Lance_S Posted May 31, 2011 Report Share Posted May 31, 2011 Hey guys, Curious what you are seeing with the 3.90 gear swap. I heard it was 500 rpm at around 3,000. So with the swap that would be 3500 rpm? Any solid stats out there? Thanks, L Link to comment Share on other sites More sharing options...
Blue Posted May 31, 2011 Report Share Posted May 31, 2011 Ask IntercooledFlatty Link to comment Share on other sites More sharing options...
strang3majik Posted May 31, 2011 Report Share Posted May 31, 2011 At least...theres a huge difference between 3.55s and 3.90s. And, I'm guessing if you're cruising at 3000 you have a 5 speed? Because that would also make a big difference. Link to comment Share on other sites More sharing options...
StarquestRescue Posted May 31, 2011 Report Share Posted May 31, 2011 http://www.jawsgear.com/calc_rpm.php Link to comment Share on other sites More sharing options...
Lance_S Posted May 31, 2011 Author Report Share Posted May 31, 2011 Actually not as bad as I thought. Hmmmmm. with 3.55: 60 mph is 2319 rpm70 mph is 2706 rpm80 mph is 3093 rpm90 mph is 3479 rpm with 3.90: 60 mph is 2548 rpm70 mph is 2973 rpm80 mph is 3400 rpm90 mph is 3822 rpm The difference @ 60 mph is 230The difference @ 70 mph is 267The difference @ 80 mph is 307The difference @ 90 mph is 357 That is with a 245/45/16 which is what I run on the rears. Either way, not nearly as bad as thought. i was thinking it was a 500 rpm bump at 60 to 70. This just became an option. At a cruise of 80 I might be able to live with a 300 rpm bump. Anyone want to chime in on the conversion? Have you done it? Do you like it? would you do it again? etc. L Link to comment Share on other sites More sharing options...
Lance_S Posted June 1, 2011 Author Report Share Posted June 1, 2011 So where do I get 3.90 gears and how tough is the swap????? Link to comment Share on other sites More sharing options...
The Rabbit1 Posted June 1, 2011 Report Share Posted June 1, 2011 So where do I get 3.90 gears and how tough is the swap?????got mine from a mighty max pickup. PQ has a complete walkthrough on how to R&R the diff. from what I have read the shims should be the same ones that are in the diff. not the ones that come from the donor diff. Mitsubishi shimmed each diff as to the difference in the casing and the gears are cut equally. That is what I have read. Your results may vary, so be sure to check your backplay and your gear mesh. Link to comment Share on other sites More sharing options...
Lance_S Posted June 2, 2011 Author Report Share Posted June 2, 2011 Rabbit, The numbers I posted above seem to make sense? I have plugged my tire size and speed in, then went out and checked it and am getting a bit different numbers. What I am really interested in is the cruise difference numbers. Can you give me your tire size and tell me what your rpm reads at 60,70 and 80. Either in 4th or 5th, doesn't matter. Thanks, L Link to comment Share on other sites More sharing options...
The Rabbit1 Posted June 2, 2011 Report Share Posted June 2, 2011 Rabbit, The numbers I posted above seem to make sense? I have plugged my tire size and speed in, then went out and checked it and am getting a bit different numbers. What I am really interested in is the cruise difference numbers. Can you give me your tire size and tell me what your rpm reads at 60,70 and 80. Either in 4th or 5th, doesn't matter. Thanks, LI have an auto, so do you mean final drive? the final drive ratio isn't that bad. 0.36 is minimal. Now my camaro going from 2.73 to 3.90 is a big jump. Link to comment Share on other sites More sharing options...
StarquestRescue Posted June 2, 2011 Report Share Posted June 2, 2011 I come up with 3188 rpm at 70 mph with 3.909 gears, 245/45/16 tires and .856 5spd 5th gear ratio. Check your input data. Click the gear link in my signature. Link to comment Share on other sites More sharing options...
Lance_S Posted June 2, 2011 Author Report Share Posted June 2, 2011 My inpit data wasn't the problem. The problem was the website that was linked above assumed .80 reduction for 5 speed manual and I don't know what our final drive reduction is with a 5 speed. But I think that is where the discrepancy was coming from. If I use their .80 reduction my car in 5th comes out at 3.90 gear numbers but I thought that might be wrong. I went back out and checked the numbers again at 60,70 and 80 in 4th thinking our 4th is 1:1. That time they were actually about halfway between 3.55 and 3.90. I looked in my manual but didn't see our gear ratios for a 5 speed, anyone have them? I know my speedo is correct, I have been roadside radar and have confirmed its accurate so maybe my tach is off a couple hundo. Either way, if someone has our syock 5 speed gear ratio it would help. Thanks. Link to comment Share on other sites More sharing options...
StarquestRescue Posted June 2, 2011 Report Share Posted June 2, 2011 (edited) .856 5spd 5th gear ratio. Click the rear gear link in my signature.1st 3.3692nd 2.0353rd 1.3604th 1.0005th .856 Use the Specific Trans Ratio calculator further down the link page, where you can enter the trans gear ratio. The "3.90 gears" are actually 3.909, or really more like 3.91 than 3.90 Edited June 2, 2011 by StarquestRescue Link to comment Share on other sites More sharing options...
