NewToVibe wrote: ↑Fri Apr 05, 2019 7:31 am. For example going 80 mph the car is revving at 3.5 rpm in sixth gear. I asked a mechanic about this and he said that since the tires are so small it makes perfect sense... I just feel it makes sense for a five speed not a six??? Does anybody have thoughts on that?
tldr summary:
80 mph at 3500 is about right for a GT. see below if not avoiding math
I think the # of gears and gear ratios in the 6th speed were designed to allow upshifting near redline and staying in lift
In a manual transmission car with no clutch slipping, or an auto with the torque converter locked up there is a direct mechanical connection between the engine and the tires. The speed the tires spin depends on the engine RPMs, the overall gear ratio and the tire diameter.
The relationship between RPMs and miles per hour depends on transmission, gear selected, and tire size.
For the 1st gen Vibes (03-08) there were 4 transmission options: Base 5 speed manual, GT 6 speed manual, base 4 speed auto, AWD four speed auto. The gear ratios for these are
Manual 6 speed
1st : 3.17
2nd : 2.05
3rd : 1.48
4th : 1.17
5th : 0.92 (overdrive)
6th : 0.73 (overdrive)
Reverse : 3.25
Final Drive Ratio: 4.53
Manual 5 Speed:
1st : 3.17
2nd : 1.90
3rd : 1.31
4th : 0.89 (overdrive)
5th : 0.73 (overdrive)
Reverse : 3.25
Final Drive Ratio: 3.94
Auto (FWD):
1st : 4.01
2nd : 2.21
3rd : 1.43
4th : 0.98 (overdrive)
Reverse : 3.27
Final Drive Ratio: 2.96
Auto (AWD)1st : 2.85
2nd : 1.5
53rd : 1.00
4th : 0.70 (overdrive)
Reverse : 2.34
Final Drive Ratio: 4.24
In this case, the gear ratio means the ratio of speed between the input and output. A gear ratio of 2 means the input spins twice as fast as the output. After the selected gear ratio, the transaxle has a final drive ratio that works the same way. If the gear ratio was 2, and the final drive ratio was 3, the engine will spin 6 times faster than the axle.
After that, you need to factor in the tire circumference (or pi * tire diameter), and throw in a unit conversions of 60 minutes/hour and 1 mile/63,360 inches to get from engine RPMs to vehicle MPH
from a calculator at
https://tiresize.com/calculator/ the standard tire diameters are
205/55R16 - 24.9"
215/50R17 - 25.5"
putting this all together using the most common 205/55R16 tires
Engine RPM / gear ratio /final drive ratio * wheel diameter in inches * pi *60 min/hr *(1mile/63,360 inch) = miles per hour
To simplfy this
RPM /gear/final drive * (24.9*3.1416*60/63,360)
or RPM/gear/final * 0.07408
using the most common 205/55R16 tires
With all that, a GT at 3500 RPM in 6th gear should be moving at 3500/0.73/4.53*0.07408 or 78.4 mph, close enough to the 80 you observed
A side note, since the early 5 speeds had reliability issues, some have swapped in the 6 speed, but given the difference in final drive ratios,
for the same speeds in top gear, the 6 speed car will be running about 15% higher rpm than the 5 speed (4.53/3.94)