big66440 Posted November 1, 2016 Share Posted November 1, 2016 Hey guys, I decided to mess around with the RPM/gear ratio calculator many online sites offer and noticed something isn't right with my set up. At 75MPH on the highway my tachometer shows 2700RPM ( and I must say it runs BEAUTIFUL)meaning my ring and pinion would be close to a 4:56 ratio? I've never actually tried to figure out the gear ratio but if I'm not mistaken the 4:56 ratio was only available with the 2.5 from the factory? is it possible that I may have an issue with my transmission overdrive or torque converter lock up ? the transmission shifts perfectly fine, the transmission fluid is Dex/Merc and it's 100% new since I had it flushed this September. One last thing I checked the fuse under the dash in the passenger side and it's good,thanks. Link to comment Share on other sites More sharing options...
Tex06 Posted November 1, 2016 Share Posted November 1, 2016 Count your shifts. Should see your tach drop on each one. Sent from my SM-G930T using Tapatalk Link to comment Share on other sites More sharing options...
HOrnbrod Posted November 1, 2016 Share Posted November 1, 2016 What's your tire size? If your AW4 is going into O/D, that translates to about a 27" tire @ a 4.56 ratio. Link to comment Share on other sites More sharing options...
big66440 Posted November 1, 2016 Author Share Posted November 1, 2016 235/75/15 - so 29" tire ? Link to comment Share on other sites More sharing options...
Tex06 Posted November 1, 2016 Share Posted November 1, 2016 Or 3.55 with 33" tires if it's not going into OD Sent from my SM-G930T using Tapatalk Previous owner could have changed out the gear (or however the tranny relays the speed info) to correct the speedo. You should use your phone or a GPS and check the accuracy of it. Link to comment Share on other sites More sharing options...
HOrnbrod Posted November 1, 2016 Share Posted November 1, 2016 235/75/15 - so 29" tire ? 4.56 w. 29" tires @ 75MPH = approx. 2600 RPM. Sounds about right to me. Plus your tach could be off a little. You need bigger meats mate. Link to comment Share on other sites More sharing options...
big66440 Posted November 1, 2016 Author Share Posted November 1, 2016 I'll pop cover and count the teeth when I get a chance, I'm actually kind of excited to see if I do have 4:56's :) ,,,, but then again if I don't???? Link to comment Share on other sites More sharing options...
HOrnbrod Posted November 1, 2016 Share Posted November 1, 2016 Regear (not) or bigger meats (yes). :thumbsup: You should get (rent?) a handheld tach and check out your dash tach too. Link to comment Share on other sites More sharing options...
Tex06 Posted November 1, 2016 Share Posted November 1, 2016 Easiest way is to clock your speed with a GPS, note the RPMs, find your tire diameter (28.88 inches), and count that gear you're in. Then figure it all out. I use the garage buddy app, makes it all easy. Just remember overdrive is .75 for the AW4. 3rd gear is 1:1. Another possibility, OD works 4.56 gears(4.56*.75 OD is 3.42), previous owner had 32" tires on it and corrected for it. Sent from my SM-G930T using Tapatalk Link to comment Share on other sites More sharing options...
Tex06 Posted November 1, 2016 Share Posted November 1, 2016 You should get (rent?) a handheld tach and check out your dash tach too.X2. They're notorious for being inaccurate in almost every car ever produced. A lot of higher end timing lights have tachs built-in. If you can find a timing light these days, lol. Sent from my SM-G930T using Tapatalk Link to comment Share on other sites More sharing options...
big66440 Posted November 1, 2016 Author Share Posted November 1, 2016 ^^^ Previous owner had 4 different size tires( no where near 32") with 4 different tread patterns and depths :hmm: . I got a full cluster off of an XJ at the Junkyard as well since mine was originally the "idiot" light one. I think I may have a tach laying around in my garage so I'll try to dig it up just to cross reference the readings. Link to comment Share on other sites More sharing options...
Tex06 Posted November 1, 2016 Share Posted November 1, 2016 I meant at one time he (or someone before him) could have changed things around. Remember, it's a Jeep. People modify these things ;) Sent from my SM-G930T using Tapatalk Link to comment Share on other sites More sharing options...
big66440 Posted November 1, 2016 Author Share Posted November 1, 2016 I meant at one time he (or someone before him) could have changed things around. Remember, it's a Jeep. People modify these things ;) Sent from my SM-G930T using Tapatalk I hear you loud and clear :thumbsup: Link to comment Share on other sites More sharing options...
Eagle Posted November 1, 2016 Share Posted November 1, 2016 235/75-15 with 3.55 gears at 75 MPH should run at 2395 RPM in overdrive, 3195 in 3rd gear. With 3.73 gears it would be 2525 RPM, and with 4.10 gears it would be 2775. Those numbers were calculated based on the actual revolutions per mile of a 235/75-15 tire, NOT based on a theoretical calculation for a "29-inch" tire. Link to comment Share on other sites More sharing options...
