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Top Speed, Speedo & Tach Accuracy & Power

By Rich Ying


Here are some findings in regards to top speed, speedometer error,   tachometer error and wheel horsepower required to achieve speed. From my own investigations, I found that BOTH my speedometer and tachometer were off. I checked the tachometer with a Fluke 85 DVM in the frequency mode utilizing a clamp on inductive pick up and here are the results:

Indicated        Actual
2000                1950
3000                2850
4000                3840
5000                4800
6000                5700
7000                6630

I suspect that probably other if not all SE-R tachometers would be optimistic in their readings also. If I were the manufacture, I would have left it this way to prevent over exuberant revers from having a few valves kiss or worse - blowing up their engines.

The speedometer error varies with what tires that are mounted in front. Also, the error is not linear - that is a 2 mph error at an indicated 50 is not a 4 mph error at an indicated 100. With the original Dunlop D-87 (185-60) tires I was observing that the speedometer would read about 3 mph high at an indicated 60 mph and about 10 or 12 mph high at an indicated 120 mph (actual speed closer to 110). Currently, I am running 195-60/14 AVS-I and the errors observed are less. Basically, the error goes up geometrically. I am not sure why. This is also observable for other cars when observed at the extreme. What I used to verify the speedometer is an electronic speedometer based on a crystal controlled clock and a bunch of CMOS logic and counters. I used a Hall effect sensor and a single rare earth magnet mounted on the inside of the rear wheel to monitor the time for one wheel rotation. Accuracy is about +/- 0.2 mph. I know the electronics on this is accurate. Actual top speed measured with the car stock is 124.5 mph with no wind and ambient temperature at about 40 F. The speed listed is the least number. There is one phenomenon called tire growth (centripetal forces causing the tire to increase in diameter) which may add maybe 1 mph. Tire growth was a lot worse with the older style bias ply and bias belted tires. If any of you know a tire engineer, you might want ask how much is this a factor. My speedometer was reading 135+ at the time (I was mostly paying attention to the electronic one and the road).

My classic has a mechanical speedometer and what is observed here may not apply to electronic speedometers. I would expect the later electronic speedometers to be more linear but not necessarily more accurate (unless the manufacturer wanted it to be). I have since added a Stillen intake, exhaust and JWT ECU, but have not checked the top speed since these modifications were installed.

Here is the relationship between wheel hp and 1/4 mile speed I have gathered from experience with my classic.

                        Wheel HP             Mi Speed
STOCK                      119                     87 
+Intake & Exhaust          124                     88.2 
+ ECU                      129                     89.4 
+ Stillen Header           137                     91.2 
or HS Header               141                     92 

The 1/4 mi. speed was based on again a SE-R classic with a curb weight of 2600 lbs. and a 150 lb. driver. I have verified the 1/4 mi. speed with the car in stock, +intake & exhaust, + ECU but not with the header. I have the header but need to install. In regards to 1/4 mi. speed and times, you must note if there is a headwind or a tailwind. A 5 mph head wind can knock off 1 mph on your trap speed. To give you an example of how wind affects trap speed, my car will do 15.5 @ 89.3 mph +/- 0.3 mph with no wind and at an ambient temp of 60 F. This has been repeated a few times at 3 different drag strips. With a 10 mph tail wind, I have turned a 15.38 @ 91.30 mph. Going from 89.3 mph to 91.3 mph would indicate that a 7% increase in horsepower had occurred. In reality nothing changed - just the wind!

The following is the amount of wheel horsepower required to reach a top speed. The top speed was calculated from coast down measurements and some curve fitting software. You need to keep in mind that this half empirical and half theory. To reach top speed, your gearing must be such that when maximum horsepower is obtained, the car needs to be at that speed. For example, many of you have reported maximum power on the dyno's at around 6500 rpm. With 195-60/14 AVS-I's, this would work out to be 138 mph - very close to achievable with those engines that are putting out 140+ wheel hp.

Top Speed           Wheel HP Required
        Classic SE-R (Cd .35)  NX2000 (Cd .31) 
125                119             105 
130                131             119 
135                143             132 
140                157             145 
145                171             160 
150                188             175 

The above data applies to a Classic SE-R which has a drag coefficient of  0.35 or that was listed in the 1991 sales brochure. I suspect that the 200SX has a lower drag coefficient but not as good as the NX2000 (Cd = 0.31). Since several of the SE-R's have been dyno'd in the high 130 hp to the low 140 hp range, top speed of 135 real mph for the classic is very possible. Above that, we are talking about turbos, NOS or superchargers.

Car & Driver in the 91 Road Test Annual indicated that the SE-R did 125 and the NX2000 did something similar. In a later test, the NX2000 did 134 I believe. Unfortunately, none of the American car mags give all the conditions of the test like the old British publication Motor. Motor Trend reported that their SE-R did 131. I read the article carefully and they mentioned high winds which may have contributed to what I call a higher than normal top speed. So looking at what kind of horsepower is needed to go 150 mph is presently beyond what a NA, non-NOS SR20DE can produce.

I hope this discussion was a value to the list.