WyoCherokee Posted February 15, 2009 Share Posted February 15, 2009 what rpm's are you running with 3.55's, or 4.10's? with a 205/75/15, or 125/75/15 tire. i looked up a rpm caluclator and it says I'm just over 3000 rpms at 70mph with the 3.55's and a 205/75/15(27.1" tire). that seems a bit high to me Link to comment Share on other sites More sharing options...
wallisek Posted February 15, 2009 Share Posted February 15, 2009 That is the correct calculation assuming you are in 4th gear (1.0 ratio). Link to comment Share on other sites More sharing options...
aemsee Posted February 15, 2009 Share Posted February 15, 2009 Well, I've never gone 70 in my truck. Not sure it would do that :D . Link to comment Share on other sites More sharing options...
WyoCherokee Posted February 15, 2009 Author Share Posted February 15, 2009 hmmmm ok. seems high to me but i guess I'm used to v8's running slightly lower rpm's Link to comment Share on other sites More sharing options...
mvusse Posted February 15, 2009 Share Posted February 15, 2009 3000 rpm for a 4 cylinder is not high. Small displacement can't make a lot of torque, so to make up for that and still make some HP they rev higher. With a smaller rotating mass, it is easy for them to rev higher also. I would like to know what red line for the 4.0 is, but without a tach, it shouldn't matter anyway. My Ford 302 was built to handle 6500 rpm, but the valve springs were too weak, causing the valves to start floating around 6200. That worked as a built in rev limiter that probably saved the engine a few times when I was in a bad mood. Good thing I have mellowed out a bit as I've grown older. Link to comment Share on other sites More sharing options...
WyoCherokee Posted February 15, 2009 Author Share Posted February 15, 2009 ok thanks. thatl ease my mind when flooring it in third to get up the pass :clapping: :cheers: Link to comment Share on other sites More sharing options...
wolfpackjeeper Posted February 15, 2009 Share Posted February 15, 2009 when I was commuting up to AL for the week I was running 80 or 85 and the RPM's did not seem that high. I have not taken my dash back out and turned the little screw to correct the cluster from the 6cyl to my 4 cyl so it does read a little low. At Idle it is at about 400 to 500 Rpm. At 80 or 85 it is only showing about 2500. I have a 4spd with 205-75-r15 tires. Link to comment Share on other sites More sharing options...
Geonovast Posted February 15, 2009 Share Posted February 15, 2009 My 2.5 in 4th with 3.55 is right around 2800 at 65 Cruising at 75-80 she stays well above 3200 and doesn't seem to mind. I dunno if they changed it to a direct signal from the ECU for the Chrystler system, but my tach in my 2.5 came straight from a 92 4.0 HO XJ and I never turned an adjuster, but it reads accurately. Link to comment Share on other sites More sharing options...
vaquaro Posted March 8, 2009 Share Posted March 8, 2009 :ack: O.K. I've been all thru this thread & the 2.5 gauge pod swap one as well ....I swapped a 89 Eliminator pod into my 90 2.5 ax4 2wd .I knew the reading was off by about 1/3 ,idle varies between 700-1000 , although it sounded faster . Sooo I tried the adjustment suggesting "turn screw full left for 4 cyl. ".......WRONG :headpop:( idle now reads under 500 ) Pull the whole freekin' pod apart again , try turning screw full right ( idle now at 1250-1300 ) Have no idea if thats correct ? Of coarse , I never bothered to calibrate with a hand-held tach for reference :dunce: ( when I worked for the O.K. Ranch , cowboss Al would always tell me a Short-cut was the longest way anywhere )....The delema now is ,if the reading IS correct , what do I do to get the idle to a reasonable range ? :hmm: Link to comment Share on other sites More sharing options...
Eagle Posted March 8, 2009 Share Posted March 8, 2009 what rpm's are you running with 3.55's, or 4.10's? with a 205/75/15, or 125/75/15 tire.i looked up a rpm caluclator and it says I'm just over 3000 rpms at 70mph with the 3.55's and a 205/75/15(27.1" tire). that seems a bit high to me My spreadsheet doesn't include 205/75 tires, but for 215/75R15s, with 3.55 gears 4th gear would be 3114 RPM at 70 MPH, and 5th gear would be somewhere around 2242 RPM. With 215s and 4.10 gears, 70 MPH would be 3607 RPM in 4th gear and around 2597 in 5th. These are based on actual tire manufacturers' revolutions-per-mile specifications, not a theoretical calculation based on tire diamater that doesn't account for sidewall compression in service. Link to comment Share on other sites More sharing options...
Eagle Posted March 8, 2009 Share Posted March 8, 2009 The delema now is ,if the reading IS correct , what do I do to get the idle to a reasonable range ? :hmm: Who cares about idle? The purpose of a tachometer is to tell you when to shift. You need it to be accurate in the range where you normally drive. I don't know any way to get it right other than to calibrate it against a known-good tachometer. Link to comment Share on other sites More sharing options...
vaquaro Posted March 8, 2009 Share Posted March 8, 2009 The delema now is ,if the reading IS correct , what do I do to get the idle to a reasonable range ? :hmm: Who cares about idle? The purpose of a tachometer is to tell you when to shift. You need it to be accurate in the range where you normally drive. I don't know any way to get it right other than to calibrate it against a known-good tachometer. Thanx Eagle , you and cowboss Al would get along well ...I will do it properly as you suggest :yes: .....I did try one more short-cut though,ran it down the hiway at 104 kph ( 65 mph ) in 4th ,tach read 3100 rpm . Thats fairly close I think ,so idle is a little high @ 13-1500 rpm , but its been the same for 2 years since I originally installed the gauge pod . :cheers: Link to comment Share on other sites More sharing options...
Eagle Posted March 9, 2009 Share Posted March 9, 2009 It sounds like you're fairly close. Maybe your truck idles fast. Before you go ripping the cluster out again, get your hands on an idle tach (or a regular add-on tach that hasn't been installed) and use that to check the RPMs showing on your cluster tach through the range covered by the idle tach. You can take the feed from the smaller of the diagnostic plugs under the hood. I posted a pin-out for that connector a couple of weeks sgo. Link to comment Share on other sites More sharing options...
vaquaro Posted March 9, 2009 Share Posted March 9, 2009 It sounds like you're fairly close. Maybe your truck idles fast. Before you go ripping the cluster out again, get your hands on an idle tach (or a regular add-on tach that hasn't been installed) and use that to check the RPMs showing on your cluster tach through the range covered by the idle tach. You can take the feed from the smaller of the diagnostic plugs under the hood. I posted a pin-out for that connector a couple of weeks sgo. Will do :thumbsup: Link to comment Share on other sites More sharing options...
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