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Poll.. What RPM = What Speed in What Gear? And what tire?
Posted: Mon Dec 13, 2004 5:05 am
by douglas vincent
The faster the speed, the greater the discrepancy in the speedo. But the rpm range should stay near the same for the actual speed (or so I think). And tires are related to this of course.
So....
Go to the freeway.....
Put it in fourth and go 3000 rpm, 4000 rpm and 5000 rpm. record what the speedo reads at these marks.
Shift to 5th and go to 3000 rpm, 4000 rpm and if you are gusty and stupid enough, 5000 rpm. Record what the speedo reads at these marks.
Report back here.
Include your tire size.
I suspect that we are going to see some big differences!
Posted: Mon Dec 13, 2004 7:41 am
by douglas vincent
Bridgestone RE92s 205/55/16's
5th gear
3000rpm = 73 ish
4000rpm = 97.5 ish
5000rpm = 120 mph
4th gear
2000rpm = 35 ish
3000rpm = 55 ish
4000rpm = 72 ish
Posted: Mon Dec 13, 2004 6:03 pm
by THAWA
MPH = (((RPM * Tire Diameter)/2)/168)/(Gear Ratio * Final Drive Ratio)
Tire Diameter = (Cross Section * Aspect Ratio *2)/25.4 + Wheel Diameter
For example, using your trans and 205/55-16
Diam = (205*.55*2)/25.4 + 16
Diam = 8.878 + 16
Diam = 24.878
4th gear = .972
Final Drive = 4.111
MPH = (((3000*24.878)/2)/168)/(.972*4.111)
MPH = 222.125/3.996
MPH = 55.59
or thereabouts
Posted: Mon Dec 13, 2004 8:13 pm
by douglas vincent
Arrrrgh. It just doesnt make sense. In the context that my speedo was running faster with the smaller diameter tires than when running the larger diameter tires, but the smaller diameter tires would have me running slower at the same rpm compared to the larger tires in reality. Or is this correct and just the way the speedo reads out? I'm lost here cause when I was running 185/70/14 tires, my speed was almost 10% off. I would get a trap speed of about 80 mph with these tires, but speedo read close to 90 mph. With the 205/55/16 tires, the speedo and the trap speed were withen 2 mph of each other. Thats a big difference and one I don't understand.
Posted: Wed Dec 15, 2004 3:52 am
by jamal
That's how it's supposed to be. A smaller tire has to turn more times to go the same distance, so the indicated speed will be higher that what you're actually going.
With 185/70 14s, in my 4eat with a 3.9 rear diff, 3000rpm = indicated 86mph.
With the 205/55 16s I have now, it's the same, but now I'm going faster than that.
