Here's how I figured my theoretical speed at various rpm's:
Mark a spot on one rear tire and roll the car so the mark touches the
pavement. Mark the pavement at this point. Roll the car forward with you in
it (for highest accuracy) until the mark on the tire is again on the ground.
Mark this spot and measure between them. This is the distance the car moves
forward for each rotation of the axle, call it "C". In my case, C=69 5/8" =
69.625".
With a 4.2 rear end, the drive shaft will turn 4.2 times to turn the rear
axle one revolution. Assuming a fourth gear ratio of 1:1, then at 6K rpm,
6000/4.2 = turns of rear tire per minute = 1428.72. Multiply by 60 to get
turns per hour, = 60 x 1428.720 = 85732.2
This number times the number of inches forward for each turn you figured out
earlier(C)tells you how many inches forward for each hour at 6k rpm in
fourth. Divide by the number of inches in a mile(63360) and you should get
miles per hour.
{(rpm/rear end ratio)x 60 x C }/ 63360
So for a 4.2 rear end, 69.625 inches per turn of tire, fourth gear:
(6000/4.2) x 60 x 69.625 / 63360 = 94.18 mph, at 5k rpm = 78.49mph
I'm sure someone with a better memory of math will correct me, but luckily I
play with DNA for a living and don't do gobs of math.
> I havn't seen anybody talk about tire size which can have a very
>difinite
>effect on speedo error and correct top speed. Look at any tire chart and
>you will see a big difference in tire circumference with the tires sizes
>very close together if not the same size. Don't forget to take this into
>consideration. Does anyone one out there have the formula for determining
>top speed using tire circumference, axel ratios, and rpm's?
>
>
>BOB
>65 Midget vintage racer
>________________________________________________________________________
>
>
Regards,
Glen Byrns
'59 Bugeye
|