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Calculating RPM, axle ratios & tire size

(1 March 2008)
Allan Franklin's 1954 Chevy 1/2 ton has a rebuilt '61 235 engine, '57 3-speed transmission with overdrive, (Borg Warner, not working now). When it was working, it seemed OK. Open drive shaft with a '59 rear end. It can't go faster than 50 mph. He wants it to work at freeway speeds (at least 65 mph). What to look for?

Sooooo.... You just finished the frame-off resto on your truck and rolled it out for its gleaming and triumphant debut in the driveway for the family and neighbors to see just what you've been slaving away on all these (months/years/decades). It just screams at you to "take me for a DRIVE, big daddy!!!" So you do and you discover that you forgot just how sloooooooooooooooooow the ole truck is...

       You have two options -- you can leave it as is, thumb your nose at the modern rat race and enjoy the truck's originality (many people do! And it is no crime to do 48 MPH in a 75 MPH world), or you can update your drive train through axle gear replacements, bigger tires, modern engine and driveline or any number of things.

       If you decide to start upgrading aspects of the drivetrain to get more speed out of the ole bus, here's a few ideas, places to start, provided by your fellow 'Bolters.

First things first

       Before you can start playing with axle/tire combos, you have to know what you are starting with -- what is the combo on your truck now? Start with your current axle's ratio. If it is the original axle to the truck, you can easily find what the ratio is by looking around this site or consulting your trucks service manual or the dealer data book. If you don't have access to those resources, or the axle isn't original, just jack the truck up, put the rear end on jack stands so you can safely turn the wheels while watching the driveshaft. Put a piece of tape or other mark on the driveshaft and then slowly rotate the wheel. Count the number of turns the wheels make while the driveshaft makes one turn. If the wheel makes 3.55 turns for each turn of the driveshaft, you have 3.55 rear end.

        Also, you can look for the serial number on the rear end. If you can find it, there are web sites (for example Nova Resources page on Axle Codes) that can decipher it for year, gear ratio, etc. The numbers are usually stamped on the axle tube near the differential. Unfortunately, it was not consistent over the years (left side, right side, front, back, top, bottom) - so you might have to search all over the housing.

       Then, once you know your truck's current configuration (rear axle ratio, tire size, transmission and engine specs), you can use several tools available to help you find a new configuration that will help you achieve safe, modern speeds with your rig.

Determining a new set up

       There are several resources available to help you determine optimal set ups (rear axle ratios, tire sizes, etc) given your current (or desired) engine and tranny set up. Try any of these:

       

One last note -- For safety's sake, if you upgrade your truck for modern highway speeds, PLEASE make sure your brakes and steering are up to the task, as well!

 

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A big Stovebolt Mahalo to Aryln Unrein, Gordon Hillsden & Grigg Mullen for contributing to this article!

Good luck and BE SAFE out there!

Be sure to check out our extensive Forums discussions -- from General Truck talk, Electrical Bay, Big Bolts, Panels and Burbs, Engine and Driveline, Paint and Body, Interiors, Tool Chest -- The Stovebolt Collective can help in your quest and walk you through the mire and magic of working with old iron!

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