The following spreadsheet can be used for comparing the effects of Tire Size, Rear Gear Ratios and Engine Speed for the 4L60E automatic and 3 different T56 transmissions. I put it together when I was going through the rear gear selection process for a T56 swap and I would be glad to share it with fellow wagon lovers.

Below is a screenshot of the spreadsheet TAB with tables that calculate Engine Speeds at various Vehicle Speeds per gear for the different transmissions. Also calculated and graphed is Max Vehicle Speed for the entered value of Max Engine Speed per gear.


Another TAB has a set of tables that calculate Vehicle Speed for an entered Engine RPM per gear. It also has a set of tables that calculate Engine RPM for an entered Vehicle Speed per gear as shown below.


The cells that are highlighted in yellow are variables that can be changed by the user. These include Tire Size, Rear Gear Ratio, Max Engine RPM and Manual Transmission Option. The other cells are locked so you should not be able to mess them up .

The Manual Transmission Option allows you to enter the transmission that you want to compare ("1" for MN6, "2" for M29, "3" for M28 and "4" for an option so you can enter custom transmission ratios).

Send me a PM request with your email address if you want a copy of the spreadsheet. As I said above, I would be glad to share.

Anyone serious about getting the most out of their cars, need this chart. Don has done a great job. Have him email you an interactive version.

Spent the last 1/2 hour playing with the numbers, and also plugging in the values for the 4L80. Interesting in that in my case, the 6 speed would be useless unless I was pulling at least 4.11 gears.

Thanks Don

Bill

Nick:

Was the T56 ever used in any of the trucks? I looked up things such as clutches and hydraulic throw-out bearings and got totally confused as to clearances, flywheel thickness requirements and so on. It would be nice if we could find a direct bolt in replacement that will work with the diesel.

Also has anyone had any problems with the stock drive shafts spinning at the higher speeds? With the 4.88 gears, at 60 mph, the shaft will be spinning at about 2,200 RPMs. Alot of centrifical forces working here.

Seems to be alot more then figuring out how to mount an extra pedal. Still leaning towards a 4L80 but am open to the possibility of rowing gears again. Really only trying to get a couple of useful gears when towing the RV on the highway. The 4l60 can only tow in third and the spread between third and second is way too steep. The 4L80 will allow me to tow in fourth with third being available without running the engine up against its govenor.

Bill

Not concerned about the engine, been there done that with the Roadmaster. Its the interface between the diesel and the T56. Would like to avoid the dual mass flywheel of the later 6.5 as they were troublesome, but rather use the stock flywheel for the 6.2.

My question was about the hydraulic throw out bearing thingy they keep talking about on the T56 aftermarket parts sites. Getting rid of the clutch fork and external slave makes a lot of sense, assuming the combo throw-out bearing, clutch slave will fit between the tranny and the engine.

Haven't looked closely at the Caddy's driveshaft but if I were to spin it twice its normal speed, I would still be concerned about balance. Not too worried but still better safe then sorry. Remember the Caddys shaft is 4 inches longer. 1/2 way to Florida is not the place to regret not doing it right.

Bill

I had 4.56's in my Fleetwood with 26" tall tires and I did 100MPH stretches with no vibes from the stock shaft.

And Don, way cool chart!

I added a driveshaft speed calculation to the three tables.





PM your email address if you want me to send you an interactive copy of this spreadsheet.

http://www.impalassforum.com/vBulletin/showthread.php?t=233667

This came in handy when I was doing my calculations.