Gear Ratio Calculator

[snowblur17]

Saw this in an off-road mag from 2000.

First: Recommended engine RPM for a highway speed of 60 MPH

*4 cylinder: 3,000-3,400 rpm
*V-6 : 2,600-3,200 rpm
*Inline Six: 2,000-2,600 rpm
*Small Block V8: 2,400-3,000 rpm
*Big Block Engine: 2,200-2,800 rpm

Next: To determine what gear ratio your truck needs to maintain the previously listed engine rpm at 60 mph:

Gear Ratio = (rpm x Tire Diamter) / (mph x 336)

Example:
Inline six engine at 60: 2,000-2,600 rpm (use the mean of 2,400rpm to get a middle value)
Tire Diameter: 33"
so: Gear Ratio = (2,400 x 33) / (60 x 336)
= (79,200) / (20,160)
= 3.928 ~ 3.93, so 4.10's (likely not available in 3.93's)

That is a basic way of doing it. If you prefer to calculate for optimum rpm, taking into account the transmission type:

* For Manual Transmissions without overdrive:
RPM = (Gear Ratio x mph x 336) / tire diameter
Example: Truck with 4.10s 30" tires traveling 60mph will have an engine speed= (4.10x60x336)/30 = 2,755 RPM

* For Automatics without overdrive:
RPM = (Gear ratio x mph x 335)/ tire diamter
Example: Truck with 4.10s, 30"tires, going 60 mph will have an engine speed = (4.10x60x335)/30 = 2,911
* Note that at the same speed as the manual the auto has a slightly higher engine speed because of the auto's inherent slippage, even 60 mph

*For autos and manuals WITH Overdrive:
RPM = (gear ratio x OD percent x mph x 336) / Tire diameter
Example: Jeep with 3.55's, 33" tires, an automatic overdrive (0.7) going 60 mph should have an engine speed =(3.55 x 0.7 x 60 x 336) /33 =1518.11 rpms

[Ktmracer419]

fuck math

the ktmracer 4.0 aw4/ax15 gear chart:

31's- 4.10
32-4.56
33-4.56-4.88
34-4.56-4.88
35-4.88-5.13
36-5.13-5.38
37-5.38
anything bigger and peeps should know what gears they want to run

[snowblur17]

weird how it told me to run 3.92's. basically comes down to preference. I'd rather have 4.10s with what I've got now.

[DrThunder]

I run my big block at 1500 rpm


I am constantly doing this calculation,

Engine RPM / Gear ratio / trans ratio / 336 X Tire size = Speed

Chevelle doesn't have a speedo that operates and the trucks speedo is way off to

[snowblur17]

Where does the trans ratio come from?

[rckymntn05]

2400 to 3000 seems REAL high for a small block V8. I have never even seen one above 2000 on the highway!

[snowblur17]

Yeah me either, my brothers small block runs around 1700 at 65 I wonder what they were basing these numbers on

[DrThunder]

Where does the trans ratio come from?

Depending on what gear you're in. 2.80, 1.53, 1.00, .75. for the AW4. that order is in 1st to 4th.

[snowblur17]

ahhh I see