Reflash computer after gear-swap

Cursedtofirst45

Cursedtofirst45

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Oct 29, 2008
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So I just had a shop swap out the gears in my rear end from 4.10s to 3.55s and now the tach still reads a bit high. My buddy told me that I needed to have the computer reflashed to read the 3.55s, which makes sense. But I can't find confirmation online from what I've searched. Anybody have to do this?

Truck is a 1998 12 valve
 
Tach is run off the engine, rear gearing should not affect the tach.

Speedo is a different story. Yes the dodge dealer can make the change. I had it done to match my larger tires.
 
Weird, it just seems to read a little high from the trucks I've had with 3.55s. It's at about 2050 rpm at 65 mph, and I'm only running 33" tires. The other 12v i've had with 3.55 run about 1700 at 65. Is there something i'm missing, I can definitely tell the difference when I'm driving.
 
I've only ever had manuals. Is there something the guy who installed them could've done wrong? I know he said he was having trouble setting them
 
Unless he knocked teeth of of the tone ring, improper setup wouldn't affect the speedo reading. Tach could be off, speedo reading wrong. Checked with GPS?
 
I want to check with a GPS. I think the tach is off, even with 4.10s, it showed about 2200 rpm at 60 mph
 
So I just downloaded a speedometer app on my phone that runs off of a GPS signal. It shows that I'm going about 68-69 mph when my speedo reads 60. So that explains it. How do I go about fixing this, is it possible to reflash for a speedo adjustment?
 
Awesome, I'm bringing it to the dealership in the morning, hopefully doesn't charge more than 0.5 hours of labor.
 
I can change it with a Snap On scanner. You can use a website like Tires.com and use their calculator to figure revolutions per mile according to tire size. That is what the computer uses to calculate MPH with.
 
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