BKrois Posted December 17, 2015 Share Posted December 17, 2015 I'm going to be changing the gearing on my B61SX tri-axle when I change the transmission and am trying to figure out the best gear ratio.The plan is to put in the 12 speed Direct transmission going to 24.5 tires. I know a lot of people run the 12 speed OD trans with 4.64 gears. With the direct i'm thinking 4.17s, not sure if 4.42s would really hurt my highway speedNot sure where i could find the formula to figure it out or if anyone has some experience 1 Quote BryanMy Photo Albums Link to comment https://www.bigmacktrucks.com/topic/43159-gearing-question/ Share on other sites More sharing options...
220cummins Posted December 17, 2015 Share Posted December 17, 2015 http://www.roadranger.com/rr/CustomerSupport/Support/Calculators/index.htmI used this when I decided to make my RT a RTO. I know it's a road ranger site but you enter you specs to determine road speed. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/43159-gearing-question/#findComment-316214 Share on other sites More sharing options...
Green Dash Posted December 17, 2015 Share Posted December 17, 2015 RPMx60/tire rotations per mile x 4.17. I'm pretty sure an 11x24.5 has 476 rotations per mile. If so:2100 x 60=126,000.476 x 4.17=1984.92.126000/1984.92=63.48 Quote Jim Link to comment https://www.bigmacktrucks.com/topic/43159-gearing-question/#findComment-316215 Share on other sites More sharing options...
Green Dash Posted December 18, 2015 Share Posted December 18, 2015 If you have an OD trans, you would add the top gear ratio to the equation after the axle ratio. I skipped that step because with a direct trans you would be multiplying by 1. Quote Jim Link to comment https://www.bigmacktrucks.com/topic/43159-gearing-question/#findComment-316218 Share on other sites More sharing options...
Vladislav Posted December 20, 2015 Share Posted December 20, 2015 What gears are you going to get rid of?I'd be interested in a set off a B model of about 5.5 - 6.5.Any speed is a trip per time. Your wheel makes a trip doing a turn. Every turn it makes it's overall outside diameter x3.14. Or just turn a scale over the tire, more sharp. You need to convert inches into miles. An engine turns wheels. You can see revs. The most gears get them to wheels slower. So you need to divide engine revs by gear ratio. First divide by a gearbox gear used, than again by an axle gear. Those revs are still per minute. Multiply with 60 to get them per hour. That is how fast your wheels turn, revs per hour. Multiplying wheel's revs with the wheel's 1 rev trace we get miles per hour. Quote Никогда не бывает слишком много грузовиков! leversole 11.2012 Link to comment https://www.bigmacktrucks.com/topic/43159-gearing-question/#findComment-316564 Share on other sites More sharing options...
terry Posted December 20, 2015 Share Posted December 20, 2015 I'm going to be changing the gearing on my B61SX tri-axle when I change the transmission and am trying to figure out the best gear ratio.The plan is to put in the 12 speed Direct transmission going to 24.5 tires. I know a lot of people run the 12 speed OD trans with 4.64 gears. With the direct i'm thinking 4.17s, not sure if 4.42s would really hurt my highway speedNot sure where i could find the formula to figure it out or if anyone has some experienceI don't think a direct trans with 4.17's even with 24.5 tires is going to give you a good road speed, what do consider a good road speed? I can tell you for sure I had a DM dump with a 12speed overdrive 4.17's and 22.5 tires and at 1600 RPM would run about 65, plus I had the motor set to run 2300 RPM, it would really fly! Quote Link to comment https://www.bigmacktrucks.com/topic/43159-gearing-question/#findComment-316571 Share on other sites More sharing options...
smorse54 Posted December 20, 2015 Share Posted December 20, 2015 Back in the day we ran 237's and 300's with direct 5 speeds with 4.17 rears.We even had 1 R model 237 with 4.50 rears, They had good road speed and got the job done. I wouldn't hesitate to run the direct 12 speed with 4.17's for your truck. Steve Quote Link to comment https://www.bigmacktrucks.com/topic/43159-gearing-question/#findComment-316612 Share on other sites More sharing options...
terry Posted December 20, 2015 Share Posted December 20, 2015 Back in the day we ran 237's and 300's with direct 5 speeds with 4.17 rears.We even had 1 R model 237 with 4.50 rears, They had good road speed and got the job done. I wouldn't hesitate to run the direct 12 speed with 4.17's for your truck. SteveI drove many 237 with 5 spd. with 22.5 tires you are lucky to get 68 MPH with the engine wound tight. Quote Link to comment https://www.bigmacktrucks.com/topic/43159-gearing-question/#findComment-316617 Share on other sites More sharing options...
b61mack Posted December 20, 2015 Share Posted December 20, 2015 I agree with Terry a 5 speed with 4.17 wound tight would get you around 65-68. 3.86 70-72. Quote Link to comment https://www.bigmacktrucks.com/topic/43159-gearing-question/#findComment-316620 Share on other sites More sharing options...
Freightrain Posted December 21, 2015 Share Posted December 21, 2015 Yes, mine 4.10 in OD is about 72 mph. Take about 6-7 mph out for Direct and you are only in the 65 mph range. Fine if you don't plan on any long distance trips on the highway. Mine was 4.62 with OD it was still only 65 mph. Now I can run about 65 mph at 1800 rpm. I have 22.5 rubber.If you have enough granny gear, go 3.90? Quote Larry 1959 B61 Liv'n Large...................... Charter member of the "MACK PACK" Link to comment https://www.bigmacktrucks.com/topic/43159-gearing-question/#findComment-316679 Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.