Understanding Gear Ratio's vs. RPM's
By Doug Chartier

(Or attempting to anyway…:-))

The Jeep is basically TWO different vehicles made for TWO different purposes.  The transfer case makes it change from an off road vehicle to a highway vehicle (shift low to high).  Off road for the same engine rpm we want to go SLOW.  On the hwy we want to go FAST with the same rpm.

Within each of these purposes there are TWO major variables, Automatic transmission and 5 speed standard tranny.

AUTO: 
At low speed regardless of the position of the transfer case lever (high or low) the first gear and second gear are  VARIABLE ratios.  In other words it slips as if you were slipping a clutch in a standard shift.  You are sitting still with the engine running and the tranny in low.  No movement, but the engine and wheels are connected.  The tranny is slipping 100% so there is a ZERO gear ratio.  As you increase the engine rpms the vehicle will start to move at some point.  At that point there is finally a relationship (ratio) between engine rpms and wheel rpms, but it continues to change because the tranny does not lock up (In general; there are some auto trannys that can lock up in 1st and 2nd gear.).  This makes the auto very versatile in low speed operation.  Makes it possible to achieve the same results on the trail as a standard tranny with a very low gear ratio.  The down side is that this causes excessive heat in the auto, and since the auto does not lock up in low gear, it provides very little engine braking.  The tranny slips when trying to stop just like it slips when trying to start vehicle movement.

On the highway, when the Jeep is in high gear, the tranny IS locked.  It no longer slips like it did in low gear.  There is a specific, one and only, gear ratio in high gear (on automatics without overdrive).  It is 1:1.   So for each revolution of the engine the transmission causes  the drive shaft to turn one time. - (1:1)

Auto tranny gear ratios.  First and second gear use the torque converter, which is where the "slippage" comes from.  Will not get into detail about that except to say that it can generally be considered a "variable" gear ratio.  Not locked like the 3rd gear.

1 ........ 2.74:1 (but it slips - it's variable - as much as 4x to as little as .5x)
2 ........ 1.54:1 (also slips)
3 ........ 1.00:1 (locked at about 35 mph)

MANUAL: 
The standard shift tranny has 5 specific gears.  The following are the specific ratios for the 4 cyl and the 6 cyl engine TJ.  The only way to get a different relationship (ratio) between engine rpms and wheel rpms is to change gears and/or slip the clutch.  Too much clutch slipping and you have to buy a new clutch.

Gear ...... 4 cyl ........ 6 cyl.
1 ............ 3.93:1 ..... 3.83:1
2 ............ 2.33:1 ..... 2.33:1
3 ............ 1.45:1 ..... 1.44:1 (These could be the same, like 4.10 & 4.11)
4 ........….1.00:1 …..1.00:1 (Same as high in the Auto)
5 ......….... .85:1 ...…  79:1 (Overdrive)

From this you can see right off that the standard has an advantage over the auto on the highway.  The overdrive.  With the a/t you have one choice in high - 1:1 gear ratio.  The standard gives you two options - 1:1 or .85:1/.79:1 depending on engine.  This will make a difference in the axle ratio you choose IF you want to keep approximately the same engine rpm with either transmission. 

There is an additional concern in high gear with the auto.  Just as it builds up heat in low gear when it is slipping, it also builds up heat in high gear when the engine is straining.  For this reason you don't normally want to gear an auto with low engine rpms.  Example:  Stock 3.07:1 standard tranny will produce about 70 mph at about 2,000 rpms.  If you need extra power, you can shift into 4th gear, which is the same as the auto.  This builds up the rpms and puts less strain on the engine so the engine does not heat up.  If you geared the auto for the same 2,000 rpms at 70 mph and needed some extra power, hill, trailer, head wind etc., there is no gear to pick except second.  By looking at the two charts you can see that second in the auto is about where 3rd is in the standard.  Neither of these two gears are 70 mph gears.

So, bottom line, you need to gear the auto so that it runs in the mid to upper 2k range at about 70 mph.  Otherwise you will take the chance of often over heating the transmission.

TIRE SIZE, AXLE RATIOS, SPEEDS ETC. ETC.

Getting back to our TWO purpose vehicle –

1)        Go slow in low range
2)        Go fast in high range.  Here's what we need.

TRAIL:  It's all the same for every vehicle - SLOW speed without slipping the clutch

HIGHWAY: 
4 cyl, 5 speed or auto:  The 4 cyl engine needs more rpms to attain its power than the 6 cyl.

