Top speed run. Can this be right?

[jonnyk]

Originally posted by smithbling:


exgmguy

5400 RPM in is what you reached but your speedo only reads somewhere lower. Go out and do 1800RPM in 5th. Record the speed. Say 50MPH. You can simply triple the speed and RPM to get 5400RPM and 150MPH. This works only if you remain in the same gear and it also works because there is a linear relationship between speed and RPM in a given gear. This method is FAAAARRRRRRRR more accurate than the calculators.

The calculators are numerically based. Your speedo is not always accurate especially if you have changed gear or tire heights. Therefore, I would say the calculators are FAAAARRRRRRRR more accurate than "guesstimating" using a crappy stock speedo setup. If that 5400 rpm reading came from an aftermarket tach, then 5400 in 5th gear (0.68) with 3.73 gears and a 28" tire = 177.32 mph. I have created a nice little spreadsheet that calculates this.
The formula I used is:
RPM / (rear gear * tranny gear)*60*(tire circumference in inches) / 5280 / 12


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'91 LX Hatch, 17 ROH ZR6's, lowered, 3.73's, offroad H pipe, SN Cobra cat-back, ASP pullies, 14° timing, C&L 76mm MAF, 170A 3G, B&M Ripper on a T5
Best ET: 14.593 @ 94.62 2.292 60'
1991 LX Hatch 5.0L