fl70 rear gear advice needed please

[barkoguru]

The difference in the gear ratio between auto and stick is the reason you're getting both recommendations for the 3.73 and 5.86 rear end ratios. It HAS to be a stick truck he's mentioning. I'd be curious to know exactly which one so I could look up the ratios for it.

That's what I kinda figure also, in the 3.73 range would put me way down in the rpm range best I can figure, a search on the net does show most used diffs are in that ratio range though.

 

[greasytshirt]

That's what I kinda figure also, in the 3.73 range would put me way down in the rpm range best I can figure, a search on the net does show most used diffs are in that ratio range though.

I'm assuming that manual transmission trucks have traditionally been produced in higher numbers than automatics, hence the greater number of used diffs in that range.

I looked up a Fuller transmission similar to the one in your truck, the 0.78 ratio in 6th gear would give a 70 mph engine rpm of 2184 when using a 4.88 rear, and an rpm of 1669 when using a 3.73.

The suggestions we're getting are all valid, but they are incomplete without knowing the gear ratio from the transmission.

 

[sewmun]

I'm assuming that manual transmission trucks have traditionally been produced in higher numbers than automatics, hence the greater number of used diffs in that range.

I looked up a Fuller transmission similar to the one in your truck, the 0.78 ratio in 6th gear would give a 70 mph engine rpm of 2184 when using a 4.88 rear, and an rpm of 1669 when using a 3.73.

The suggestions we're getting are all valid, but they are incomplete without knowing the gear ratio from the transmission.

He says earlier that it's a Allison 3060 which is direct drive in 4th,.75 OD in 5th & .65 OD in 6th.

 

[greasytshirt]

He says earlier that it's a Allison 3060 which is direct drive in 4th,.75 OD in 5th & .65 OD in 6th.

Yes,I know. I wanted to know the ratio in high gear for a Fuller 6 speed, so that the 3.73 gear argument could be understood.

 

[sewmun]

Oops sorry,I reread it & realized your talking about his other truck.
The problem Fuller has a 6 speed stick w/OD & the more common trans is these trucks doesn't have OD,so a 3.73 running on a truck w/o OD could get decent mileage too.

 

[greasytshirt]

Oops sorry,I reread it & realized your talking about his other truck.
The problem Fuller has a 6 speed stick w/OD & the more common trans is these trucks doesn't have OD,so a 3.73 running on a truck w/o OD could get decent mileage too.

Roger that. It sounds like the 5.86 gearset will be on par with what is considered normal. If I could find a graph of his engines torque/horsepower, I might possibly be able to justify going to the 5.29 gearset. I'd hate to tell him that it'll work but have the thing downshift whenever the slightest grade is encountered or if he'd got a 5 knot headwind.