Need Speed

[try2truck]

Hey guys, I just bought a Mack RD688SX with a allison 5 speed automatic trans and a 672 rear end. Max hwy speed is 53 mph. I'm a scrap iron dealer located in the Houston, Tx. area. I need to get this old ride running at least 70 mph. I have contacted the local Mack dealership and they tell me there are no parts available for re-gearing the chunks. Their solution to my problem was to replace the 65000 lb rears to 44,000lb with a 520's ? something rears. They also said that I could leave everything like it is and add a 2 speed aux. gear. Anybody have any experience with this problem ??

[rhasler]

Hey guys, I just bought a Mack RD688SX with a allison 5 speed automatic trans and a 672 rear end. Max hwy speed is 53 mph. I'm a scrap iron dealer located in the Houston, Tx. area. I need to get this old ride running at least 70 mph. I have contacted the local Mack dealership and they tell me there are no parts available for re-gearing the chunks. Their solution to my problem was to replace the 65000 lb rears to 44,000lb with a 520's ? something rears. They also said that I could leave everything like it is and add a 2 speed aux. gear. Anybody have any experience with this problem ??

It sounds like you probably have 95/96 series differentials. They used to be available down to 5.22 ratios but the parts may not be available. Changing to the lighter differentials will require different housings, etc. Check on the front right side of the housings for a CRD number to determine the model