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One other thing, your truck does require a 12V feed to power the transmission control module. The 60 amp gen and aftermarket 24V alternators do NOT have a 12V feed. If you try to go that route you will have to figure out some way to feed 12V to the rear battery to keep the TCM functional and not...

measure the diameter of the pulley. To me it looks too small, but without a good size reference we could be wrong.

if it was me I'd do it from underneath the truck, aimed at the crankshaft pulley. That's pretty much the only thing that is guaranteed to be at crankshaft speed.

pulley size definitely affect it. The earlier trucks used a drive off the oil pump for the tach. With the turbo on there there is no room for that drive, so instead it takes a signal from the generator, and since that's belt drive, changing the pulley size affects how fast the generator spins...

The tach is a handheld unit. You just point the end at the moving object to see how many times per minute the shiny tape you put on it goes past. In this case you'd want something on the crankshaft, as that's the speed you are trying to determine. However, the added information that you have...

just to verify, what generator do you have in your truck? As the other thread said, if it's the 400 amp then it has a smaller pulley and will change what the tach reads.

the one he linked there just requires a shiny spot on a moving object in order to get it's RPMs. It comes with a strip of shiny tape. You put a piece of tape on one side of the rotating object, and every time it goes past a light from the tach gets reflected back at it. You'll likely have...

I suppose it could be the the transmission slipping somehow, but if it's really slipping that much it's probably already toast, and the smell of burnt transmission fluid should be pretty obvious when you pull out the dipstick. Once you hit lockup on the transmission there shouldn't be any...

This will sound a little rude, and I apologize if it does. Everything points towards the tachometer giving you a bad reading. This is the second thread you have started for your issue. The information we have to go off of is: You bought a truck with a hacked up wiring harness. the...

So how to verify any of this? Block the front tires, both in front of and behind, to keep it from rolling anywhere. Put the transmission into neutral. Can instead put the transfer case in neutral instead, but transmission is easier usually. Jack up one rear tire of your truck. Put a mark on...

Now you're talking about a differential gear ratio of about 5.05:1 A work truck will "feel" a different speed than a hmmwv. You're at different heights from the road, different viewing angles, different lots of things. A work truck built for the civilian market will have a lot different "feel"...

2nd gear with 40 inch tires would max out at 4000 rpms at 55mph. This sounds like where you were at. What did the RPMs read on your tach? doh, would be over max. How about this to check it out. Put the truck in first gear. Drive at 25mph. Your tach should read right about 3k rpms. Or do...

the ECV/REV manual only has one part number each for ring and pinion.

okay, so with 40 inch tires if you are in 3rd gear it would put the differential ratios about 3.18 at 3300 rpms. If the tires are actually 39" diameter instead of 40", then it puts it right back at 3.08 where it's supposed to be. 67mph at 3300 rpms would be 39.9" diameter tires back at 3.08...

All the ECV/REV trucks should be geared 3.08. I haven't heard of lower gearing than that in any military HMMWV, but I suppose it's possible somehow. Always the chance the wrong gear for the speedometer is in the transmission, or maybe you're getting stray signals to the tachometer causing it...