45MPH /200r4 / 2000 RPM?????

[Blew86442]

Update 45mph/200r4 / 2000 RPM?????

I bought a 442 with a SBC in it, 200r4 CZ, 3.73 running stock size tires.

My car runs about 2000 RPM at 45 mph. Is that a little high RPM for that speed? I don't believe my TC is locking up. Will this cause damage to the trans? I plan on installing a manual lock up kit soon if need be.

I now have my TCC locking up. My car cruises about 50mph @ 2000 rpm in OD locked up. I measured my tires, the way they are currently worn measure about 25in. Does this sound more accurate?

Your car is rejecting the Baboon heart transplant :lol:. Seriously, the lock up only drops the rpm 200 to 300 rpm in my experience. What does it run at 60 mph? Mine runs at 1850 rpm at 60 mph, 3.42 gears with short 25.6" tires.

Haha....my car will have a fresh BBO waiting on the transplant table when the sbc fails!

 

[King_V]

I'm with the other posters - that sounds like about what sort of RPM you'd have with your gearing if the trans was in 3rd gear rather than 4th.

I know (from a car I had long ago), that mathematically (from published material), with lockup, a 1984 Buick Regal with 2.41 gears, a 3-speed auto trans with lockup, and 195/75R14 tires does 1965 RPM at 60MPH.

3.73 (your gears) divided by 2.41 (my reference gearing) = 1.55

1965 RPM * 1.55 = 3045 RPM @ 60MPH

45 MPH / 60 MPH = 0.75

3045 RPM * 0.75 = 2284 RPM.

195/75R14 is 25-1/2 inches in diameter.

You say your tires are 25 inches in diameter... 25/25.5 = 0.98

2284 RPM * 0.98 = 2239 RPM

Your car is in 3rd gear, with lockup. The tach isn't 100% accurate, which may account for the 239 RPM discrepancy if you're seeing exactly 2000 RPM.

The numbers do not lie. A 200-4R has an overdrive ratio of .67, so at the same speed, 2239 * 0.67 = 1500 RPM (the RPM you should see if your car were really in OD and lockup at 45 MPH).

 

[Blew86442]

Thanks King V,

I'm just going to buy an aftermarket tach. I am more or less playing a guessing game here.

 

[King_V]

You're welcome . . and, yes, I am a bit of a math geek. 😛

 

[drogg1]

viewtopic.php?f=14&t=40029#.UkgF7YZwrhc

Sort of explains.

MPH = ((engine RPM + 300 if lock up engaged)X(Tire diameter in inches))/((Transmission Gear Ratio)X(Rear Gear Ratio)X(336.1352))

engine RPM + 300 if lockup engaged = ((MPH)X(Transmission Gear Ratio)X(Rear Gear Ratio)X(336.1352))/(Tire diameter in inches)

Tire diameter in inches = ((MPH)X(Transmission Gear Ratio)X(Rear Gear Ratio)X(336.1352))/(engine RPM + 300 if lockup engaged)

Transmission Gear Ratio = ((engine RPM + 300 if lock up engaged)X(Tire diameter in inches))/((MPH)X(Rear Gear Ratio)X(336.1352))

Rear Gear Ratio = ((engine RPM + 300 if lock up engaged)X(Tire diameter in inches))/((MPH)X(Transmission Gear Ratio)X(336.1352))

The number 336.1352 takes care of the conversions between units of RPM and MPH.

 

[DRIVEN]

Yet another way to get there -- http://www.angelfire.com/fl/procrastination/rear.html