Is it possible to put higher OD in a rockwell 9

My dad just bought a 1993 mack ch midroof that's in really good condition for local/regional runs. The trans is a rockwell 9 over against the dash. I've been looking all over and can't find the spec sheet for the gear ratios. My dads thinking the big hole is a .86 or .80 but hes not positive. If it helps the specs are 3.91 with 22.5 lo-pro, at about 70 shes at 1750 or so.

He wants to keep the 9 instead of dropping a 13 or 18 in it. Most of the runs will be about 50-65k and coming home empty most of the time. Would it be worth it to put a higher od? If its not worth it hes gonna bite the bullet and drop a 13 double over in it

 

@Oxbow Ok that's what I was looking for so it probably does have a .73 or .74 its a pretty big gap between 8th and 9th at 1800 she drops back to around 1250-1300. It needs new drives so would it be better to go with taller rubber or go a head and drop a 13 OO in? Probably just better to go with a 13 since the trans needs a bearing kit anyways

Bob tailing 1500 miles from texas we got about 8.5-8.7 running about 1650 that seems to be the sweet spot for the e7

 

Finally found a chart. It seems like either a .72 or .74 not sure weather it's an "A or B" model though. Since it needs new drives will go with taller rubber and rebuild the trans here at the house call it good. 1675 @ 70 should do pretty for economy. That old e7 sounds like a real diesel compared to the new dd15 I've been driving

 

For right now my dad's gonna run it the way it is. I think 3.42 or 3.55 will be too high. It's only a 12L 350 so not winning any races here but at least it doesn't have any emission bs on it. For a 1993 its in very good shape, unheard of for a mack that old, usually turned into a dump truck by now.

Up until now I've only driven the dd15 and isx with the epa crap. This mack feels like it has some bark that the new stuff is missing