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2002 Isuzu NPR only goes 45mph


New Recruit

I joined because i saw several references to this forum on other forums. I just bought a 2002 Isuzu NPR stake bed 5spd , it smoked a little but it had been sitting a long time, super dirty but i am happy to clean it up. It starts great and ran nice, the test drive I did not think much of how low the 1st gear was, most trucks we have had are larger trucks and 1st is always made for heavy loads but the problem comes later once you try 5th. I hit the highway to come home and i am already in 5th on the on ramp. It tops out at 45 and not because of lack of power it just feels like there should be another gear or maybe even 2 more geats. It feels a lot like when you have a 2 speed rear axle and you are in low range. The speedo says 70mph on the dash, gps says 45mph. I got a great deal on it and know i will need to do a little work.

I have begun looking all over the truck for a reason for this, but thought I would just ask if this sounds familiar to anyone? Did they make different rear ends for these or even a 2 speed version? Does anyone know a good reference to check the rear diff for a stamp to indicate the gear ratio so I can confirm the truck has the right rear axle, maybe the axle got replaced and it went to auction without anyone figuring it out?