I got my son a 2002 Dodge Ram 1500 for his first truck a couple of years ago. The engine, radiator, and all belts, hoses, etc., under the hood were replaced before I put the keys in his hand. While driving down the road this past Sunday, his truck stopped. Turns out, the rear end went out. The mechanic said it would save $500 to replace the 3.92's with 3.55's but said the MPH will be off a little. If anyone has done this, can y'all tell me the difference in MPH (and perhaps performance) you noticed, if any?
I'm not sure why your mechanic is telling you there is such a large price difference between the gear ratios.
I buy the 3.55 ring and pinion for the same price as the 3.92 ring and pinion. The biggest difference in price is whether you have a limited slip diff. or an open diff.
If you shop around for the best price you can get a limited slip aka Trak-Lok differential for under $400. Even the Dana Spicer Trak-Lok is only $342 from Summit Racing and a 3.92 gear set from Motive Gear is $254 and can be purchased from Summit Racing.
There are better prices to be had with some Google-fu and some handy interwebs shopping. I build these things quite often and the only difference in price for my customers is whether or not the truck has a limited slip.
Also, you will notice the performance loss going from a 3.92 gear to a 3.55. The RPM difference running down the highway is about 300rpm in 4th gear.
If for some reason you do decide to go with the 3.55's, which I wouldn't, the speedometer can be corrected with a proper scan tool. If you can locate a dealer who still has a working DRB-3 or a proper working DRB-3 emulator available with the WiTech 1.0 application, they can do it.
If not, a Snap-On, MAC/OTC scan tool can do the job. The adjustment needs to be made in the ABS Module. What needs to be changed is called the "Pinion Factor". You will need to ask the shop if they know how to adjust the pinion factor if you do change from 3.92's to 3.55's.
