40-series Gear ratio, RPMs and Speed

[AlexJet]

Interesting data I found when looked closely at US-spec 49-series in regards to Gear ratio, Engine RPMs and Speedometer gauge readings.
I used the data collected on this forum and applied to my '74 as an example.

Stock tire size:
H78-15
Conversion to modern size:
215/75-R15
Tire data:
28" diameter, 8.5" width

H42 Transmission:
1st Gear - 3.555:1
2nd Gear - 2.292:1
3rd Gear - 1.410:1
4th Gear - 1.000:1
Reverse Gear - 4.271:1

H42 Transfer Case:
Hi Gear - 1.000:1
Low Gear - 1.992:1

Recommended Speed Limit by the original Operator Manual book:
1st Gear - 20mph / 30kmh
2nd Gear - 35mph / 55kmh
3rd Gear - 55mph / 85kmh
4th Gear - 63mph / 100kmh

 

[AlexJet]

My findings...


If taking 1st Gear as a starting point and stock configuration:

3.555 (1st Gear ratio) x 1.000 (Transfer Case Hi Gear) x 3.73 (Differential ratio) = 13.26 (Final ratio)
28" tire diameter => Circumference (PIxD) 3.141592 x 28 = 90"
1 Engine revolution will give travel => 90 / 13.26 = 6.79"

Doing the same math to 4.56 diff gears and 35 tires:

3.555 (1st Gear ratio) x 1.000 (Transfer Case Hi Gear) x 4.56 (Differential ratio) = 16.21 (Final ratio)
35" tire diameter => Circumference (PIxD) 3.141592 x 35 = 110"
1 Engine revolution will give travel => 110 / 16.21 = 6.79"

Finding #1: 35" tires and 4.56 differential gears are brings performance to the stock level.
P.S. I'm not counting on weight increase, etc, just pure math.



Using the same approach I have counted Engine RPMs for Recommended Speeds from the Manual.

3.555 (1st Gear ratio) x 1.000 (Transfer Case Hi Gear) x 3.73 (Differential ratio) = 13.26 (Final ratio)
28" tire diameter => Circumference (PIxD) 3.141592 x 28 = 90"
1 Engine revolution will give travel => 90 / 13.26 = 6.79"
6.79" is 0.000107mi
To cover 1mi Engine would need 1 / 0.000107 = 9'346 Revolutions
Per hour would be 9'346 / 60 = 155.77
Engine RPMs at 20mph should be 155.77 x 20 = 3'115

2.292 (2nd Gear ratio) x 1.000 (Transfer Case Hi Gear) x 3.73 (Differential ratio) = 8.55 (Final ratio)
28" tire diameter => Circumference (PIxD) 3.141592 x 28 = 90"
1 Engine revolution will give travel => 90 / 8.55 = 10.53"
10.53" is 0.000166mi
To cover 1mi Engine would need 1 / 0.000166 = 5'978 Revolutions
Per hour would be 5'978 / 60 = 100.40
Engine RPMs at 20mph should be 100.40 x 35 = 3'514

1.410 (3rd Gear ratio) x 1.000 (Transfer Case Hi Gear) x 3.73 (Differential ratio) = 5.26 (Final ratio)
28" tire diameter => Circumference (PIxD) 3.141592 x 28 = 90"
1 Engine revolution will give travel => 90 / 5.26 = 17.11"
17.11" is 0.000270mi
To cover 1mi Engine would need 1 / 0.000270 = 3'704 Revolutions
Per hour would be 3'704 / 60 = 61.73
Engine RPMs at 20mph should be 61.73 x 55 = 3'395

1.000 (4th Gear ratio) x 1.000 (Transfer Case Hi Gear) x 3.73 (Differential ratio) = 3.73 (Final ratio)
28" tire diameter => Circumference (PIxD) 3.141592 x 28 = 90"
1 Engine revolution will give travel => 90 / 3.73= 24.13"
24.13" is 0.000381mi
To cover 1mi Engine would need 1 / 0.000381 = 2'625 Revolutions
Per hour would be 2'625 / 60 = 43.74
Engine RPMs at 20mph should be 43.74x 63 = 2'755

Finding #2: Seems to be that Toyota kind of picked +/- 3'400 rpm as maximum per each gear except the 4th. This can relate to trying to keep RPMs bellow 2'800 and not over stressing the engine. In theory the engine capable to get to 80mph at 3'500 RPM. This is static data and I can see that pushing vehicle like 40-series through the air at 60+mph is not very aerodynamically, so it may not reach that theoretical maximum or may do depend on the road and weather conditions.
If apply 3'500 RPM to the ratios, then speeds for the gears will coming as 22.5 (1st), 34.9 (2nd), 56.7 (3rd) and 80 (4th).

 

[AlexJet]

When my vehicle will get out of the shop, I'd be interested to put GPS and test the actual data.

 

[exwattscop]

Yes, please post. I'm looking to swap out my stock 3 speed and gears to get my 72' 40, more freeway friendly. Looking for the right combination to mate to my Chevy 350.... Probably going all out for an automatic but want options....

 

[1911]

When my vehicle will get out of the shop, I'd be interested to put GPS and test the actual data.

I think you're pretty close with your math. I have both a GPS and a tachometer in my 40; 3.70 diffs, 2F, H41 (same 1.0 4th gear as the H42), and 33" tires. Turning 2500 rpm gives me 65 mph in 4th gear.

 

[MoCoNative]

I'm running P235/75R15 tires, so they are about an inch bigger, but my FJ-60 with that same drivetrain runs pretty close to those numbers.

If you really like math that is great, but I like this website, it seems to be pretty close to reality: Gear Ratio Calculator

You can have some fun comparing components and combos.