Hwy - high 2k to mid 3k rpm range at 70 mph.

6 cyl - 5 speed:  Hwy - low 2k  rpms at 70 mph

6 cyl - auto:  Hwy - mid 2k to very low 3k rpms at 70 mph.  The engine reaches peak torque at around 2,800 rpms so that would be a nice area to run the speed limit, 70 or 75 mph.

TIRE SIZE:
Simply a matter of doing the math.  Each revolution of the tire causes the vehicle to move a certain distance - the circumference of the tire.  The REAL circumference of the tire is often NOT what the tire maker says it is, but for this math we will assume that it is correct.  It is the relationship between tire sizes that is important anyway.  Real tire size is often over an inch less than advertised.  This causes real rpms to be higher than in the following equations.

THE QUESTION:
What is the relationship (ratio) needed to obtain about 2,800 rpms at 70 mph with each tire size with a 1:1 auto transmission ratio?

First thing we need to do is convert everything to the same dimension.  How about minutes and miles.  60 mph is 1mile in 1 minute.  2,800 rpms at 70 mph is the same ratio as 2,400 rpms at 60 mph.  So, what gear ratio do you need with either of the two tires sizes to achieve 2,400 rpms at 60 mph?

A 33" tire will roll 103.67" per revolution or 611.15 revolutions per mile.

A 35" tire will roll 109.96" per revolution or 576.23 revolutions per mile.

The ratio of 2400:611.15 equates to 3.93:1.

The ratio of 2400:576.23 equates to 4.17:1.

The 3.93 come between the 3.73 and 4.11 (available ratios).  If 2,800 rpms is a bit much for you at 70 mph then use the 3.73 and get about 2,650 rpms at 70 mph.  The 4.11 will give you about 2,925 at 70 mph.

With the 35" tire the 4.11 gears will give you about 2750 rpms at 70 mph.

With a .79:1 overdrive (6 cyl) those same gears, 3.73 and 33" tires would produce about 2,100 rpms at 70 mph.  The 4.11 would give you about 2,310 rpms at 70 mph.

For the 35" tires it works out this way.  4.11 gears - about 2,180 rpms at 70 mph.  4.56 gears will give you about 2,415 rpms at 70 mph.

If you want good highway performance, you must gear for that and let the off road performance be whatever the end result might be.  If off road performance is more important, then gear lower and go slower on the highway.

The way to get the best of both worlds is to change the gear ratio on the LOW side of the transfer case.  That way you can pick the axle ratio that fits your highway preference AND the off road performance will far exceed what you could achieve with ANY axle ratio.  Stock low in the transfer case is 2.72:1.  By changing that to 4:1 (4:1 low kit) or 4.3:1 (Atlas II 4.3 transfer case) you can almost double your crawl ratio in low.  To go a step further replace the transmission with an NV4500.  It has a low gear that is over 5:1 as opposed to the stock 5 speed of 3.83:1 and an overdrive in the low  .7's:1 as opposed to .79:1 in the stock 5 speed, and it is structurally a much stronger tranny too.

Last part of the equation:  When you make any of these gear and tire changes it changes the possible load on the axles and drive shafts.  As the lower gears multiply the engine torque it puts that much ADDITIONAL torque on the axles and drive shafts.  Make sure those components are able to take the additional torque.

- Doug Chartier

 

A simple way to calculate the gearing needed for a specific tire size to emulate (or duplicate) the gearing you have today: 

Original Gear Ratio

 

y (number you are looking for)

------------------------

=

------------------------

Original Tire Size

 

New Tire Size

So then the following assumption could be made:

y=(Original Gear Ratio x New Tire Size) divided by (Original Tire Size)

For example: y=(3.73x35)/30 or y=130.55/30 or y=4.35

Where a vehicle was stock with 3.73 gears and 30” tall tires and wanted to go to 35” tires it would need at least a 4.35 gear ratio to keep the same gearing it had stock.  Since that ratio is probably not available  one would need to decide between 4.10 or 4.56 gearing (some applications may have a 4.26:1 available).

Keep in mind that the measurements listed on the sidewalls of most tires are inaccurate compared to the actual rolling diameter and that taller tires weigh more, so even with exact proper gear ratio it may still feel more sluggish with the taller, heavier tires.