Here's some handy calculations that will let you figure the RPM at any speed
for any vehicle if you know the rear gear ratio and transmission ratios. If
anyone wants the "long" version with all the math & gory details, just let
me know and I'll post it:
Engine RPM = [(rear end ratio) x (transmission gear ratio) x mph x 336.14] /
(tire+wheel diameter in inches)
Transmission gear ratios for 1997 42/44 RE Automatics:
1st = 2.74
2nd = 1.54
3rd = 1.00
4th = 0.69
Transmission gear ratios for 1997 AX15 Manual Transmissions:
1st = 3.83
2nd = 2.33
3rd = 1.44
4th = 1.00
5th = 0.79
Transmission gear ratios for the 1997 NV3500 Manual Transmissions:
Wide Ratio Model
1st = 4.01
2nd = 2.32
3rd = 1.40
4th = 1.00
5th = 0.73
Close Ratio Model
1st = 3.48
2nd = 2.16
3rd = 1.40
4th = 1.00
5th = 0.73
As an example, my 1997 Dakota has a 4.10 rear end gear and in OD (OD= 4th
=0.69) it tachs between the 2000 and 2125 marks (say about 2062 RPM) @
60MPH. I measured the tire wheel diameter and got 27-1/4 inches (27.25in).
SO....
Engine RPM = (4.10 x 0.69 x 60 x 336.14) / (27.25) =2093 RPM which is pretty
darn close!
BTW, I ran these type calculations numerous times when I decided to go from
a 3.55 to a 4.10. Made a huge difference in the truck....especially when I
turn off OD in the city from light to light... ;)
Hope this helps.
Mike
This archive was generated by hypermail 2b29 : Fri Jun 20 2003 - 11:47:09 EDT