Hello
I've recently purchased a 2008 Superduty with the 6.4L. The guy I bought it off of was not happy with the stock difs so he put Detroit Lockers front and rear in it. He used to pull heavy loads however the most I now pull is a few sleds on a sled deck. At 100kms/hr I'm about 1750 RPM and at that speed I'd like to be about 1400 RPM. It currently has 32 inch tires and I'm going to put 35s on it after summer. My question is anyone know what gearing my dif is, I know stock it was 3L which I believe is 3.76 but I'm not sure if that is what it would be now. And would I be able to change the gearing on my difs, if so any recommendations?


Side note I run a Spartan chip in it, I always manually calculate my economy. Empty I get 7.1km/L, 2 sleds I get 5.67 km/L. My buddy with the 09 and 2 sleds gets 6.5 km/L, and when he is empty he gets 8.2km/L, however when he is at 100kms/hr he is only just over 1500 RPM.


Thanks for any input.
Cheers

 

Lockers don't affect gear ratio nor towing, but rather in an off road-poor traction situation by locking the left and right wheels together. What are you trying to accomplish? That part wasn't clear. What gears are in your truck now? The sticker inside the drivers door has the code and there are numerous posts and info about what ratio combinations work best with what tire and use.

 

My guess is that even with the Detroits put in he would have left stock gearing...so you prob have 3.7.3 in there and your buddy prob has 3.5.5 gears in his. Going to a larger tire will reduce your current gear ratio somewhat so you'll have lower rpm's on the highway.

 

I'm just playing with numbers and costs at this point, you have to save a lot of fuel to warrant paying for new gearing and the tires will come. However I figure if I put 35's on and lower down to 3.55 gearing I'd be sitting about 1400 RPM at 100km/h, drive nice and easy on the hwy probably get a bit better economy.

 

To really find out what gears you have because the previous owner has had the diff open is to pull the cover and count the teeth on the pinion and divide that number into the number of teeth on the ring gear. Only way to know for sure.

 

Keep in mind if you go to low gearing wise...you'll be lugging the engine if you tow anything.