HOrnbrod Posted November 1, 2016 Share Posted November 1, 2016 ......NOT based on a theoretical calculation for a "29-inch" tire. Oh damn! I was off ~150 RPM. Excuuuuuuuuuuse ME! :rotfl2: :rotfl2: :rotfl2: The tach is probably off way more than that............ Link to comment Share on other sites More sharing options...
big66440 Posted November 1, 2016 Author Share Posted November 1, 2016 235/75-15 with 3.55 gears at 75 MPH should run at 2395 RPM in overdrive, 3195 in 3rd gear. With 3.73 gears it would be 2525 RPM, and with 4.10 gears it would be 2775. Those numbers were calculated based on the actual revolutions per mile of a 235/75-15 tire, NOT based on a theoretical calculation for a "29-inch" tire. Thanks Eagle, with that said I guess I "may" have 4.10's in my diff then. Link to comment Share on other sites More sharing options...
johnj92131 Posted November 1, 2016 Share Posted November 1, 2016 Surprised nobody has mentioned the fact that the speedometer is also not properly calibrated. Checking your actual speed at 60 mph is the very first thing you should do. Then note you transmission gear at 60 mph. Look at your tach at the same speed. Now, along with your tire brand and size, you can accurately tell what rpm you should be running with any given final drive. Link to comment Share on other sites More sharing options...
Tex06 Posted November 1, 2016 Share Posted November 1, 2016 Surprised nobody has mentioned the fact that the speedometer is also not properly calibrated. Checking your actual speed at 60 mph is the very first thing you should do. Then note you transmission gear at 60 mph. Look at your tach at the same speed. Now, along with your tire brand and size, you can accurately tell what rpm you should be running with any given final drive. Read my first post, down at the bottom. :D Sent from my SM-G930T using Tapatalk Link to comment Share on other sites More sharing options...
Eagle Posted November 2, 2016 Share Posted November 2, 2016 Surprised nobody has mentioned the fact that the speedometer is also not properly calibrated. Checking your actual speed at 60 mph is the very first thing you should do. Then note you transmission gear at 60 mph. Look at your tach at the same speed. Now, along with your tire brand and size, you can accurately tell what rpm you should be running with any given final drive. Read my first post, down at the bottom. :D Sent from my SM-G930T using Tapatalk Sent from my SM-G930T using Tapatalk :???: :???: Link to comment Share on other sites More sharing options...
Eagle Posted November 2, 2016 Share Posted November 2, 2016 ......NOT based on a theoretical calculation for a "29-inch" tire. Oh damn! I was off ~150 RPM. Excuuuuuuuuuuse ME! :rotfl2: :rotfl2: :rotfl2: The tach is probably off way more than that............ Gotcha! :banana: Actually, my comment was directed toward the OP's post that he has 29" tires. Link to comment Share on other sites More sharing options...
Eagle Posted November 2, 2016 Share Posted November 2, 2016 Thanks Eagle, with that said I guess I "may" have 4.10's in my diff then. Or not. Remember, 235/75-15 was never an OEM tire size. Whatever your truck's original size was, 235s are bigger, which means you travel a little bit farther and faster for each revolution. Your speedo might say 75 MPH, but that may not be accurate. If (and it's a bif "if") the speedo was fairly accurate for the OEM tire size, you may be rolling up to 5 MPH faster than your speedo is telling you. The advice from other posters above, to verify your actual speed with a GPS, is good advice. Link to comment Share on other sites More sharing options...
DirtyComanche Posted November 2, 2016 Share Posted November 2, 2016 Just remember overdrive is .75 for the AW4. In 1989? Link to comment Share on other sites More sharing options...
Tex06 Posted November 2, 2016 Share Posted November 2, 2016 :???: :???: Previous owner could have changed out the gear (or however the tranny relays the speed info) to correct the speedo. You should use your phone or a GPS and check the accuracy of it. Whoops, second post. Counting is hard..... Sent from my SM-G930T using Tapatalk Link to comment Share on other sites More sharing options...
gogmorgo Posted November 2, 2016 Share Posted November 2, 2016 Having four mismatched tires on it seems like a symptom of a seller who wanted to hang onto larger tires but not break the bank just to help out the next guy. Most smartphones have GPS and you can get pretty accurate speedometer apps. Just bring along a passenger so you don't get a ticket. As far as the first few posts wondering about torque converter locked up, my experience is that with the trans computer unplugged (and therefore no TC lockup) the revs slowly creep upwards without an increase in speed. Link to comment Share on other sites More sharing options...
big66440 Posted November 2, 2016 Author Share Posted November 2, 2016 Ok sounds like I got some homework, verify actual vehicle speed with GPS and go from there. Link to comment Share on other sites More sharing options...
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