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.

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 ...

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%

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?

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