Posted by Keith in Washington [24.41.55.51] on Thursday, March 26, 2015 at 18:32:00 :
In Reply to: Increase road speed posted by Mark Race [24.165.195.136] on Thursday, March 26, 2015 at 17:41:39 :
Ouch! After all that work.
I see a couple options. Overdrive, a different engine, or different axles with better gearing. What is the RPM that your engine is turning at max speed? Do you have the original speedo? If so, the gear and tire change will make the speedo read low. I suspect that you can not get sufficient RPMs from the diesel engine to get the higher speeds.
I have the same truck. A 42 WC53, with 4.83 gears, NP200 from a 56 PW, a SM420 transmission, with 36" tires but with a Chevy 327 V8. I have front disks and rear drums and an ARB locker in the rear. My speedo is an electronic one that has been corrected to the gears and tires.
Here is what I see in MPH verse RPM (numbers slightly rounded):
50MPH = 2500RPM
60MPH = 3000RPM
70MPH = 3500RPM
80MPH = 4000RPM
Your truck and mine are similar enough for a direct comparison. The only physical difference in the drive train is that you have slightly larger tires which is a small benefit for you. Our engines are basically turning the same RPM for any given speed.
I can cruise all day at 70MPH as long as I am willing to pay for the gas. I usually hold it at 60MPH. I will kick it up occasionally to 75 or 80 MPH. It is stable at those speeds. At 70MPH I can let go of the steering and it will track true down the freeway. I have been to 90MPH once and never expect to go there again. The engine is balanced and etc so it can turn over 5,000RPM with out any issues.
The main difference between our trucks is that my engine can develop much higher RPMs which equals much higher speeds.
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