mackmodelb Posted August 11, 2006 Share Posted August 11, 2006 After getting my B-61LT up and running on it's maiden voyage, the highest road speed I can get out of it seems to be 52 mph. The rear end numbers are CRD78-460-29. It has the original size tires 10.00 x20 with the original TRDL-67 trans. Any suggestions on what ratio to switch to to get around 65mph at 2100 rpm, or somewhere in that area? Thanks. Quote Link to comment https://www.bigmacktrucks.com/topic/1404-56-b-61-rear-end-ratio/ Share on other sites More sharing options...
CML Service Posted August 11, 2006 Share Posted August 11, 2006 10.00 x 20 are 500 rev mile tires , your trans is direct in high. Divide 500 into 2100 and you will get ratio needed for 60 MPH. 4.00 ratio is very availible but low HP on a 673 may not pull a full load at speed. Chuck Quote Link to comment https://www.bigmacktrucks.com/topic/1404-56-b-61-rear-end-ratio/#findComment-7017 Share on other sites More sharing options...
Freightrain Posted August 11, 2006 Share Posted August 11, 2006 My 673 pulls 30K around at near 65 with not much problem. Little slow on the big hills. Quote Larry 1959 B61 Liv'n Large...................... Charter member of the "MACK PACK" Link to comment https://www.bigmacktrucks.com/topic/1404-56-b-61-rear-end-ratio/#findComment-7018 Share on other sites More sharing options...
jeff20878 Posted August 12, 2006 Share Posted August 12, 2006 mackmodelb,As Freightrain said, dividing 2100 by 500 yields a ratio of 4.20According to my book, a 4.28 ratio was available from Mack for the CRD-78 carrier.Good luck with the switchover!Jeff... Quote Link to comment https://www.bigmacktrucks.com/topic/1404-56-b-61-rear-end-ratio/#findComment-7022 Share on other sites More sharing options...
thermodynediesel Posted August 12, 2006 Share Posted August 12, 2006 Chuck,When you say that the 4.00 ratio is available what do you mean ? can new gears be ordered and put in an old rear end. Thanks Quote Link to comment https://www.bigmacktrucks.com/topic/1404-56-b-61-rear-end-ratio/#findComment-7023 Share on other sites More sharing options...
mackmodelb Posted August 13, 2006 Author Share Posted August 13, 2006 Thanks for all the great info. I will try to look for the right carrier for my original rearend. Is it probably cheaper to get a newer style rear end out of a newer Mack, with a gear ratio that is close? I know a few bone yards around my neighborhood. Is it fairly easy to swap complete rear ends or is there a lot of cobling involved to get it in there? Just asking. Thanks again. Quote Link to comment https://www.bigmacktrucks.com/topic/1404-56-b-61-rear-end-ratio/#findComment-7041 Share on other sites More sharing options...
CML Service Posted August 14, 2006 Share Posted August 14, 2006 Chuck,When you say that the 4.00 ratio is available what do you mean ? can new gears be ordered and put in an old rear end. ThanksThe 4.00 ratio was very popular in the single axle R and U models from 1965 production on. I'm not sure on gear set interchange with a older carrier. But a newer carrier should go into the older axle housing OK. Chuck Quote Link to comment https://www.bigmacktrucks.com/topic/1404-56-b-61-rear-end-ratio/#findComment-7054 Share on other sites More sharing options...
mackmodelb Posted August 15, 2006 Author Share Posted August 15, 2006 Thanks for the info Chuck. I will look into it.Tom Quote Link to comment https://www.bigmacktrucks.com/topic/1404-56-b-61-rear-end-ratio/#findComment-7064 Share on other sites More sharing options...
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