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Can I change gearing ratios to improve gas mileage?

I have a diesel ram truck, manual transmission. Love it. But the gearing is short. At 75mph, the engine spins at 2500rpm, and the redline is 3200rpm. Wouldn’t I get better gas mileage with taller gearing? Is this something I can change for not too much money? Or is it better to have the engine revving near the top of its range (don’t those econobox continually variable transmissions always keep their engines screaming near the top of the rev range?) What’s better?

Yes you can.
It’s gonna take lots of money.
Is it a 4x4 ? cha-ching, even more money.

Then you’ll greatly compromise its pulling power. You’ll noitice it wil barely out run its own shadow at take off.

You’ll need to change the ring and pinion gear set in the rear axle, oil, seals, gasket glue and additive if positrak.
If its a 4x4 double that for the front axle too.

Maybe you can find used parts and spend less but prove the ratio first.
Ring gear teeth divided by pinion gear teeth equals ratio.

After you spend 500 or so doing that,
when will you begin to see any monitary gain from it ?

65mph will take care of your problem and save you money instead of costing you!

Yes, I conceded to keep the slower top speed and great pulling ability for my 79 chevy.
I’ve changed nothing and just drive slower, even empty, in that truck.

They make an add-on overdrive or underdrive “splitter” for these trucks…It replaces the stock transmission tail-housing. But you will never save enough fuel to pay for it…All bets are off if it’s 4WD…

I don’t think you are going to get anywhere by comparing your Dodge Ram diesel engine to an engine in an econobox with a CVT. In my opinion, they are way too different for that comparison to be useful. CVTs keep the engine at optimal RPMs. Depending on the engine, sometimes the optimal range is high, and sometimes it is low. Personally, I think the optimal range for CVT-equipped econoboxes is low. They just sound like they are screaming because these cars are designed for optimal fuel economy, so they have little or no sound insulation and engine noise is pretty bad.

You might get better fuel economy with taller gearing, but diesels can be so different from one another, I can’t say for sure.

If your truck has a manual transmission, you could experiment with different shifting techniques. For example, many commercial diesels get the best fuel economy when they are upshifted at 1,500 RPMs and downshifted at 1,000 RPMs, while others do best with a progressive shifting pattern. For more information on this, google “progressive shifting trucks.”

Below is a graph of what a progressive shifting pattern might look like.