I have two questions..

1) I understand from reading a few different posts that my 1997 is the last year of the old 3-speed automatic transmission.

Maybe someone can verify this as fact first.

If so, can a 1998 or newer OD transmission be swapped in easily, and will fuel economy improve enough to warrant the work?

2) My rear diff is the 8.25" stock unit. Not sure what the gear ratio from the factory is, but that's what I have. What is the best set of gears to run for highway economy? I don't carry heavy loads or tow with this van but I spend a lot of time on the highway doing 65MPH.

My e-Fan installation back in early July saves me about $6 a tank in fuel, and is already completely paid for. By the end of the year I'll have come close to recouping the cost for the water pump and new radiator, too.

 

There might still be a little metal tag attached to one of the diff cover bolts. 2.9 would be a 2.91 to 1. 3.5 would be a 3.55 to 1 ect. You could also lift the rear axle and spin the driveshaft and count the revolutions until the wheels turn once.

I can't help with your other questions

 

Not sure I'd swap in a over drive transmission. From what I understand the OD is the week part of the tranny.

 

For the highway economy 2.91 or 3.21 gears would be best. Mine gets about 13.5 on highway (36RH w/3.21), what does yours get?

A newer OD tranny is not an option for our vans, the newer ones require a TCM (Transmission Control Module) that works with the PCM in order to correctly shift the transmission.

There is a web site that has instructions in putting in an older OD tranny, try this: http://www.transmissioncenter.net/SwapInformation.htm

 

Mine gets 15.5 - 16 MPG on the highway with the e-Fan conversion and a fresh tune up carrying ladders on the roof rack. I might squeeze out 17 if the ladders are removed and I drive thru an entire tank. That's roughly a 500 mile cruising range.

 

My B1500 2000 Ram Van has a 3-speed with no OD. So, I do know that the 3-speed was still offered then as well.

As for putting a tranny from 1998+ newer into a 1997, take into consideration that the engine and tranny for the 1998-2003 were moved forward 4-inches to allow more front driver and passenger room and to allow the fuel tank relocation to the mid center of the vehicle. Therefore, the drive shaft or the tranny may need to be lengthened or shortened depending on how you look at it.

As for fuel economy, I have a 2001 B2500 with the heavy HD suspension, 8-lug 16 inch wheels, seating for 12-passengers, AND the 3.92 rear. With all of that said, I'm now getting 16Hwy as of my last two fill ups.

Much of this mileage increase was due to a few things. Running with Autolite AP5224 plugs, copper/brass cap-n-rotor, higher flow exhaust, high flow CAT, new O2 sensors, 5W30, larger oil filter, synthetic rear axle 75W90 oil, Teflon (PTFE) synthetic wheel bearing grease (over pack the snot for the front inner/outer bearings is a must), and new self rear adjusters for the back brakes to reduce further drag.) Plus, I do about 60 on the interstate vs. the 75 I once did. So, I leave 10mins earlier, but the fuel savings is a win.

When my radiator goes, could be 6mn to 1 yr, I plan on the eFan then. My biggest problem is to curb the small rust issue right now.

 

You may wish to look into installing one of those electronic rust control systems. From what I understand the system stops rust dead in it's tracks when installed properly. The CounterAct unit has been featured on the Speed channel shows like Trucks! and Two Guys Garage.

 

Not so sure about the electronic gismo to fend off rust. The van has seen a few rough winters up north while being in fleet service. Rochester, NY to be more specific. The van is now here with me in Nashville, TN where the winters are really mild and the snow is seldom.

So, if I fix up the body, it will last a long time down here.

 

I think the major benefit for your vehicle is that the rust that has already started, both where you can and can't see it will be stopped. Even if you are in the most rust friendly environment on the planet the tinworm will continue to chomp away since he's already been invited to dinner.