Gear Ratio's vs. RPM's
By Doug Chartier
attempting to anyway
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.
each of these purposes there are TWO major variables, Automatic transmission
and 5 speed standard tranny.
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)
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.
........ 2.74:1 (but it slips - it's variable - as much as 4x to as little
...... 4 cyl ........ 6 cyl.
this you can see right off that the standard has an advantage over the auto
on the highway. The overdrive.
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.
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.
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.
SIZE, AXLE RATIOS, SPEEDS ETC. ETC.
back to our TWO purpose vehicle
Go slow in low range
It's all the same for every vehicle - SLOW speed without slipping the
- high 2k to mid 3k rpm range at 70 mph.
cyl - 5 speed: Hwy - low 2k
rpms at 70 mph
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.
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?
33" tire will roll 103.67" per revolution or 611.15 revolutions
35" tire will roll 109.96" per revolution or 576.23 revolutions
ratio of 2400:611.15 equates to 3.93:1.
ratio of 2400:576.23 equates to 4.17:1.
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.
the 35" tire the 4.11 gears will give you about 2750 rpms at 70 mph.
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.
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.
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.
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.
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.
simple way to calculate the gearing needed for a specific tire size to
emulate (or duplicate) the gearing you have today:
then the following assumption could be made:
Gear Ratio x New Tire Size) divided by (Original Tire Size)
example: y=(3.73x35)/30 or y=130.55/30 or y=4.35
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.