[Roadsters] Speed, gearing and horsepower

[Gordon Glasgow]

Changing the gearing won't necessarily give you a higher top speed. To get
absolute maximum speed, you need to gear it so that peak HP occurs at that
point where wind resistance + rolling resistance exactly equals peak HP. A
lot of cars now are geared WAY beyond that in top gear.

As an extreme example, My 2002 Maxima 6-speed turns 32 mph/1000 rpm in 6th.
With a 6500 rpm redline, this would be 208 mph! I don't think I'm going to
get there with 255 hp. If I remember right, 5th is 25 mph/1000 rpm, which
would be "only" 162 mph. So my guess is that I'd probably achieve the real
top speed in 4th gear.

Now, another thing to keep in mind is that the power you have to accelerate
is the difference between the power the engine is putting out at a given rpm
and the total resistance at that speed. If you do gear the car for absolute
maximum speed, you won't have as much power available for acceleration
(think about the shape of the curves for HP and drag). So it's definitely a
tradeoff.

I think the original roadster tires were about 24" diameter. Using that and
the 3.70 gearing of the 2000, the 6000 rpm power peak occurs at 139.5 mph.
That's probably past the drag curve. It might get up over 120 but would have
a hard time accelerating from there. 5500 rpm would get you about 124.5 mph.

However, 4th gear at 6500 would be 125.4 mph, which is probably achievable
(especially if you leave the top up - convertibles have bad drag
coefficients).

A couple of final points to keep in mind:

 - Aerodynamic resistance increases as the SQUARE of the speed. So there is
four times as much aero drag at 120 as at 60.

 - The tires constitute a part of the overall gearing, too. Change the
diameter, you've changed the gearing. It's all about the overall
relationship between engine rpm and vehicle speed.

Gordon Glasgow
Renton, WA

-----Original Message-----
From: datsun-roadsters-bounces+gsglasgow=comcast.net at autox.team.net
[mailto:datsun-roadsters-bounces+gsglasgow=comcast.net at autox.team.net]On
Behalf Of Matthew A. Smith

One would think that if you geared the rear end higher, you'd get a little
higher top speed. Also, if you started adding things like a header and
aftermarket air cleaners for better flow, hotter spark with better plugs,
wires, and a 12V EI (i.e. a Boone), better tires and wheels. Where is "top
speed now?