.00 x 4.11 x 63371.4) / (75 x 60) = 3762 RPM(pretty high rpm!)

        RPM (@ 120mph) = (120 x 1.00 x 4.11 x 63371.4) / (75 x 60) = 6946RPM (you just blew-up your big block V8 car and got beat by a Honda!!)

        Now let's run the numbers using a Richmond 5 speed (Doug Nash) and 3.08rear gears! The Richmond has a first gear of 3.26 and a final gear of 1.00.

        Launch = 3.08 x 3.26 = 10.04 (this is larger than the 9.04 above,and as such you would launch harder!)

        RPM (@ 65mph) = (65 x 1.00 x 3.08 x 63371.4) / (75 x 60) = 2819 RPM(mellow, and the Beach Boys can be heard on the 4 track tape player)

        RPM (@ 120mph) = (120 x 1.00 x 3.08 x 63371.4) / (75 x 60) = 5205RPM (that Honda didn't get away and you still have pistons withoutholes in them!)

        Want to get crazy, and still have that mythical 4.11 launch? Let's runthe numbers if you put in an "aftermarket" T56 and 3.42 reargears. The first gear ratio is 2.66 and the final gear (sixth) is 0.62.

        Launch = 3.42 x 2.66 = 9.10 (slightly higher than that original4.11, just about the same launch, a little better)

        RPM (@ 65mph) = 65 x 0.62 x 3.42 x 63371.4) / (75 x 60) = 1940 RPM(think about the mileage!)

        RPM (@ 120mph) = 120 x 0.62 x 3.42 x 63371.4) / (75 x 60) = 3583RPM (this is a slower engine speed that the original car had at 65mph!!)

        Hey, let's drive to Los Vegas!! But we still have to have that4.11 feeling! This time it's a GM T56 6 speed and 3.42 rear gears. Firstgear is 2.66 and final gear (sixth) is 0.50.

        Launch = 3.42 x 2.66 = 9.10 (same as above)

        RPM (@ 120mph) = 120 x 0.50 x 3.42 x 63371.4) / (75 x 60) = 2890RPM (Elvis could only dream of this! Now you know why this transmissionis popular with the guys that do things like the Power Tour!)

        For fun, I plugged the engine speed of 2890 rpm into the "speed"formula and used the gearing from that original "base-line" 60'scar. At 2890 rpm, it would only be going 50 miles per hour!!!!

        With all that said and done, what you really have to figure out is what"over-all" gear ratio you want for launch. I can't help you out!I don't drag race at all! What I'd recommend is that you just keep yourears open when listening to people that are running a car with similarengine and weight. What rear are they running and also what transmission.What are their 60 ft. times at the quarter mile? Do they "top-out"before hitting the end of the "quarter"?

        The rear-end ratios that I quoted are currently available for Chevy12 bolt rear-ends (as far as I know).

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        6. "Viper" top end gotyou down?

        A 64 Chevelle wagon is an aerodynamic brick! But let's say that we haveton's of horsepower, don't mind swapping in a Ford rear-end (they havea better selection of rear-end ratios), and just have to do 200mph!

        Let's see what can be done, and still not add an "extension"(Gear Venders) overdrive. We could run a GM T56 6 speed, Ford 9 inch rear-endwith 2.80 gears, and a 28" tall tire (the other tire values were basedon a 25" tall tire, like I have on my wagon).

        The tire circumference = 3.14159 x 28 = 88"

        RPM (@ 200mph) = (200 x 0.50 x 2.80 x 63371.4) / (88 x 60) = 3361RPM (do-able!)

        Just count on having a lot of horsepower to plow through thewind (and also think about what happens when the car wants to fly!)!!

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        Have fun!