Walt1967 Posted March 28, 2015 Share Posted March 28, 2015 Hey guys, I have a 1991 rw713.truck has 425 cat 13 eaton trans, 4.42 rears.seems like truck will do 62, 63 mph at 1900 rpms.any way to increase speed w/o changing tires.has 11r22.5s.also approx how much to change gears to 4.17 or is ther a different 13 speed I could put in it.truck is pulling 160000 gross.really don't want to switch to bigger tire cause we drop and hook different trailers all day, and they sit low.how much speed would you gain w/ 4.17 gears? Quote Link to comment https://www.bigmacktrucks.com/topic/39459-rear-end-ratios/ Share on other sites More sharing options...
Kansas farmer Posted March 28, 2015 Share Posted March 28, 2015 If you figure the percent I think you will gain about 6 mph. I found a website that had a calculator. I think it was a fuller website but can't find it on my phone. It lets you put in trans ratio gear ratio rpm and tire size and will give you a speed. Quote Link to comment https://www.bigmacktrucks.com/topic/39459-rear-end-ratios/#findComment-284368 Share on other sites More sharing options...
AZB755V8 Posted March 28, 2015 Share Posted March 28, 2015 Hi,New here but the math is EZ. What you have is 1900rpm's ~ .86 Trans overdrive = 2,209RPM's I think it is .86 for your Eaton 13 speed.2,209 ~ (gear ratio) = YTire roll out in feet = Z Z for 22.5's = 11.00 Z for 24.5's = 11.52Y x Z x 60 ~ 5280 = MPH 5280 feet in a mile 60 minute in an hourThis is like you said for what you have now499.84 x 11 x 60 ~ 5280 = 62.48MPHReducing the equationY x Z x .01136 = MPH This works for any ratio and the two tire sizes For 4.17 ratio and 22.5's529.736 x 11 x .01136 = 66.19 MPHI have 3.76 rears in my RW on 22.5's and I like them @ 73.41 MPHHope this helps as I see rear ratios have come up more than once. 2 Quote Link to comment https://www.bigmacktrucks.com/topic/39459-rear-end-ratios/#findComment-284373 Share on other sites More sharing options...
Lmackattack Posted March 28, 2015 Share Posted March 28, 2015 If what you are saying is at 1900 rpm you see 63 mph with 4.42 rears and tires with 513 revs per mile my math says you have around a .78 overdrive.If you went from a 4.42 down to a 4.17..... Your road speed would be 67mph at 1900 rpmThis is assuming you have around a .78 overdrive 1 Quote Link to comment https://www.bigmacktrucks.com/topic/39459-rear-end-ratios/#findComment-284374 Share on other sites More sharing options...
j hancock Posted March 28, 2015 Share Posted March 28, 2015 For 160,000 lbs, like the 4.42. Quote JimIt doesn't cost anything to pay attention. Link to comment https://www.bigmacktrucks.com/topic/39459-rear-end-ratios/#findComment-284387 Share on other sites More sharing options...
b615rich Posted March 28, 2015 Share Posted March 28, 2015 Right Jim. I'd think taller gears with that much weight would be less than desirable i.e. more shifting etc. What's that heavy anyway? Here it's overweight vehicles or tandem 48's and how much more top end would you need? I'm sure there is a good reason, I'm just curious. Quote Link to comment https://www.bigmacktrucks.com/topic/39459-rear-end-ratios/#findComment-284396 Share on other sites More sharing options...
Walt1967 Posted March 28, 2015 Author Share Posted March 28, 2015 It's 160000 gross eight axle trash hauling in michigan. Quote Link to comment https://www.bigmacktrucks.com/topic/39459-rear-end-ratios/#findComment-284413 Share on other sites More sharing options...
Ronnie Surrett Posted December 1, 2021 Share Posted December 1, 2021 I bought a 2013 mp7u13 Mack truck it came from the factory with a tag axle. removed the jack plate installed a dump body 16’. Suspension is air bag. I’m told it gets real shaky. Would like to stiffen it up. what can I do. Quote Link to comment https://www.bigmacktrucks.com/topic/39459-rear-end-ratios/#findComment-483856 Share on other sites More sharing options...
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