Hello All,
I have a 1984 Dodge W250 318 V8 727 Torqueflite, I believe 3.55 gears but could be wrong. Here's my confusion. The previous owner installed a tachometer, its grounded well and has power via a fuse tap. At 65 mph in third gear it's reading roughly 5000 rpm. This doesn't seem right at all to me, I used an online calculator, and with my 31" tires I should be under 3000 rpm in third gear at 65. Is the tachometer inaccurate or am I missing something and the engine really is spinning this fast? I'd like to be able to do 70-75 on the highway without worrying about rpms being too high but at this moment I keep it under 65. Thank you all for the input!

 

Wow, that engine would be screaming. Unless you have the old 5.88 gears, I can't see it spinning that fast. I'll bet you have a universal tachometer. These will have a switch for different engines. Look on the back for a little sliding switch with markings for 4/6/8. I'll bet it's still set for a 4 cylinder engine. It has a very basic computer inside that registers the primary circuit pulses. If it's counting as a 4 banger, it's reading twice as many ignition pulses for #1 cylinder. Set properly, it will probably be reading 2500 RPM.

My old '84 had 3.21 gears. The engine never really screamed. It ran on the highway just fine. If you get a chance, look at your rear differential housing. If the tag is still there, it will tell you what ratio you have. Your front will be just a little less. If your rear is 3.55, your front will be 3.54 so the chain in your transfer case will stay tight.

 

i agree that it is likely set for the wrong size engine. my 81 use to only spin at about 3600rpm at 85mph.

hard to say if your truck has 3.55 or 3.23. the 3.23's were very common until about 91 when they got overdrive and they they started using 3.55 on everything. i know my 89 came from the factory with 31's and it has 3.23 gears. bit sluggish off the line but gets around ok even without 2nd gear.