Ok, didn't mean to muddy the water. I need to focus on the high rpms first.
Not at all. Definitely was a legitimate question.
Turned out to not be a thing, but always worth asking. (ok, "almost" always worth asking!
;D)
Could low oil in the rear cause enough resistance to raise the rpms?
Nope, but one way to tell is how well it rolls.
If you put it in neutral and coast, does it stop immediately? When you take off from a stop, do you have to really push the gas pedal down? Does the engine feel under-powered (for an old school engine that is) at all?
The higher revving trans before shifting could indicate a higher load, but you'd feel it in those other aspects. If it won't coast down your driveway, you might be on to something.
Heck, even a parking brake can be left on! But here again you'd smell the culprit pretty quickly.
But a big extra resistance is not generally from low oil levels. Not until the gears are ready to melt down or freeze up.
Inside the differential is a physical connection. The only break in that hard connection between the engine and rear wheels is inside the transmission itself.
If the trans and/or torque converter are failing/slipping, you could get higher rpm levels.
The shifting points do seem high for a Bronco C4 though, as mentioned. So maybe there is something going on inside. But before major repairs are warranted it's time for some confirmations such as you are going to try with the tachometer.
Am I not accurate in that 3.5 revolutions of the driveshaft after one complete rotation of the tire equals 3.55 gears?
You are correct. Though I believe the Ford 9" differential ratio is 3.50 instead of 3.55. That's why I asked about the open vs limited-slip diff though. An open can skew the readings under certain circumstances. I've seen them halve the number, so that a 4.11 was actually only turning 2 times on the driveshaft.
But all that is neither here nor there due to the miniscule difference that .05 : 1 ratio change would make in the engine speed. And to you having a limited-slip where both wheels would turn in the same direction if they were both off the ground. Giving you the full reading.
And since 3.50 is the most common ratio found in Broncos (it was the stock ratio for V8 equipped Broncos) it's probably correct.
So we know the speed test is accurate because you used a GPS app. Probably have dogged other cars with accurate speedos too?
We know the tire size from your measurements.
We know from the experience of many, and the many questions that arise from the perception of high rpms that the "sound" of the engines makes you think it's really revving up.
We don't know if the tach is accurate yet, even though it pretty much coincides with your feelings on engine speed.
We probably know the gear ratio, assuming your test was accurate.
Which leaves only the transfer case being in a lower range, and the transmission slipping.
But the t-case being in low would result in the engine probably pushing redline at 50mph!
Leaving only the transmission if all else is correct.
And that's spelling out "expensive fix" right now, so for peace of mind, verify the tachometer readings somehow, and maybe still pull the front cover off of the front diff to count the teeth.
Make sense so far?
Paul
