Optimal cruising rpm for 3.8L
09-28-2016, 05:03 PM
#1
JK Junkie
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Optimal cruising rpm for 3.8L
Know going at 55mph is the most efficient because of the drag profile but I cruise daily at 75mph with 35s and would like to regear. Thinking 4.88 with the 6 speed. I know there is a chart and that is for 70mph so it should be far off for 75mph but are the tire sizes actual mounted size or tire manufactured size.
09-28-2016, 05:28 PM
#3
JK Jedi Master
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Yep, the charts in the faq's assume measured tire height at normal load and inflation. Not the spare. Not the mfg's spec.
They have been pretty accurate for me through 5 different tire sizes and 3 different gear ratios.
They have been pretty accurate for me through 5 different tire sizes and 3 different gear ratios.
09-29-2016, 06:40 AM
#4
JK Enthusiast
I modified the tire size chart to include a bunch more info. Here it is with the numbers for 75MPH.
I was in your situation also. I had 35's (Duratrac's) with stock gears. Most of the time I had to turn off overdrive to get any decent performance. I knew I wanted to go to 37's in the future, so I regeared to 5.38's. With that gear and 35's it was a joy to drive again. Then I put 37's on and lost some of the pep again. Oh well.
I would recommend that you go with the a higher gear than a lower one. The 3.8 wont have any problem with a little bit higher RPM.
75MPH with 35's and 4.88's
75MPH with 33's and 5.38's
75MPH with 5.38's
Unfortunately I can't attach it in Excel format. But if you send me your email I will send it to you so you can plug in your own numbers.
CRAP, I JUST REALIZED YOU ARE IN A MANUAL.
I was in your situation also. I had 35's (Duratrac's) with stock gears. Most of the time I had to turn off overdrive to get any decent performance. I knew I wanted to go to 37's in the future, so I regeared to 5.38's. With that gear and 35's it was a joy to drive again. Then I put 37's on and lost some of the pep again. Oh well.
I would recommend that you go with the a higher gear than a lower one. The 3.8 wont have any problem with a little bit higher RPM.
75MPH with 35's and 4.88's
75MPH with 33's and 5.38's
75MPH with 5.38's
Unfortunately I can't attach it in Excel format. But if you send me your email I will send it to you so you can plug in your own numbers.
CRAP, I JUST REALIZED YOU ARE IN A MANUAL.
Last edited by maxspongebob; 09-29-2016 at 07:00 AM.