At 10:09 PM 8/28/00 -0400, you wrote:
>Should I shift to 4th or let it ride? Also, what do you suggest for the topend
>so that I wont have to lose all the power when going past the rpm range in 3rd
>or losing the time by going into 4th. What do you think? Help!!??
You mentioned that your truck's stats are:
255/70R15 tires, 3.55 rearend, 83.05mph in the 1/4
Based on that, we can figure out what your finish line RPM is as follows:
Engine RPM = driveshaft RPM * gear ratio
Driveshaft RPM = tire RPM * rearend ratio
Tire RPM = MPH * 5280 * 12 / 60 / tire circumference (in inches)
Tire circumference = tire height * PI
Tire height = (tire width / 25.4 * aspect ratio * 2) + rim height
Thus:
Tire circumference: ((255 / 25.4 * .7 * 2) + 15) * 3.14159 = 91.28"
Tire RPM: 83.05 * 5280 * 12 / 60 / 91.28 = 960.78rpm
Driveshaft RPM: 960.78rpm * 3.55 = 3410rpm
Your forth gear is a 1:1 ratio, third is 1.401:1. So, your engine RPM
at 83.05mph should be 3410 in 4th and 4778 in 3rd.
Based on that, it sounds like you should be able to leave it in
third. (The best place to shift is generally about 5,000
anyway.) However, keep in mind that the mph given at the track is actually
an average over 60 feet, so when you cross the finish line, you are
actually moving faster than 83.05mph. (The "average" method of determining
velocity assumes a steady speed, but you are still accelerating. Also,
differences in tire pressure and just plain slop and rounding errors could
throw off the actual RPMs as well. So... I would say its worth a try to
leave it in 3rd, but keep an eye on the tach, and if be prepared to lift if
it gets too high...
-Jon-
.--- Jon Steiger ---- jon@dakota-truck.net or stei0302@cs.fredonia.edu ---.
| Affiliations: AOPA, DoD, EAA, NMA, NRA, SPA, USUA; Rec & UL Pilot - SEL |
| '92 Ram 150 4x4 V8, '96 Dakota V8, '96 Intruder 1400, '96 FireFly 447 |
`------------------------------ http://www.cs.fredonia.edu/~stei0302/ ----'
This archive was generated by hypermail 2b29 : Fri Jun 20 2003 - 11:53:45 EDT