Les,
a 145-13 is 566mm in diameter, the 185/70-13 is 598mm
Circumference is pi*diameter, so I can just divide 566/598=0.947
So you turn 95% as many rpm as with stock tires.
Going from 4.22 to 3.9 is 3.9/4.22=.924 or 92.4%
Now to get the rpm.
17.3mph/1000rpm for a 3.7 with 145s
1000/17.3 = 57.8 rpm/mph
80mph * 57.8 = 4624rpm in a '79 Midget
In your car:
4624 * (4.22/3.7) * .924 = 4873rpm
In my car: 3792rpm.
I'll try my top speed after I rebuild the engine :-)
I think I'll have to use 4th to get there though...
Ulix
On Sun, 8 Feb 1998, Les Myer wrote:
> So how many RPMs am I turning with a 4.22 and 185/70R-13 tires during an
> 80mph commute to work on I-94 (tach is off (reads 6500), but have
> calculated MPH by the mile markers)? Really would like to know. You gotta
> do 80 here in Michigan (70mph speed limit) to keep from getting run over by
> the 18-wheelers going from Chicago to Detroit and vice versa. I cringed
> every time until I put the bigger tires on and I think the bigger tires
> look a lot better than the little skinnies I had before. I think they are a
> viable option to swapping diffs. Plus, it softened the ride immensely.
> The only things I don't like about them is the wheel hop I get when dumping
> the clutch at RPM and I don't think it handles quite as well in the 30 mph
> range.
>
> BTW, 80 is not the top end - have had PUDDLES briefly over 100mph with
> these tires - kind of weird, the exhaust note seems to get much quieter at
> very high rpm. I try to keep the cruising speed at 70-75 mph for
> long-distance driving.
>
> Les
>
> At 10:13 AM 2/8/98 -0800, you wrote:
> >I swapped out the 4.22 for a 3.7 rear end in my car, which is equipped
> >with the 5-speed. Wow! It has transformed the character of the car on
> >the highway. WIth the ribcase and 4.22, I felt completely out of place on
> >the highway, the car felt like it was doing something it wasn't designed
> >to do. I used to drive ~60mph. The combination of 3.7 and 5-speed lowers
> >my rpm to 72% of original. At 4000 rpm, I go the same speed that used to
> >take 5500rpm, which according to me calculations should be 84mph. 74mph
> >at 3500rpm. I ride the fast lane unstressed. (All numbers based on
> >theory, not the inaccurate instruments of my car. rpm to mph conversion
> >based on Haynes manual that says 17.3 mph/1000rpm in 1500 with 3.7 rear).
> >
> >Of course there are tradeoffs. Inclines on the highway will get
> >me downshifting early, since the motor has less power at 3500 than at 4000
> >which is what I used to see at cruise.
> >The long rear end makes for a long first gear which makes for harder
> >takeoff from a stop. First gear on the Datsun trans is shorter than the
> >one of the ribcase, which helps to offset this some. This shorter first,
> >coupled with the 4.22 resulted in a very short first gear that is nice on
> >steep hills, but is also too low to make any significant speed in the
> >process on accelerating from a stoplight. I didn't notice this until now.
> >Now, after taking off, there is rush of forward motion in 1st, that wasn't
> >there. This is hard to explain, let me try again...if you take off on a
> >mountain bike in the lowest gear it has, yes - it is very easy to take
> >off, but this low gear doesn't get you to any useful speed before you have
> >to upshift...
> >
> >I have yet to try taking off on an EXTREMELY steep hill in my
> >neighborhood to test the difference in that respect.
> >
> >If you do a lot of highway cruising, I would recommend this combo (maybe
> >not with a 948).
> >
> > Ulix __/__,__
> >.......................................................... (_o____o_)....
> > '67 Sprite
> >
> >
> >
>
Ulix __/__,__
.......................................................... (_o____o_)....
'67 Sprite
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