jeepjack54 Posted May 1, 2015 Share Posted May 1, 2015 I recently did the popular gauge cluster swap. The gauge cluster came out of my retired 95 Jeep Cherokee 4.0 with auto trans. When it was in the Cherokee i'd cruise down the highway at 75 and 2800 rpm. Now that it is in my 91 Comanche (4.0 h.o. manual trans) when I cruise down the highway at 75 the engine spins at around 1500. This seems low to me and when I rev the engine the tacometer barely reaches 2500 rpm. Is there an issue with the tacometer or does a manual trans jeep just rev lower? Thanks, Jack Link to comment Share on other sites More sharing options...
HOrnbrod Posted May 1, 2015 Share Posted May 1, 2015 A 95 4.0 HO tach into a 91 HO 4.0 should work fine if the speedo gear is right. What size tires do you have on the 91 MJ? Link to comment Share on other sites More sharing options...
Eagle Posted May 1, 2015 Share Posted May 1, 2015 Find your tire size and gear ratio here: https://docs.google.com/spreadsheets/d/1pHFuhGgj6dQDfzyfFJH5z7NCDLW2KX3ABQgAJr3lBvM/pub?hl=en&hl=en&output=html Link to comment Share on other sites More sharing options...
glundblad Posted May 1, 2015 Share Posted May 1, 2015 This may not be what you are talking about if you can't even rev your engine but many comanches have 3.07 gears which is way too high IMO. If I am driving down the highway, most of the time, I won't even go into 5th gear. If I recall correctly, 4th is around 2200rpm at around 60mph. For years, I always thought there was something wrong with my Comanche until I installed a tach and could see what was really going on. Link to comment Share on other sites More sharing options...
Marine1Texas Posted May 1, 2015 Share Posted May 1, 2015 in 5th gear at 75mph my truck sits at 2300 rpms. unless taking off my revs never get over 2800 to 3000 rpms. When driving it almost always below 2200. I find best power for eco in my truck is 1800 to 2200. When I cruse at 75 to 85 I see big drops in eco. Link to comment Share on other sites More sharing options...
jeepjack54 Posted May 1, 2015 Author Share Posted May 1, 2015 My tires are 205/75/15 and the gears are 3.07. I think these tires are too small. My speedometer is off by about 5mph as well. Link to comment Share on other sites More sharing options...
Marine1Texas Posted May 1, 2015 Share Posted May 1, 2015 well put 215 225 or 235 on. I have 235/75r15 since day one Link to comment Share on other sites More sharing options...
HOrnbrod Posted May 1, 2015 Share Posted May 1, 2015 With that gear ratio and tire size you should have a 31 tooth speedo gear. Take a peek at the one you have. Link to comment Share on other sites More sharing options...
Blue XJ Posted May 3, 2015 Share Posted May 3, 2015 A speedometer gear is not going to change the tach reading, it will only adjust your speed. Link to comment Share on other sites More sharing options...
87MJTIM Posted May 3, 2015 Share Posted May 3, 2015 Was the cluster original to the 95? Could it have come from a 2.5 prior? I remember reading that the 4.0 and 2.5 count pules differently for the Rev counter. If the PO of the 95 had swapped clusters from a 2.5, you may have taken over his/her problem. Link to comment Share on other sites More sharing options...
HOrnbrod Posted May 3, 2015 Share Posted May 3, 2015 A speedometer gear is not going to change the tach reading, it will only adjust your speed. True, but he may be going slower (or faster) than the speedo indicates and the indicated revs may not be as far out as they seem. Link to comment Share on other sites More sharing options...
jimoshel Posted May 3, 2015 Share Posted May 3, 2015 I never blindly trust instrumentation until I've checked and verified it's accuracy. Your tach shows 1500 RPM but is that really correct? Same for the speedo. Temporarily install a 2nd tach, or use a test bench one to check the tach. A stop watch and mile marker will verify the speedo. In non precision lab instruments some times up to a 20% error is acceptable. Link to comment Share on other sites More sharing options...
Eagle Posted May 3, 2015 Share Posted May 3, 2015 A speedometer gear is not going to change the tach reading, it will only adjust your speed. Of course the speedo gear won't affect the tachometer. But the tachometer only tells you the engine speed, not the road speed. Unless the speedometer reads correctly, you can't know how many RPMs the engine is turning at _X_ MPH. Any time you change tire size or gear ratio without changing the speedometer gear, you make the speedometer inaccurate. Link to comment Share on other sites More sharing options...
Blue XJ Posted May 4, 2015 Share Posted May 4, 2015 He's talking a 1300 rpm difference though. If he doesn't realize that he is going 10+ mph faster/slower than he thinks he is going, there are other issues to address. Link to comment Share on other sites More sharing options...
HOrnbrod Posted May 4, 2015 Share Posted May 4, 2015 He's talking a 1300 rpm difference though. If he doesn't realize that he is going 10+ mph faster/slower than he thinks he is going, there are other issues to address. Probably doesn't. Link to comment Share on other sites More sharing options...
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