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Old 02-10-2018, 11:23 PM   #1
Cam-Shaft
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8.1L 4L85E 5.13 gears 37” tires

Have a 91’ K5 with 8.1, 4L85, 5.13s and 37” tires. This truck will eventually see some decent highway time and possibly some towing 5-8k. Currently cruises at 70mph in the 2200-2300 range. Is this a healthy number for towing/ extended highway time or should I be looking to make some changes ?

Thanks it advance


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Old 02-11-2018, 12:09 AM   #2
4x4k30
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Re: 8.1L 4L85E 5.13 gears 37” tires

you should be fine as-is the 8.1 will pull whatever you want and 2000-2300 rpm at 70mph is not bad
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Old 02-11-2018, 12:13 AM   #3
mongocanfly
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Re: 8.1L 4L85E 5.13 gears 37” tires

according to my calculator if your trans has a .75 od you should be turning 2446rpm at 70...good enough I wouldn't change anything. ...and not worth the cost to change it....
http://www.csgnetwork.com/multirpmcalc.html
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Old 02-11-2018, 12:26 AM   #4
Cam-Shaft
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Re: 8.1L 4L85E 5.13 gears 37” tires

For some reason I thought OD was .70, but I stand corrected (thank you for the calculator). I guess I’ll leave it be
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Old 02-11-2018, 12:31 AM   #5
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Re: 8.1L 4L85E 5.13 gears 37” tires

my quick search on the od ratio said the 4l80 and 4l85 are the same at .75....
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