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I have an 84 f-250 with axle code 33. The decoder says my rear end gear is 3.54, tire size is 235/85 r16. I’m looking to get a gear that would be better for highway speeds, it’s currently running 3,000 rpm at 70mph. Any suggestions on a gear ratio? Thanks.
 

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Per my favorite RPM calculator the math says you should be at 2625 RPM at 70 MPH with that tire size and a 3.54 gear ratio with a 1:1 final drive in the trans.

Personally I wouldn’t go to a numerical lower rear axle ratio for you’re application. If you went to a 3.00 or a 3.25 gear your going to lose a lot of mechanical advantage when taking off from a standstill or when towing.
 

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Per my favorite RPM calculator the math says you should be at 2625 RPM at 70 MPH with that tire size and a 3.54 gear ratio with a 1:1 final drive in the trans.

Personally I wouldn’t go to a numerical lower rear axle ratio for you’re application. If you went to a 3.00 or a 3.25 gear your going to lose a lot of mechanical advantage when taking off from a standstill or when towing.
Hmm alright my tach must be off a bit. Thanks.
 

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Per my favorite RPM calculator the math says you should be at 2625 RPM at 70 MPH with that tire size and a 3.54 gear ratio with a 1:1 final drive in the trans.

Personally I wouldn’t go to a numerical lower rear axle ratio for you’re application. If you went to a 3.00 or a 3.25 gear your going to lose a lot of mechanical advantage when taking off from a standstill or when towing.
What would you say is the max rpm of a completely stock 460?
 

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You may not be able to assume an overall 1:1 with a C6. I've got a 1976 F-250 with the Dana rear axle.
91984

It's got a 3.07 rear gear and 9.50R16.5 LT tires on it. I don't have a working tachometer on it right now, so I can't provide any RPM numbers, but it seems "comfortable" at highway speeds.
 

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Very true, the converter could be slipping which would raise the rpm’s at cruise. Unless the OP does A LOT of highway driving the return on investment in fuel savings from a gear swap is going to be a long way down the road. Also the off the line performance will suffer.
 

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The first thing I would do is grab my phone and download one of the free GPS speed apps. Go drive the truck and verify that the speedo is accurate all the way up to highway speeds.
 

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Lots of slip in the converter... nature of the beast. For the same reason years ago, I swapped to a TKO 5spd... no slip, lower 1st gear ratio (3.27 vs 2.72 w/ the wide gear kit) plus 0.68 OD for low rpm cruising.

Now there is an auto trans conversion with lockup converter and OD. More expensive but an option.

If it won't be used to tow much, high 2's or low 3's final drive gears will help reduce rpms... but the C6 slip gets worse with lower rpms and higher loads. May want to try a tighter converter before swapping out the gears. It should bring rpms down modestly (~2800) while retaining towing capabilty with current gears.
 
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