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DanielG

Need 5 mph more!!!!!

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Could someone please tell me how I can get another 5 to 8 mph out of my Mack please. I have an R688ST with a E6 300 engine and a T2090 trans, 4.64 rears, fuel turned up a bit but still can only get this truck up to 65 mph no matter what I do (except downhill). Plenty of pulling power at 1600RPM , actually pulls unbelievable for a 300. If I am sitting still I can get it up to 1900 rpm to the floor, but going down the road in any gear it doesn't seem to go above 15 to 1600 rpm ?? Any help would be appreciated. I only want a 5 to 8 mph more. Thanks again.

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short of changing rear end ratios  a good pump shop could pull your top rpm up to 2100 making all necessary adjustments that go along with that setting.  or change tire size to larger .either way is not cheap

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what is the high idle spec. for the 300?  I haven't been around them, but most of the other 6 cyl. would turn around 2100.  the tag on the valve cover  should say . If the eng wont free rev to that speed, adjust the linkage till it does then test drive.

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Keep in mind that this truck was built when the speed limit was 55 MPH...

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you might have a low rpm engine. Mack for a few years offered engines set for 1800 RPM (loaded) if this is the case a simple pump adjustment would do the trick

 

I did the math and if you have 11R22.5 tires your engine will be turning 1800 RPM when traveling 66MPH

 

If you bump up the Engine RPM to 2100 you will be able to get to 77MPH.

 

Also look to see if that Transmission has a device to limit engine RPM in top gear...

 

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Have someone step on the throttle pedal while you check the pump arm is traveling all the way. I've seen worn out linkage cause incomplete travel. 

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