Thanks for the help. Hopefully the larger pulley isn’t too expensive or I might just have to get one of those tiny tachs. It would just be nice to verify some other way so I don’t accidentally blow my engine. It makes sense but I need to verify.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 and screws up the tach signal.
Not something you really think about until you're affected. I'm just glad Coonass77 chimed in with the answer you were looking for.
As for top speed, they aren't really made to go fast. As it is, once you get the tach straightened out, you're talking a top speed over 70mph easy, and reports have doing over 80 no problems.
What point could I use with one of those gauges to measure rpm’s? I assume a lot of the pulleys aren’t spinning at engine rpm’s or I mean, they are different sizes.