dek1581 Posted August 21, 2015 Share Posted August 21, 2015 I recently saw some posts re: changing rear axle ratios to get better mpg and/or higher top speed. I know there is a formula to estimate top speed based on tire size (revs per mile), engine rpm's, and rear end ratio. now, what I want to know is there a limit, so to speak, on how low (numerically) you can go before it becomes difficult to take off from a standing start w/o revving the hell out of it and burning the clutch? I have driven some dump trucks where you can let off the clutch while in 1st w/o giving any gas at all. I can only imagine what rear they had in them. Quote Link to comment https://www.bigmacktrucks.com/topic/41441-rear-axlehow-low-can-you-go/ Share on other sites More sharing options...
Maddog13407 Posted August 21, 2015 Share Posted August 21, 2015 depends what truck your talking about, im runnin a 711 na with a 15 speed triplex and a 4.00 rear. nice on the highway. gets to about 75+++ miles per hour but pull something with it and it falls on its ass to about 30mph pretty quick. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/41441-rear-axlehow-low-can-you-go/#findComment-300876 Share on other sites More sharing options...
mrsmackpaul Posted August 21, 2015 Share Posted August 21, 2015 Yeah all depends on what you wanna do there used to be a formula to work all this out back in the late 70's you put in what weight what height how fast you wanted to travel ect and could work out what size motor gear ratio's gear box ect that you would need to obtain the best balance between economy and power Paul Quote Link to comment https://www.bigmacktrucks.com/topic/41441-rear-axlehow-low-can-you-go/#findComment-300898 Share on other sites More sharing options...
hatcity Posted August 21, 2015 Share Posted August 21, 2015 put more gears at the transmission.18 spd will give you a good low and plenty at the topor add a 2 spd. auxillary Quote Success is only a stones throw away.................................................................for a Palestinian Link to comment https://www.bigmacktrucks.com/topic/41441-rear-axlehow-low-can-you-go/#findComment-300899 Share on other sites More sharing options...
RowdyRebel Posted August 21, 2015 Share Posted August 21, 2015 You've also got to take into consideration where the engine likes to run. Mack engines prefer the 1400-1450 range, so spec'ing a rear that is TOO "fast" in order to run 1100 or 1200 RPM down the road isn't doing you any favors. Find out where the "sweet spot" of the engine is, then decide how fast you want to cruise down the road. Then, pick a transmission and rear gears so that your final drive ratio will put you there. If you have a direct-drive (1:1) top gear in the transmission, you'll probably have higher (lower numerically...faster) gears in the rear to make up for it, where an overdrive transmission (.71:1) will have lower (slower) rear gears to compensate. It is all 1 system working together...engine, transmission, rear gears, and tire size. Changes to any one of them could force changes in others to become necessary. Engine might have a different "sweet spot"...and behind that, every change affects the overall ratio determining what speed the engine turns at what road speed. Spec it around the engine's sweet spot for the road speed you want to cruise. Quote When approaching a 4-way stop, the vehicle with the biggest tires has the right of way! Link to comment https://www.bigmacktrucks.com/topic/41441-rear-axlehow-low-can-you-go/#findComment-300913 Share on other sites More sharing options...
dek1581 Posted August 21, 2015 Author Share Posted August 21, 2015 hey rowdy, I know it's only math and there are other variables BUT I ran the numbers and with the setup I put together it revved at 1400 rpms at 75 mph. 5th gear (overdrive) would only be used at 55+ mph. that should give me better mpg then now. BUT correct me if I'm wrong or missing something. Quote Link to comment https://www.bigmacktrucks.com/topic/41441-rear-axlehow-low-can-you-go/#findComment-300943 Share on other sites More sharing options...
RowdyRebel Posted August 21, 2015 Share Posted August 21, 2015 1) What engine do you have, and where is it's sweet spot? 2) What transmission do you have, and what are the ratios in low (startability) and top (cruising) gear? 3) What rear end gearing do you have now? 4) What size tires do you have on the truck? Until that is known, there's no point in offering up suggestions or opinions on how to go about making changes. 1 Quote When approaching a 4-way stop, the vehicle with the biggest tires has the right of way! Link to comment https://www.bigmacktrucks.com/topic/41441-rear-axlehow-low-can-you-go/#findComment-300949 Share on other sites More sharing options...
kt_Engineer Posted August 24, 2015 Share Posted August 24, 2015 You need to calculate the desired startability index based on Clutch Engagement Torque, Tire Revs/mile, overall gear ratio (transmission start gear x axle ratio) and GCW/GVW. For a typical highway truck you want to have startability to be no lower than 16% to be on safe side but if you are going to drive turnpike only (<3% grades) majority of the time, Eaton suggests a startability of 12%. I have seen one customer happy with a 10.5% startability recently.If it is a vocational truck (especially a garbage hauler) the minimum suggested startability is 25% which would require a deep axle ratio. Quote Link to comment https://www.bigmacktrucks.com/topic/41441-rear-axlehow-low-can-you-go/#findComment-301348 Share on other sites More sharing options...
dek1581 Posted August 24, 2015 Author Share Posted August 24, 2015 great info kt! thanks a bunch, it's starting to get clearer now haha. with a 237 motor (plus the other figures) I came up with 15.61%. so I will go from there. Quote Link to comment https://www.bigmacktrucks.com/topic/41441-rear-axlehow-low-can-you-go/#findComment-301353 Share on other sites More sharing options...
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