pennsyjeff Posted June 4, 2006 Share Posted June 4, 2006 Hi guys, I have my '64 B-61 on the road now and it runs well, but I have one problem. With the 300 plus under the hood, coupled up to the original quad box tranny, I can only get 50 MPH and the engine is tach'ed out at 2100 rpm's.. How do you tell the ratio's of the diff's??? do any of you have any idea of what ratio I should drop in to get 55-60 mph on the road. I want to pull my trailer with my old cats, but I don't have a whole week to get to the shows!!! I will post some pictures so you guys can see it, it really turned out nice..thanks. Quote Link to comment https://www.bigmacktrucks.com/topic/1161-b-model-differentials/ Share on other sites More sharing options...
HK Trucking Posted June 4, 2006 Share Posted June 4, 2006 The ratio is stamped in the carrier housing, on the flat area on the front side to the bottom right. If yours is on 11.00x22 rubber it prob. has 7.49 ratio, that was common in lots of B61 10 wheelers back then, & would give approx 52-54 mph at 2100. If it's on 11.00x24 rubber it may have 8.24 ratio, that would also put you in the same ballpark as far as mph. 6.34 is an available ratio for the crd 112 & 113 carriers (which I'm just assuming here is whats in there) and we had a truck with that ratio on 11.00x22 which if I remember correctly was capable of about 65 mph @ 2100 rpm. Also, if you go to the Eaton Fuller Roadranger website there is a ratio/mph calculator there to determine road speed @ a given rpm depending on trans high gear ratio, rear axle ratio, tire size etc. Hope this helps, looking forward to seeing the "completed" pics.Herb Quote "If You Can't Shift It Smoothly, You Shouldn't Be Driving It" Link to comment https://www.bigmacktrucks.com/topic/1161-b-model-differentials/#findComment-6000 Share on other sites More sharing options...
pennsyjeff Posted June 6, 2006 Author Share Posted June 6, 2006 The ratio is stamped in the carrier housing, on the flat area on the front side to the bottom right. If yours is on 11.00x22 rubber it prob. has 7.49 ratio, that was common in lots of B61 10 wheelers back then, & would give approx 52-54 mph at 2100. If it's on 11.00x24 rubber it may have 8.24 ratio, that would also put you in the same ballpark as far as mph. 6.34 is an available ratio for the crd 112 & 113 carriers (which I'm just assuming here is whats in there) and we had a truck with that ratio on 11.00x22 which if I remember correctly was capable of about 65 mph @ 2100 rpm. Also, if you go to the Eaton Fuller Roadranger website there is a ratio/mph calculator there to determine road speed @ a given rpm depending on trans high gear ratio, rear axle ratio, tire size etc. Hope this helps, looking forward to seeing the "completed" pics.HerbThanks Herb for your help, I am going to find out what the ratio is and then look for something "taller" if I can...it runs well but is just too slow...as soon as I get the pictures downloaded I will get them on here... Quote Link to comment https://www.bigmacktrucks.com/topic/1161-b-model-differentials/#findComment-6012 Share on other sites More sharing options...
Rob Posted June 6, 2006 Share Posted June 6, 2006 Thanks Herb for your help, I am going to find out what the ratio is and then look for something "taller" if I can...it runs well but is just too slow...as soon as I get the pictures downloaded I will get them on here...My yellow dog has 10:00X20 rubber with 6.34:1 rears and runs a "blistering" 58 MPH against the governor.Rob Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment https://www.bigmacktrucks.com/topic/1161-b-model-differentials/#findComment-6014 Share on other sites More sharing options...
pennsyjeff Posted June 11, 2006 Author Share Posted June 11, 2006 Hey y'all, Herb was correct, I looked on my diff's and it is 7.49 ratio......what do you think I should go to for some road speed..I have in on 11.00 x 24.5 tubeless radials, but it originally came with 11.00 x 22 rubber....tell me what you guys think..thanks, Jeff. Quote Link to comment https://www.bigmacktrucks.com/topic/1161-b-model-differentials/#findComment-6067 Share on other sites More sharing options...
Packer Posted June 12, 2006 Share Posted June 12, 2006 4.17's should put you about 65 mph, give or take, in direct. Overdrive will be closer to 75 mph.Packer Quote Keep a clutchin' Link to comment https://www.bigmacktrucks.com/topic/1161-b-model-differentials/#findComment-6068 Share on other sites More sharing options...
Superdog Posted June 12, 2006 Share Posted June 12, 2006 At 50 mph with 7:49's on 24.5's,now,I believe I'd go to a 5:73,should put you about were you want.My B83 has 4:17's on 24.5's and it is way too fast,100+ mph Quote Link to comment https://www.bigmacktrucks.com/topic/1161-b-model-differentials/#findComment-6071 Share on other sites More sharing options...
pennsyjeff Posted June 12, 2006 Author Share Posted June 12, 2006 Hey guys, I mentioned I would get some photos on here of the truck when completed, well here they are....... Quote Link to comment https://www.bigmacktrucks.com/topic/1161-b-model-differentials/#findComment-6075 Share on other sites More sharing options...
HK Trucking Posted June 14, 2006 Share Posted June 14, 2006 Extremely beautiful! Looks just like it probably did in 1964.I believe Superdog is correct about the 5.73 ratio.I had a couple of DM 690's with 5.73 rears on 10.00x20 with T2070 7 speed which has an overdrive which is comparable to 5th hi split in a quad box. They would run 68 mph @ 2000 rpm, but on your 11-24.5 it'll be a bit faster, also your engine runs 2100 rpms, so that'll give a little more speed too.Might want to check with your local Mack Dealer, the 5.73 gears may not work in your current 7.49 carriers. Could be a difference in the pinion housing due to the depth being different due to the different diameter of the ring gear.Perhaps you could exchange the present carriers for 5.73 carriers.I have purchased exchange rear differentials from Weller truck parts in the past. Next day service, they deliver a rebuilt & pick up the old one. Can find their phone # if you need it, otherwise they advertise in most of the truck sale magazines.Herb Quote "If You Can't Shift It Smoothly, You Shouldn't Be Driving It" Link to comment https://www.bigmacktrucks.com/topic/1161-b-model-differentials/#findComment-6086 Share on other sites More sharing options...
therabbittree Posted June 16, 2006 Share Posted June 16, 2006 phew I just checked em on my b81 ..9.02 ratio if i am reading the right #.. this is a 58k set CRD 93 - 133-14 it was lowbed tractor for moving heavy cranes/ machinery etc.. man a 9.02 i bet 45mph would be high speed in this truck..end673 natural with quad box..what is the top overdrive gear ratio in the quad box?.. .68?... I will definately be swapping em out ... and putting in set of 4.17 or even 3.90 if i can get em.. i plan on runnning it with a 12v71 and allison 740..if it works out ok..heck even if i leave it mack engine and trans i still would like about 85mph top speed if possible..what ratio would i need then it has 11x24 rubber now i probably will swap to 24.5 tall radials...keep the thread goingThanksDeo Quote Keep on keeping on... Link to comment https://www.bigmacktrucks.com/topic/1161-b-model-differentials/#findComment-6117 Share on other sites More sharing options...
Packer Posted June 16, 2006 Share Posted June 16, 2006 That 24.5 rubber was my downfall!!! 10X20 and 18 speed (double over) is 83 mph with 4.17 at 2100 rpm.Packer Quote Keep a clutchin' Link to comment https://www.bigmacktrucks.com/topic/1161-b-model-differentials/#findComment-6120 Share on other sites More sharing options...
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