So if the engine was spinning 3,000 rpm at 70 mph-in overdrive it will now spin at 3,000 x 0.7 = 2,100 rpm. Stated another way, this overdrive will reduce the engine rpm by 30 percent. So in the case of a 700R4 or a 4L60E (they are the same with regard to the overdrive), the fourth gear ratio is 0.70:1.
In order to determine the overall gear ratio in overdrive, the math calls for multiplying the rear gear ratio by the overdrive ratio. The variables that we have to deal with include:Īll of these affect the selection of the rear gear ratio.įirst, let’s cover some basics. There are multiple variables to deal with, and while you didn’t tell us how fast you intend to cruise, or the height of the rear tires (which affects gear ratio), we can still get pretty close based on some estimates. It sounds like you have a great combination, and with a little bit of simple math, we can offer several options. Jeff Smith: We can certainly help you with this. Is there a way to figure out what gear I should run? Right now, the car has a 3.08:1 rear gear but I’m guessing that won’t work well when the trans shifts into overdrive. I don’t drag race this car but I do want to be able to cruise down the freeway at a decent speed-somewhere around 2,200 rpm. The car right now has a mild big-block 454 and I’m looking to put together a nice running street and highway car. I have an early 70s Chevelle and I am planning for either a 700R4 or maybe a 4L60E.
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