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Registered
Join Date: Apr 2004
Location: Bend Oregon
Posts: 227
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pls check my math
I was wondering if my speedo on my 74 911 (NOT AN S) was accurate cause the tires are so different from the stock ones. 195.65.15 vs 225.50.15
I have not owned a 74 n 12 yrs and do not recall 75mph being at 3600 rpms. But then I may be comparing it to my last SC which cruised 75 at 3200rpms. Combined section height of the stock 1974 911 tire ---a 195.65 is 253 mm. Combined section height of my current 225.50 tires is 258mm. that is less than 5mm from top of wheel to bottom of wheel, which amounts to a speedo discrepancy of aboiut 3% or not worth worrying about. I know the stock 911 only had 140hp which is why the gearing is so aggressive but I would have thought fifth gear would be more like an overdrive. Appreciate comments. Oh, and the reason I don't like the Rs is cause the six webers suck big time---gas that is ---at that rev and at $3.40 gal where I live or $40 every 200 miles is a lot of money for the fun.
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"zumwoll" German for "to well being"---what happens when you drive a Porsche! |
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06-10-2006, 09:22 PM
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Registered
Join Date: Dec 1999
Location: TX USA
Posts: 9,804
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The stock tires for a '74 base 911 were 185/70-15, which were 25.5" tall, and about 815 revs/mile.
The 225/50-15 tires are only 23.9" tall, and 870 revs/mile. You may want to recheck your math ...
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Warren Hall, Jr. 1973 911S Targa ... 'Annie' 1968 340S Barracuda ... 'Rolling Thunder' |
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06-10-2006, 09:58 PM
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Tire Math 101:
195 65 15 Width = 195 mm = 7.72 inches aspect ratio = 65% of the width 15 inch wheel So the height of the tire in inches is 2*7.72*0.65 + 15 = 25.03 inches The circuference of the tire is PI* diameter or hieght = 78.64 inches The 225 50 15 tire has a circuference of 74.95 inches This is an error of 4.7%
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Rex 1975 911s and 2012 Range Rover Sport HSE 1995 BMW R1100RS, 1948 Harley FL |
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06-10-2006, 10:07 PM
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Registered
Join Date: Jun 2004
Location: Rio Rancho, New Mexico
Posts: 1,325
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zumwoll,
With Warrens info, The 225/50-15 tires are only 23.9" tall, and 870 revs/mile. A '74 915 5 speed transaxle used a 7:31 pinion and ring gear, with a 29:21 fifth gear. 31/7=4.42857 final drive ratio. 21/29=.724133 fifth gear ratio. 4.42857 X .724133= 3.2068 final in fifth gear. At 3600 rpm/ 3.2068 = 1122.5 output rpm at wheel. 1122.5/870 revs per mile= 1.29 1.29 x 60 = 77.42 mph @ 3600 motor rpm. 3600/77.42= 46.5 rpm constant. 65 mph x 46.5 = 3022 engine rpm. 70 mph x 46.5 = 3255 engine rpm. I can't resist these numbers problems, edit: I've got a problem with 870 revs per mile for a 23.9 inch tire. I get 843.8 revs per mile using pi x 23.9. If 843.8 is true then at 3600 you are doing 79.89 mph. Help Warren, why the discrepancy?
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DOUG '76 911S 2.7, webers, solex cams, JE pistons, '74 exhaust, 23 & 28 torsion bars, 930 calipers & rotors, Hoosiers on 8's & 9's. '85 911 Carrera, stock, just painted, Orient Red Last edited by 2.7RACER; 06-10-2006 at 11:17 PM.. |
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06-10-2006, 11:00 PM
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Registered Usurper
Join Date: Sep 2005
Posts: 13,824
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zumwoll, et al,
Check out this cool calculator! Just delete the 993 Turbo no.s in the boxes, plug your car's no.s in, move your mouse arrow up & down over the graph and, VOILA - every speed/rev combo is dynamically displayed in a little box as you move about! http://www.geocities.com/z_design_studio/transmission_993_bit.html
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'82 SC RoW coupe |
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06-10-2006, 11:23 PM
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1975 Porsche 911s
