RPM at 60mph / Tach issues

[T-K]

Hello!

351 C 4v and 4 speed toploader with 3:50:1 rear end gears, correct factory options.

Anyone else running the same that can tell me what RPM`s you have at around 60mph, or 50mph?

My tach seems a little high, almost 4000rpm at 60mph.
It seems that above 3000rpm the tach lives its own life.

When cruising at 50 - 60 mph and then give full throttle the tach drops about 1000-1200 before it starts climbing again, and when letting my foot off the pedal the tach will move up about 1000rpm.

Engine runs great.

Running stock distributor and coil.

[Steven Harris]

To answer your question exactly you will also need to plug in your tire diameter. I would guess that you should be running about 2,230 RPM at 55 MPH or 2,433 at 60 MPH.

Here is how to figure it our exactly:

First find your tire diameter. I will use a 235/75R 15 for an example. Take your metric width size (235) and divide by 25.4 to convert to inches. You now have 9.25" (235 / 25.4). The 75 of the above tire size is a width/height ratio. Take your height of 9.25" and multiply by .75 (the ratio) and you end up with 6.94". Multiply that by 2 (the upper and lower portion of a tire height, now we have 13.88" and then add our rim height of 15" and we end up with an overall tire height of 28.88" Of course the actual height will be a little less as all pneumatic tires have flex.

So our 235/75R 15 translates to a 28.88" height and 9.25" width.

Take those know figures and plug them into an engine RPM calculator such as this (I'll not bore you with more calculations). http://www.csgnetwork.com/multirpmcalc.html

So with the above tire you should be running between 2,230 to 2,270 RPM's at 55 MPH and between 2,433 to 2,475 RPM's at 60 MPH.

[T-K]

Thanks

Im supposed to run at 2822 rpm at 60mph, according to the calculator.
That makes sence, because when my tach reads almost 4000rpm at 50-60mph, it droppes 1000-1200 when i push the pedal to the floor, making around 2800 +/- rpm at 60mph +/-.

Its tempting to make a Tremec T5z WC swap