When I'm at 50mph my rpm is at 2000. It goes into 3rd gear at around 40 and when I hit 48 it shifts down 200 rpm and it reads 50mph at 2000rpm. Any faster I go the rpm goes up with it. What could cause this? I've been topping out at 50mph because I'd rather not go above 2000rpm while traveling on the highway.

 

You are not getting into OD.

Did you have another thread on your tranny? (There are so many recently I cannot keep them all straight.)

 

When you maintain a speed the transmission torque converter will go into lock up and the RPM will drop 200 to 300 RPM. This is normal for a lock up torque converter. Do you know what gears that the rear differential has, 355 or 373 ? Because that will also change your RPM readings going down the road. I also notice that you didn't say if the transmission was going into overdrive.

 

Wouldn't he have to have tiny wheels and tires to be running 2000 rpm at 50 mph even with 355? I am turning about 2100 at 77 mph (according to a couple fellers in black and white cars with fancy lights...)

 

Most truck owners will go with bigger tires, I know i would. But not knowing history or being able to touch the truck, I would be only guessing and using what i know of the product. I would say we need more information from the owner.

 

I have 3.55. It shifts 1-2,2-3, and then it drops 200rpm as if it changes into 4th (3-4)

 

tires are 265/75/16

 

you're experiencing t/c lock up, is all.. it locks at the top of third and fourth/od gears..

there is a mathematical principle that can't be argued with.. if you know your forward speed, your overall drive ratio, the RPM's can be given precisely.. if you know any of those two, the third can be figured..

fluid coupler/torque converter has an effect on ratio, the transmission gearing has effect on ratio, the axle, and the tire height all play into overall ratio.. you can calculate what RPM you SHOULD be running at any given speed, in a known gear and t/c position, and through your leverage (gearing mechanisms including tires).. if you're not, you gots yourself a problem somewhere..

 

ok, but is my rpm and speed good? i feel as if the engine is revving too much if i hit even 60mph

 

nope.. you're running too high an RPM for 50mph on those tires, even if you're running 3.92 or 4.10 gears.. you should be between 1500~1700 @ around 50mph depending on what gears you have..

first off, check your speedometer against a GPS.. see if it's lying to you..

second, do you know if it has been re-geared? 4.88:1's would put you near that on those tires.. 5.12:1 and it would be maybe a little over that.. to check ratio, jack up rear of truck.. mark the drive shaft and the tire, put it in neutral and spin the wheel one revolution- count the number of times the shaft spins.. this would approximate.. to know for certain, and if you suspect you've been re-geared, pop off the diff cover and start counting teeth on the R&P..

a slipping transmission would allow you to turn more RPM's than what is being applied through the leverage of overall gear ratio, but I would hope you could hear and feel that slippage..

edited to add: a slipping pinion gear on the face of the ring cog would also allow you to turn more RPM's.. but you would hear that for certain- it would whine like a baby mule.. your o/d could be failing, too- disallowing the actual reduction..