Compression Ignition Addict
Join Date: Feb 2010
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did the 3.73 to 4.30 swap yesterday
guys with larger tires: do it. i have been having 3.73s in my truck since i bought it about 3 years ago. since then it has had a 37" tire. never paid any mind to the gears because the truck was running good and all vitals were good. just recently i swapped injectors and turbo and noticed i had a significant increase in egt (about 100 degrees) while cruising. in my quest to figure out why someone brought up putting the truck in o/d and see what the egts did. well they dropped significantly, about 70 degrees.
so i started my search for 4.30s. to have my local shop do it would have run about 1500 for the rear and 1200 for the front using yukon parts. this is not a project i was willing to tackle myself. i have seen a bunch of post lately about guys wanting better mileage so i figured id see if anybody wanted to swap rears, cheap for both parties. found a guy about 3.5 hours from me who wanted 3.73 and he had 4.30. drove out yesterday and swapped complete rear ends. now i have a factory ford built rear end with 4.30s for about $100 in diesel. cant beat that.
the results: so much easier to drive. power seems like it is just sitting there waiting to be used. the first 50 miles back were hilly, never once did it down shift or feel like it was struggling. egt dropped about 70 degrees cruising at 70 (ambient temp was the same coming and going). about a 200 rpm increase at 70, right at 2k now. boost and ebp are much closer now, used to be close to 2 to 1 now they are about 1.5 to 1. vgt % dropped about 10%. no increase in exhaust tone that i could tell. havent refueled yet so i dont know about mileage but it looks to be very close. the only thing that it didnt help was my eot, raised about 4 degrees (running BPD oil cooler). the temp is still well withing spec so im not saying anything bad just giving results. temp going was max 187, coming home was max 191. like i said not bad at all just a little change. i guess with the rpm increase the oil doesnt sit in the cooler as long so it cant cool as much.
overall i am very happy with the decision and would suggest it to anyone who is running larger tires, your engine and trans will thank you. hopefully city mpg will have a slight increase due to it being easier to get going, if not oh well. hopefully the member i swapped with has the results he is looking for. i cant foresee any negative side affects for him, he is running factory tires and was looking for a decrease in rpm at interstate speeds
06 f350: 8" ICON 2.5 resi's, ouo ala's, pmf track bar, dual stabilizers, custom 5" exhaust, 37" BFG km2's, 4.30s, 08 tailgate w/step, 08 tail lights, coolant filter, BPB oil cooler w/p 58v ficm and radiator, ARPs, 175cc, MTW s1 billet, ported intake, egr delete, onboard air and gearhead tunes
All the above just to haul the 900hp street mustang
Last edited by 94slowbra; 06-10-2013 at 04:45 AM.