Hey guys I currently own a 2004 six speed cummins with a 3.73 rear end. I am debating on upgrading to a 2006 with the g56 six speed. I have heard complaints about the sixth gear ratio and was wondering if anyone had 285's on with their g56 and what their rpm's were at 70. Just wanting to compare. thanks..
ps. any other problems I should know of with that trans?

 

I have an '07 with the G56 and 285/70/17 BFG MTs. I don't drive 70, but mine turns about 1950 rpm at 65. Hope that helps.

By the way, I love the truck. For daily driving I think I would rather have a 5 speed like the NV4500 in my old Chevy. Sometimes I forget whether I'm in 3rd or 5th with the six speed.

 

Hey thanks for the reply. Thats just what I was looking for for a response. Seems it would be just a little over 2000 then at 70.

 

2250rpm at 70mph

 

285's on mine. 70mph = 2,200rpm

 

Did you guys have to put on a leveling kit or any other lift to fit the larger tires?

 

35 12.50's will fit on a stock truck with stock rims with zero rubbing............

 

33-12.5-17's - - - - - 64mph ='s 2,000 RPM! 70 MPH ='s 2,200 RPM so about 50 RPM lower than stock 265's! GPS on my Truck is still correct at 70 mph..... I still need to get the Tires/Truck lined back up!

 

Man! I was hoping that 265s would get me down to around 2000 rpms at 70mph!!! I guess I won't waste my money on that venture.....

 

With a G56, even 315s won't get you down to 2000 RPMs at 70mph true speed. It still runs closer to 2100 RPM than 2000 RPM indicated.

The G56 trans appears to be a good one, most think a little knotchy when cold. It is a truck tranny, certainly does not feel like a slick shifting sports car. Some fellows that sled pull with highly modified motors will break them.

You can do some simple math to figure what tire size you need to reach a target rpm. If you want to reduce your 2300 RPM indicated by 300 RPM at a certain sped, thats a 300/2300 or ~13% drop, so you need a 13% larger diameter tire.

On a 3.73 G56, to reduce your rpm lower than 2000 at 70 you need a 13-14% larger diameter tire than the stock 265s. You won't fit these and not rub without also doing some suspension mods. For reference a BFG 315 is about 10% larger in diameter than a BFG 265 and I think is about as large as you can go without rubing and without other mods.

 

315s will rub just a little bit at full turn. . . .not enough though for me to add spacers or a lift just yet.

 

Here is a handy little chart that works with the g56 and 3.73 rear end, I *think* these numbers are right.

..............60.......65.......70.......75....... 80....mph
=======================================
265.......1927....2088....2248....2409....2569.... rpm
285.......1862....2017....2173....2328....2483.... rpm
295.......1845....1998....2152....2306....2459.... rpm
305.......1806....1957....2107....2258....2408.... rpm
315.......1771....1918....2066....2214....2361.... rpm
37x12....1691....1832....1973....2114....2255....r pm