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E6 300 4Valve need more speed.



Have 1985 R688ST with a e6 300 and a T2090 trans, also has 4.64 reas. Has fuel turned up and pulls excellent at 1600 rpm, no complaints there . But my issue now is I can't get this truck to go more than 65 mph?? Any thoughts, ideas, or anything that may help me squeeze another 5 mph out of this thing now. Thank you

Mack 21 024.jpg

Mack 21 026.jpg


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Sounds like you are gear bound. Need higher rear gears or different transmission with higher final ratio.  Something like a 4:17 ratio will get you 70 mph with a final trans output ratio of .79.

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 what RPM does it turn? at 65mph?

I ask because at 1800RPM with a T2090  9 speed (.71 overdrive) and 4.64 rears on 11R 22.5 tires (41.5" tall) you will be at 67mph.

If you run it at 2100 your top speed is now 79mph.


Does the truck have Macks "low RPM"  motor? it may just need a pump RPM adjustment to get the road speed up.



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The truck is set up perfect for it's dump truck life as is. I don't see where the engine is an EM-6 series and wouldn't push the envelope myself. Stretched rods and parts operating out of design criteria don't tend to hold up too well long term if the envelope is pushed. A swap to a lower numerical number gear ratio is required for additional road speed in this case. The 4.64 Mack rears are very good sound rears in their current application and 4.17 is getting on the fringe for soft dirt work with longevity considered. 4.17's will work fine but not hold up as well as 4.64's is another way to say it.

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