erasedhammer
Active member
- 843
- 60
- 28
- Location
- Maryland
Did some math, or I should restate, a website did some math.
I went to a couple different engine rpm calculators to see what rpm my engine would be doing at a certain speed with certain ratios.
All the ones with plenty of input data were consistent, so heres the one I used: http://spicerparts.com/calculators/transmission-ratio-rpm-calculator
I WAS pretty sure I had 2.73 diffs, but now I am not so sure.
For that website I threw in 5.24 for the rear end ratio (2.73*1.92) and 0.75 for the transmission ratio and 37 for the tire size. For the engine RPM I put in 2400, because according to my tach thats what I'm doing when I'm at 60 (measured with GPS speedo).
Click calculate and it shows my speed as being 67!! Then I decided to put in the ratio for a 3.08 diff, 5.91 (3.08*1.92) and I get the correct number, 59.62 @ 2400 rpm (which is what my tach and speedo say).
Short of taking apart my diff, is this an accurate way of knowing what diff ratio I have? It seems unlikely for a m998a1 to have a 3.08 in it?
Also that calculator isn't showing the 200-300 rpm reduction of the torque converter lock up.
Something doesn't add up, I suppose that air friction causes the truck to have to rev higher to attain the same speed? (since these trucks are basically a piece of plywood being dragged through the air)
I went to a couple different engine rpm calculators to see what rpm my engine would be doing at a certain speed with certain ratios.
All the ones with plenty of input data were consistent, so heres the one I used: http://spicerparts.com/calculators/transmission-ratio-rpm-calculator
I WAS pretty sure I had 2.73 diffs, but now I am not so sure.
For that website I threw in 5.24 for the rear end ratio (2.73*1.92) and 0.75 for the transmission ratio and 37 for the tire size. For the engine RPM I put in 2400, because according to my tach thats what I'm doing when I'm at 60 (measured with GPS speedo).
Click calculate and it shows my speed as being 67!! Then I decided to put in the ratio for a 3.08 diff, 5.91 (3.08*1.92) and I get the correct number, 59.62 @ 2400 rpm (which is what my tach and speedo say).
Short of taking apart my diff, is this an accurate way of knowing what diff ratio I have? It seems unlikely for a m998a1 to have a 3.08 in it?
Also that calculator isn't showing the 200-300 rpm reduction of the torque converter lock up.
Something doesn't add up, I suppose that air friction causes the truck to have to rev higher to attain the same speed? (since these trucks are basically a piece of plywood being dragged through the air)
