luke andres Posted December 13, 2020 Share Posted December 13, 2020 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 Quote Link to comment https://www.bigmacktrucks.com/topic/65243-speed/ Share on other sites More sharing options...
mrgumby Posted December 13, 2020 Share Posted December 13, 2020 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 Quote Link to comment https://www.bigmacktrucks.com/topic/65243-speed/#findComment-465337 Share on other sites More sharing options...
luke andres Posted December 13, 2020 Author Share Posted December 13, 2020 But will going from an overdrive tranny to a non overdrive tranny drop my top end speed back lower than it was. Quote Link to comment https://www.bigmacktrucks.com/topic/65243-speed/#findComment-465347 Share on other sites More sharing options...
Mark T Posted December 13, 2020 Share Posted December 13, 2020 An 8LL is an overdrive. Quote Link to comment https://www.bigmacktrucks.com/topic/65243-speed/#findComment-465350 Share on other sites More sharing options...
luke andres Posted December 13, 2020 Author Share Posted December 13, 2020 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 Quote Link to comment https://www.bigmacktrucks.com/topic/65243-speed/#findComment-465372 Share on other sites More sharing options...
Mark T Posted December 13, 2020 Share Posted December 13, 2020 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. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/65243-speed/#findComment-465374 Share on other sites More sharing options...
luke andres Posted December 13, 2020 Author Share Posted December 13, 2020 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 Quote Link to comment https://www.bigmacktrucks.com/topic/65243-speed/#findComment-465379 Share on other sites More sharing options...
Mark T Posted December 13, 2020 Share Posted December 13, 2020 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. Quote Link to comment https://www.bigmacktrucks.com/topic/65243-speed/#findComment-465383 Share on other sites More sharing options...
luke andres Posted December 14, 2020 Author Share Posted December 14, 2020 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 Quote Link to comment https://www.bigmacktrucks.com/topic/65243-speed/#findComment-465397 Share on other sites More sharing options...
steeler Posted December 14, 2020 Share Posted December 14, 2020 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? Quote Link to comment https://www.bigmacktrucks.com/topic/65243-speed/#findComment-465414 Share on other sites More sharing options...
Mark T Posted December 14, 2020 Share Posted December 14, 2020 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. Quote Link to comment https://www.bigmacktrucks.com/topic/65243-speed/#findComment-465415 Share on other sites More sharing options...
Mark T Posted December 14, 2020 Share Posted December 14, 2020 That's a transmission more for a smaller truck, like maybe a ten wheeler with a DT466 or something like that. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/65243-speed/#findComment-465416 Share on other sites More sharing options...
terry Posted December 14, 2020 Share Posted December 14, 2020 For what you are wanting to do with this truck, you need a bigger series of transmission for sure. terry Quote Link to comment https://www.bigmacktrucks.com/topic/65243-speed/#findComment-465418 Share on other sites More sharing options...
luke andres Posted December 14, 2020 Author Share Posted December 14, 2020 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 Quote Link to comment https://www.bigmacktrucks.com/topic/65243-speed/#findComment-465419 Share on other sites More sharing options...
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