Gas vs PSD
Elder User
Joined: May 2010
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From: WV
I'll check mine tonight too... should only be a 200-300 more RPMs then a 4.10 at those speeds.
Postmaster
Joined: Oct 2006
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From: SE Texas
it is. my truck has 265/75r16 with 7.3, 4.10, e40d in overdrive at 75 mph im turning somewere around 2900 rpms. when it had the stock 235/85r16 i believe it was just under that. i dont remeber its been a while but i know it was somewhere in the area of 2700-3000 rpms
All of those sizes calculate to be ~2300 rpms with a 4.10 axle and a .71 OD gear.
JL
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From: SE Texas
JL
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From: WV
Mine should be about the same then, since my fifth gear is .71 I think. Now that I really think about it, mine is between 2500 and 3000 at 80 mph (our highway speed limit is 70mph)
Posting Guru
Joined: Aug 2009
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From: emporium
im not saying your math is bad. it may be right but i my tach doesnt read that at 75 mph
Postmaster
Joined: Oct 2006
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From: SE Texas
JL
Posting Guru
Joined: Aug 2009
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From: emporium
well how do you know any tach is correct then? i highly dought that most of the 7.3 trucks have bad tachs in them
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Elder User
Joined: May 2010
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From: WV
That's why I would say most OE tachs are +- at least 200 RPM
More Turbo
Joined: Jan 2010
Posts: 680
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From: Las Vegas, Nevada
Im running 2800-2900 rpms at 70 mph with 3.73 on 35 inch tires. I know my tach is pretty close cause it reads exactly 1200 rpms on my high idle. The high idle is what i used to replace my rpm needle when i put in my white face gauges.
Postmaster
Joined: Oct 2006
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From: SE Texas
Even in Drive with a 35" tire-you shouldn't be much over 2500.
You hosed up the tach putting on that bling.
JL
Elder User
Joined: May 2010
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From: WV
The alternator has a frequency it admits as it turns. It is a faint frequency, but it's there. So the higher the frequency is, the faster alternator is turning, the faster the alternator is turning the faster the engine is turning. Hence from the alternator frequency you can find your RPM. Problem is that it is not a direct measurement, so there are many places for it screw up, including the calibration of the tach itself. I would imagine newer cars and truck just read directly from the computer, but I'm not sure.