Here's How I calculated my speed.
1) check the tire circumference using my wife's sewing tape measure, calculate
how many rotations would be required per mile.
2) with a 3.9 rear end figure the drive shaft will turn 3.9 times for each tire
rotation them assume a forth gear ratio of 1:1, - oh wait, I did just install a
210 5-speed (sh#$ !).
3) Throw out all previous calculations
4) drive beside my wife on the freeway, and have her tell me how fast she's
going! :-)
OK - Here's what I got in 5th gear with 165/65-13 tires, and a 3.9 rear end:
RPM MPH
3400 ~ 60
3650 ~ 65
3900 ~ 70
- Bryan (59-bugeye 1275)
Sna Jose, CA
>X-Sender: grbyrns@mailbox.ucdavis.edu
>Mime-Version: 1.0
>To: spridgets@autox.team.net
>Subject: calculated speed
>
>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
>
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