Gear Ratio Education Help

Old October 18th, 2018, 05:56 PM
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Gear Ratio Education Help

Hi,
I have no shame in admitting I know next to nothing about appropriate gear ratios but wanted to ask and get some advice. My 70' CS is a slightly modified 350 4 BBL with a TH350 automatic trans and stock rear end. I haven't found the stamp code on the axle yet (gonna wire brush it soon) but I believe the 70' CS was either 2.56 or 2.78 stock.

As my driving style is fairly conservative with no drag racing and mainly all weekend, city and some highway driving, I haven't felt the need to change the gear ratio to a different setup. What are the pros/cons and for a driver and not a racer, is the stock setup a good match for the car?

On a slightly related note, I don't have a tach hooked up yet to know what my RPM's are at cruising speeds but I assume they're in the normal range. If my understanding is correct, the RPM should be around the 2500-2800 range @ 65-70 mph cruising speed. Am I correct or in the ballpark? I'm confident the trans is shifting into all 3 gears smoothly, so when I do hook up a tach, if the range is higher than that amount, does that indicate an issue with the stock ratio and perhaps not ideal for my setup?
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Old October 18th, 2018, 10:18 PM
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The rear tire size will affect the overall effective gear ratio, so a shorter tire will result in higher RPM and a taller tire will result in lower RPM with the same rear gears.

With a 2.56 rear gear and 27" tall rear tire, the engine should be turning around 2400 RPM at 70 MPH.
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