[oke]

Hi. I could use some recommendations what ring and pinion to use for 8 3/4" rear-end on my setup. Driving on nice sunny days and ~1000 miles a year on highway with 75 mph limit. Fuel economy is important, but I'm after performance as well just like we all do.

This dyno slip is from same type of 6.1L, but not from my engine. Both are stock except some modifications to intake manifold and 3" exhaust. If I end up swapping cam then this info is of course more inaccurate, but should still give some ballpark figure I hope.

Transmission is 5 speed automatic (545RFE) with 1st 3.00, 2nd 1.67, 3rd 1.00, 4th 0.75 and 5th 0.67.

I tried to put numbers in excel and for example with 4.11 gears I should end up with same final drive ratio as with 2.76 using 727 transmission?

From factory these 6.1L engines come mated to 5-speed auto transmission (W5A580) with 1st 3.59, 2nd 2.19, 3rd 1.41, 4th 1.00 and 5th 0.83. Factory rear is 3.06 with that engine and transmission combo. Stock tire outside diameter is 29".

[drewcrane]

ya know dave r will know about that , a buddie and i were talking about that and the one that is interesting is the dyno dosent know what kind of gears you have , and changing gears, to a lower ratio nets more torque, and hp, lets see what dave says

[Eddie]

4.30

[patrick]

75 MPH limit on highway's. Good performance. Fuel economy somewhat important. Cruizer, not racer... I'd say 3.23 Sure Grip. Maybe, get a couple of different unit's, and speedo gear's. Depnding on the mood. Just my two cent's worth. Pat

[dave-r]

Download this spreadsheet.

http://challenger.mpoli.fi/web/AUTOMATICchart.xls

Modify it to suit your transmission ratios (maybe just change the top gear ratio) but keep in the % of slippage.

Enter your tyre size and then play around with the gear ratios.

That torque curve you have there is for a very high rpm engine. A race engine really.

Best mpg is when the engine is working at its best efficiency. Which happens to be around about where you make best torque (for that reason). In your case best torque is very high up around 4500rpm or something.
So to get best mileage at 75mph in overdrive you are going to have to use a very serious rear gear or cruise at a much higher speed!

Basically you are going to have to make a compromise somewhere. The transmission is obviously designed for a high speed car (pushing over 180mph) and there is no way your Challenger's aerodynamics can cut the air at that speed without a struggle.

Personally I would use a 4:30 gear as Eddie suggests. This is the gear I use with the GV overdrive.

[transman]

What you've got to remember is the 45RFE, the Mercedes 722.6 and the other 4/5/and 6 speed transmissions(68RFE series) Chrysler(and others) are now using have been made with one major goal in mind. "Fuel Economy". A recent seminar I attended said that in the future 7 and 8 speed transmissions will be available. Again, for Fuel Economy. A normal 4 speed automatic transmission in overdrive, will lower engine rpm by about 30 percent,(compared to a normal 3 speed of 1 to 1 ratio )would have and while I don't have the figures available, I would be willing to bet the 5 speed will lower that rpm even more.
I recently had my friends 2008 5.7L Hemi Charger out for a little road test (a blast by the way), and was amazed at the RPM the engine was turning over at highway speeds. Not very damn many. As I recall at a little over 70 miles an hour the engine was turning over around 16/1700 rpm. He and his good lady took an extended trip to visit relatives, and told me he got about 700KM's on a tank of fuel, and I believe he told me the tank held 72 litres.
If you go to the website for Randy's Ring and Pinion, you'll find a calcultor there that will show you the engine rpm at a given speed.
Fill the blanks in as far as ratio, tire height, and highway speed and it will give you what the engine will be turning over with your combination. I did this with my Challenger, and it is very accurate. The chart will show you the rpm with a normal 3 speed transmission, an AOD(automatic overdrive)transmission and other types. There is also a chart for recommended highway cruising speed.
Welcome to the new era.

transman