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
>
>
>
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