The Rabbit1 Posted June 2, 2011 Report Share Posted June 2, 2011 some good info: My link Link to comment Share on other sites More sharing options...
The Rabbit1 Posted June 2, 2011 Report Share Posted June 2, 2011 (edited) --------------1st---------2nd---------3rd---------4th--------5thKM132-----3.369:1---2.035:1---1.360:1---1:1-----.856:1L4N71B----2:842-----1:542-----1:1------686:1 Edited June 2, 2011 by The Rabbit1 Link to comment Share on other sites More sharing options...
Lance_S Posted June 3, 2011 Author Report Share Posted June 3, 2011 Thanks Rabbit. That is what i was looking for. there is a discrepency between what the calculator says and what we are seeing on the dash board. The calculator says at 70 we should see 2700 rpm (with my 245/45/16) and what I am seeing is 2950. The link you sent says that most, with stock tires, just a hair shorter than mine, are seeing right at 3000 rpm at 70. So where is the 300 rpm??? At first I thought I might have 3.90's in my car and didn't know it because on the calculator it says I should be seeing just under 3000 rpm at 70 with 3.90's....which i am. But everyone is seeing around 3K with stock gears so what gives? Has anyone double checked our rpm gauges to see if they are accurate? I know my speedo is right on, i have been clocked by one of those road side radar that flashes your speed so I know I am on the money for my speedo, that only leaves RPM.... Any ideas? Thanks, L Link to comment Share on other sites More sharing options...
The Rabbit1 Posted June 3, 2011 Report Share Posted June 3, 2011 the stock speedo and tach are only accurate up to a point, hell you can adjust them by tapping at them Link to comment Share on other sites More sharing options...
Shelby Posted June 3, 2011 Report Share Posted June 3, 2011 3:90 gears is not gona hurt your engine heck trucks run 4:56 and higher , an 4:11's was normal for chev pickups look at a Raider or montero they run up to 4:62s sure they have taller tires but you can bet the engines are rev'd up plenty good Link to comment Share on other sites More sharing options...
notfried Posted June 4, 2011 Report Share Posted June 4, 2011 (edited) In fifth I get while using a gps: 55=2350, 60=2500, 65=2750, 70=3k, 75=3200. Fourth: 40=2k, 45=2250, 50=2500 60=3k. Third: 30=2k, 35=3500. Stock gears Mind you that this is a stock tach, but I drive by the tach and did for years and always am exactly on the speed limit...Tim has followed me through cities and on interstates through about five or six states and so have a few others...never had a speedo and always right onnit, yo Edited June 4, 2011 by notfried Link to comment Share on other sites More sharing options...
Tim_C. Posted June 4, 2011 Report Share Posted June 4, 2011 I recommend lighter internals and the balance shafts be eliminated on the bottom end of the engine build. If you are building a performer, you need more mods for hi-rpm if you run the steeper gears. The 4WD with steep gears are not very good on the highway. I guess it depends on what kind of driving you will be doing. I suggest mods for high rpm use if you plan to drive it on the highway much. Or run 255/50/16's or a tall tire to minimize the increase. I went to the 3.90's and shortened the tire and it seems like a big jump to me. Mine likes it, but I do have major mods in the bottom end to raise the red line of the engine, and supporting mods all over to run high too. When I first switched over, I thoght any engine would be fine with it, but I can tell when driving my car with stock internals, that it wouldn't take too kindly to running the steeper gear. Stock rods, heavy pistons with the older/heavier piston pins too. it even has a BSEK on it, and I still wouldn't want to put the 3:90's in it. Switching to 3.90's is more of a question of what will it do in a race situation. The rpm's are gonna be quite a bit higher, and you really need something that can run with power up there. Link to comment Share on other sites More sharing options...
StarquestRescue Posted June 4, 2011 Report Share Posted June 4, 2011 Bottom line is they are really hard to live with in a 5 spd car. The time and money is better spent making more power. With extra power it becomes much easier to pull the stock gears. Link to comment Share on other sites More sharing options...
mstieg Posted June 9, 2011 Report Share Posted June 9, 2011 3.90 & 4.22 Gear setup Not that I know how to do this...just reposting a link I have saved for later. 3.90 Gear Performance Thread circa 2003 - good info Link to comment Share on other sites More sharing options...
IntercooledFlatty Posted June 9, 2011 Report Share Posted June 9, 2011 70 MPH, GPS confirmed, auto in OD 3.90 gears http://i154.photobucket.com/albums/s263/aiche_photos/DSC05248.jpgspedo is off because of the 3.90 gears Link to comment Share on other sites More sharing options...
IntercooledFlatty Posted June 9, 2011 Report Share Posted June 9, 2011 same day http://i154.photobucket.com/albums/s263/aiche_photos/DSC05255.jpg Link to comment Share on other sites More sharing options...
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