3.31 gears at 40,000 lbs gross.
Just to give a bit of a feel for how different gear ratios would behave while towing I locked out 10 then 10th and 9th then added them back again while travelling on a straight stretch of road w...
Forum Discussion
RoyJ wrote:blt2ski wrote:
Roy,
Probably a good thing we can't get multiple transmission options, along with axle gears. You'd get some crazy like me, order the 3.31 gear sets, but put a 10 so with a single OD, but have lower gears than the on Ford uses. I would have the same overall low, but at the top end, not have to worry about the DOD and TOD blowing up!
We do have some different folks south of that line!
Marty
I believe that's actually the trend in Class 8s these days: ultra tall rear end, with direct or mild OD in the transmission.
Theoretically it would make slightly more power, as ring and pinion gears of lesser difference have lower parasitic loss.
Think We can agree a point: we can't evaluate the best rear end ratio, without also taking transmission ratios into account.
A lot of the "get 4.1 to tow, 3.55 to cruise" mentality dates back to the Turbo 400 days with emissions choked 170hp 350 SBCs. With modern diesels and 8/10 spd transmissions, a 3.31 can drag a house off a foundation, and sufficient for all but the harshest fleet duties.
This is called dowspeeding and it wreaked havoc on drivelines back when the trend first started for some of the same reasons I stated here. We sold a lot and I do me a lot of clutches during this time. Many of my customers found out that the fuel economy benefits were washed away by driveline repairs. In many cases it even cost more than what they would have saved in fuel.
"Dana claims a downsped engine at cruise speed increases torque in the driveline by 57%. Fortifying the driveline is the only way to protect it against the resulting long-term torque stresses."
Downspeeding can Destroy Driveline
And for those who claim it is only engine torque that goes through the driveline. The max engine torque from these class 8 engines is 2,050 lb-ft and the added torque is way more than that.
This is for class 8 trucks that generally cruised between 1,200- 1,400 rpm.
These are all things we found out many years ago in my industry and is the main reason why I say taller axle ratios put more stress on the driveline.
