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I have a Thomas 1991 school bus (36K gvwr) with a CAT 3116 engine and an Allison MT 643 transmission. I purchased it quite recently and when driving it on the highway, the engine RPMs are at 4000 driving at 55mph.

I don't believe the transmission has an overdrive gear, but the rpms seem like a lot to a newbie like me. In fact, 4000 rpm is the furthest possible point on the tachometer.

  1. is this harmful to the engine?
  2. is this normal?
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    Are you sure it's making it to fourth gear? Like you said, there is no OD in the transmission. 4th gear is 1:1 ratio. 4k rpms has to be the absolute maximum for that engine, over if you ask me. You don't want to run it at that speed all day long. The CAT 3116 makes 205hp @ 2400 rpm and 350 @ 2800 rpm. 4k is pushing that thing long/hard up the rpm scale. – Pᴀᴜʟsᴛᴇʀ2 Apr 11 at 23:08
  • Has someone fitted smaller tires or the wrong ratio differential? – Solar Mike Apr 12 at 6:19
  • @Pᴀᴜʟsᴛᴇʀ2 I'm not totally sure (it's at the mechanic's now) but based on your feedback, I'll ask them to check it out. – Joe B Apr 12 at 6:53
  • @Solar Mike The tires are the regular size for the vehicle with stock rims too. – Joe B Apr 12 at 6:53
  • Did someone fit a replacement axle... You need to check everything until you find the cause... – Solar Mike Apr 12 at 6:55
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I had the mechanic look into it and he was able to determine that the rpm sensor on the engine was bad. He replaced that and a grounding wire and now the RPMs read perfectly.

I also found out from the mechanic that the max rpms for this engine (CAT 3116) are 2800 RPM.

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