Hi guys:
For what it's worth, I was rooting through an old Road and Track
technical correspondence page and found the following:
MPH= RPM x C/ G x F x 88.
F= final drive ratio
G= gear ratio
C= driven tire's circumference, in Feet.
The guys who rebuilt my tach used the tach to calibrate the speedo and
THEY say that with the torque converter in lockup mode at 65 mph I'm
turning about 3400 r.p.m.
Using the formula above and given that I'm SUPPOSED to have a 4th gear
overdrive (without lockup) of 0.75, I OUGHT to be turning 3100 r.p.m. at
65 (I guess a 300 r.p.m discrepancy isn't too bad--it's an LBC after
all). Which either means that my tach isn't accurate (likely), my speedo
isn't accurate (probably) or the published data on my automatic
transmission is wrong (not likely). I think I need to start by
re-ckecking my tach.
Anyway, hope the formula helps.
Regards
David Sosna
65 S4 GT V6
P.S. Rob, using the above formula: at 60 mph, I get 3523 R.P.M. for your
car. Looks close enough for me to what you came up with.
P.P.S. Your 70" circumference corresponds to my 155-80-13 tires. If I
upsize to a 175-80-13 I get a circumference of 6.23 feet and at 60 mph
with a 1:1 top gear and 3.89 axle the formula gives me 3296 r.p.m. Call
it 3300. So the engine turns about 200 r.p.m. slower with the larger
diameter tires. And the car accelerates slower. Guess everything is a
tradeoff. Good luck!
robert nanzig wrote:
>
> Scott, Roughly the first 11,000 SV's were equipped ( rather
> ill-equipped ) with the 4.22 rear end , overdrive or not. I changed
> mine to a 3.89 this past winter and it helped. It's easier to swap the
> whole differential than to change the ring and pinion. Once you pull the
> axles you can simply unbolt , remove the old and insert the new. No need
> for shims and guages. You can find a used one for $65. I did some
> testing last week with stop watch,mile markers,and hand held tach and
> determined that I needed 4100 rpms to get 60mph. According to the shop
> manual I should be getting 17.6mph per 1000 rpm. That would have me
> traveling 71 mph , not 60. I don't get it. Could my tire size make
> that much difference?
> Okay, the rest of you get out your calculators. Tell me if this
> makes sense. 1rpm on the engine = 1 turn of the drive shaft in 4th gear
> ( no OD ) . So 3.89 turns of the drive shaft = 1 turn of the tire. If my
> tire is 70" around and there are 63360 inches in a mile then it takes my
> tire 905 revolutions to travel 1 mile. Multiply by 3.89 and get 3521
> drive shaft and engine rpms per mile. Travel this distance in 1 minute
> and you get 60 mph at 3521 rpms. Are these correct assumptions? Then
> why does it take me 4100 rpms to get 60mph unless both my tachs,my watch
> or the mile markers are reading incorrectly?
>
> Rob Nanzig
> 67SV
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