speed

[luke andres 23]

I have a 1991 mack dm 690s with a mechanical em7 engine.   It has an overdrive eaton fuller 8ll which i installed years ago and 4.42 gears that also i install years ago.  Tires are 22.5 on the steel wheels that have the wedges.   I have a parts mack that has alum 24.5 wheels and 4.17 gears.  I also have an eaton fuller 8ll tranny that isnt an overdrive.  My question is which of the setups will run the faster speed.  The dm like it is now or if i switch out the three items ...tranny gears and tires

[mrgumby 149]

4:17 will give you more highway speed  no matter what size tire you put on the 24.5 will give you the most high way speed 

[luke andres 23]

But will going from an overdrive tranny to a non overdrive tranny drop my top end speed back lower than it was. 

[Mark T 60]

An 8LL is an overdrive.

[luke andres 23]

i dont think all 8ll are overdrive    the one in my truck is an rto      the used one that is in better shape with less miles is an rt     no o which stands for overdrive

[Mark T 60]

RTO  or RTX..... it's an overdrive.  Personally, 4.42s 24.5s and an 8LL is a fairly well working arrangement in my opinion.  Same thing with 4.17s will go faster though.

[luke andres 23]

tag on transmission says    rt8908ll      no O or X      according to my eaton fuller manual the rt and the rtf are not overdrive transmissions      am i correct on this 

[Mark T 60]

That is correct. I just assumed it was a larger series.  An 89 is I small transmission.  Not physically small, just not something you'd find in trucks that are bigger. 

[luke andres 23]

so the insides (gears) are small ?     should i not use this as an option in my dm     its a tri axle dump truck      gross 68000 lbs usually and have it off road quite a bit 

[steeler 34]

An 8908 would be an 8,000 lb transmission, where the more common 14908, or 16908 would be 14,000, and, 16,000. Can't say I've ever seen an 8908 in a tri axle though. What did it come out of?

[Mark T 60]

The "8" series is only rated to handle @ 800 lb/ft of torque from an engine. Likely only has a  inch and three quarters input shaft (?)  I don't think it's a good choice for the truck you want to use it in. 

[Mark T 60]

That's a transmission more for a smaller truck, like maybe a ten wheeler with a DT466 or something like that.

[terry 526]

For what you are wanting to do with this truck, you need a bigger series of transmission for sure.   terry

[luke andres 23]

i bought it used from a guy local     it was in a tandem or tri axle dump as well but it may have originally been a road tractor      not sure     can the input shaft be switched out ?        or is there a lot more to it than just that?      he told me he wasnt keeping it for a back up tranny for other trucks because it wasnt an overdrive         said truck had like 3 something gears in it