Below are charts for my truck as it is stock, 305E V6, SM-420 4-speed, 4.57:1 rear axle & 225/75R-16
tires, (7.50-16 old style numbering) & with the Spicer 5831-B 3-speed. For stock speeds with out
the over-drive, just look at the final speed for Fourth Gear. The GMC V6 are Power engines, so I
figure around 3500 for max RPM. I have also made a chart for the hoped for future setup of the GMC
401M V6, NP-540-GL 5-speed, Spicer 5831-B 3-speed & rear axle ratio of 3.73:1. The Green Gears Ratios
would be the over the road shift pattern, & are the gears shown in the Final Ratio column.
When you start thinking about gear ratios you need to look at the whole picture, not just transmission
gearing, but rear axle gearing & tire size play a lot into the equation too. there is a formula or
two for this, This is the one found in the 1965 GMC Dealer data book;
RPM x 60 divided by Final Ratio x tire revolutions pre mile.
So lets look at a GMC 1/2 ton truck with rear axle ratio of 3.54:1. We also know this info; 4 speed SM420
transmission, 1st = 7.06:1, 2nd = 3.58:1, 3rd = 1.71:1, 4th = 1.00:1. With this set up, that will give
you a final drive gear ratios of; 1st = 24.99:1, 2nd = 12.67:1, 3rd = 6.05:1, 4th = 3.54:1. Engine max
RPM of 3500, 15 inch tire will have a revolutions pre mile of around 745. So our top number will be
3500 x 60 = 210000, bottom number in 4th gear 3.54 x 745 = 2637.3, so now take 210000 / 2637.3 = 79.63 mph.
Top speed in low gear would be 210000 / 18617.55 = 11.28 mph. Just for the record, 16" tires = 705 rpm.
So now that you know who to do the math, there is a faster way,
Here is a Gear/MPH Calculator, just plug in your numbers.
The Stock rear axle of 4.57:1 in Forth gear will do 55 MPH @ 2900 RPM, & 65 MPH @ 3400 RPM.
The Stock rear axle of 4.57:1 with the Over-Drive will do 55 MPH @ 2400 RPM, & 65 MPH @ 2900 RPM.
A rear axle gear of 3.73:1 in Fifth gear will do 55 MPH @ 2400 RPM, & 65 MPH @ 2800 RPM.
A rear axle gear of 3.73:1 will cruise in Over-Drive at 65 MPH @ 2400 RPM, & 75 MPH @ 2700 RPM.
SM-420 4-speed + Spicer 5831-B 3-speed + 4.57:1 Rear Axle
|
Auxiliary
Main |
Under-Drive |
Direct-Drive |
Over-Drive |
Final Ratio |
MPH @ |
|
First 7.06:1 |
16.59:1 |
7.06:1 |
6.00:1 |
First |
First |
|
Second 3.58:1 |
8.41:1 |
3.58:1 |
3.04:1 |
Second |
Second |
|
Third 1.71:1 |
4.0:1 |
1.71:1 |
1.45:1 |
Third |
Third |
|
Fourth 1.00:1 |
2.35:1 |
1.00:1 |
0.85:1 |
Fourth |
Fourth |
|
Reveres 6.78:1 |
15.93:1 |
6.78:1 |
5.76:1 |
Over-Drive |
Over-Drive |
NP-540-GL 5-speed + Spicer 5831-B 3-speed + 3.73:1 Rear Axle
|
Auxiliary
Main |
Under-Drive |
Direct-Drive |
Over-Drive |
Final Ratio |
MPH @ |
|
First 7.41:1 |
17.41:1 |
9.52:1 |
6.30:1 |
First |
First |
|
Second 4.05:1 |
9.52:1 |
4.05:1 |
3.44:1 |
Second |
Second |
|
Third 2.40:1 |
5.64:1 |
2.40:1 |
2.04:1 |
Third |
Third |
|
Fourth 1.48:1 |
3.48:1 |
1.48:1 |
1.26:1 |
Fourth |
Fourth |
|
Fifth 1.00:1 |
2.35:1 |
1.00:1 |
0.85:1 |
Fifth |
Fifth |
|
Reveres 7.85:1 |
18.45:1 |
7.85:1 |
6.67:1 |
Over-Drive |
Over-Drive |
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