Explanation of engine RPM and rear axle ratio

[pflaming]

I found this in a Studebaker forum. It reviews most of what has been discussed in our forum's threads but I like this because the discourse is in one report. I hope it is helpful to some. I am not an expert on this so if some see 'fault' in the article, some kindly suggests are always appreciated. 

 

By: Jim Pepper; with some editing.
Most people are used to the quiet modern vehicles of today. Engine and vehicle speed can no longer be sensed by sound. It is quite common for someone to think his or her engine is roaring, when in fact it is not. Speedometer and / or tachometer error can also contribute to this misconception.

This leads people to change final drive ratio when it might not be necessary. What is the best or most efficient RPM for our engines to operate? That is a question not easily answered. You should realize that any choice you make will have compromises. A safe choice is one that puts the engine in the optimum RPM range to accomplish the task the vehicle is used for the majority of the time. During average use, the rear wheel horsepower required to drive a given car at highway speed is constant. If that car has a 6 cyl., it might use a 4.10 to 4.27 ratio. A 259 will use a 3.54 to 3.73 and a 289 will usually have a 3.31 to 3.54 ratio. Each successively larger engine makes the required power at a lower RPM, thus the change in ratios.

Studebaker, for example, did build many cars using a 3.07 ratio behind a 259. They readily admitted it was not ideal but was an effort to achieve top fuel economy.  Average use, as used above, is defined as 50% highway/city use, gross loads including occupants not exceeding 600 pounds, and at 3000 ft. or less elevation. (a)Transmission selection, ( body weight and shape,©  and deviation from average use greatly influence ratio choice.

Higher numerical ratios increase rear wheel power, but also increase engine speed and noise, can limit top speed, and can reduce fuel economy. Lower numerical ratios generally do the opposite with one exception. They can also hurt fuel economy.

Consider isolating the cabin from engine noise before changing ratios by sound deadening. Then you have the opportunity to tailor your vehicle to your precise use and eliminate some of the compromises. So how do you calculate the final drive ratio once you determine your target RPM? A few simple formulas will do the job.

 

First is to measure the tire diameter. Diameter affects the revolutions per mile; thus it is a necessary bit of information.

The formulas are as follows.
GEAR RATIO =  ( RPM x tire dia.) divided by ( MPH x 336 ) 

MPH =  ( RPM x tire dia.) divided by ( gear ratio x 336 ) 

RPM =  ( MPH x gear ratio x 336 ) divided by ( tire dia.) 

TIRE DIA. =  ( MPH x gear ratio x 336 ) divided by ( RPM ) 

 

Let’s use a Lark as an example. The known information is, 3.73 rear end, 215-70R15 tire ( 26 1⁄2” dia.), and 4 th gear is direct ( 1 to 1 ). I’ll use 65 MPH as a speed. RPM is the unknown. 

 

(65 x 3.73 x 336 ) / ( 26.5 ) = ( 81463.2 ) / ( 26.5 ) = 3074 RPM 

 

If I install a 235-75R15 ( 28” dia. ), it looks like this. 

 

(65 x 3.73 x 336 ) / ( 28 ) = ( 81463.2 ) / ( 28 ) = 2909 RPM 

 

The tire change is nearly the same as a change to a 3.54 gear.

 

( 65 x 3.54 x 336 ) / ( 26.5 ) = ( 77313.6 ) / ( 26.5 ) = 2917 RPM 

 

A 3.31 gear will produce 2728 RPM with the 26.5” tire but will be back to 2892 RPM with a 25” ( 215-60R15) tire 

To factor in the effect of overdrive, multiply the ratio times either the RPM or the gear ratio. The OD ratio of both the T-85 and T –96 is 0.7 

3.73 x 0.7 = 2.61 final drive ratio. 3074 RPM x 0.7 = 2152 RPM

Edited April 20, 2014 by pflaming

[Plymouthy Adams]

torque......all you need to know...

[55 Fargo]

Flaming, are you trying to give yourself an aneurysm, what an analyser you are ......LOL

 

Here use this for all your ciphering needs....   http://www.4crawler.com/4x4/4LoCalc.shtml

[Don Coatney]

Tire size is all you need to know.

[pflaming]

". . . .Flaming, are you trying to give yourself an aneurysm, what an analyzer you are ......"  

 

I taught English Lit in the early 60's to mid '70's at a time when there were very few men teaching English. I taught Seniors, Aristotle, Shakespeare, Milton, Bacon, Hardy, Austen,and of course the poets. Most English teacher's 'preached' English, very authoritarian, i.e.," this is what the poem means!" Case closed. I did not, I looked at poetry and asked, what can I SUGGEST this poem is about from the evidence in the poem and it's historical setting. The students loved the approach.

 

I look at a vehicle the same way, why is it the way it is, it's construction, relationship to other parts, and that interests me. The mechanical is how the theory is proven, i.e., played out. Know the theory and the mechanical choices multiply. I'm having a great time, trust others are also, but I'm doing this for myself. 

 

So aneurysm or no I will continue to dig. 

[55 Fargo]

". . . .Flaming, are you trying to give yourself an aneurysm, what an analyzer you are ......"  

 

I taught English Lit in the early 60's to mid '70's at a time when there were very few men teaching English. I taught Seniors, Aristotle, Shakespeare, Milton, Bacon, Hardy, Austen,and of course the poets. Most English teacher's 'preached' English, very authoritarian, i.e.," this is what the poem means!" Case closed. I did not, I looked at poetry and asked, what can I SUGGEST this poem is about from the evidence in the poem and it's historical setting. The students loved the approach.

 

I look at a vehicle the same way, why is it the way it is, it's construction, relationship to other parts, and that interests me. The mechanical is how the theory is proven, i.e., played out. Know the theory and the mechanical choices multiply. I'm having a great time, trust others are also, but I'm doing this for myself. 

 

So aneurysm or no I will continue to dig. 

Fill yer boots Lad, hope you are enjoying it...

[Merle Coggins]

For most accurate calculations you should use the loaded radius of the drive tires. With a normal load measure from the ground to the center of the axle and multiply by 2 for your diameter measurement. The normal squat of a tire under load effectively reduces it's rolling circumference slightly. So using a measurement that takes that into account will be the most accurate. It may only be a few RPM difference, but...

[48Dodger]

How fast your moving is all you need to know.

 

 

48D