Speedometer Error

[Carguy!]

When returning home from NSXPO and talking on the radio I realized that my speedometer was reading high. After timing between milemarkers I noticed that my '00 NSX-T was reading about 8% high. I have the stock tires and rims and the tire pressure was checked the day before the trip.

I guess my question is has anyone else experienced this? Is there any type of adjustment? I want to get this taken care of pretty soon as my mileage is also going to be off by the same percentage.

Thanks in advance!!!

 

[19inchNSX]

Could be wrong, but maybe its because the wear from your tires. The radius of the tire probably got reduced from the trip there and if you did any track driving. If the radius is reduced, it might cause the car to read at a higher speed.

Originally posted by Carguy!:
When returning home from NSXPO and talking on the radio I realized that my speedometer was reading high. After timing between milemarkers I noticed that my '00 NSX-T was reading about 8% high. I have the stock tires and rims and the tire pressure was checked the day before the trip.

I guess my question is has anyone else experienced this? Is there any type of adjustment? I want to get this taken care of pretty soon as my mileage is also going to be off by the same percentage.

Thanks in advance!!!

 

[nsxtasy]

The maximum amount of error that tire wear can account for is 2 percent.

When street tires are new, they come with 10/32" of tread depth. They need to be replaced when the wear bars show, at 2/32" tread depth. Thus the maximum difference is one quarter inch of tread depth, or half an inch difference in the (roughly) 24-inch diameter of an NSX tire. IOW, roughly 2 percent.

 

[lemansnsx]

Originally posted by Carguy!:
When returning home from NSXPO and talking on the radio I realized that my speedometer was reading high. After timing between milemarkers I noticed that my '00 NSX-T was reading about 8% high. I have the stock tires and rims and the tire pressure was checked the day before the trip.

I guess my question is has anyone else experienced this? Is there any type of adjustment? I want to get this taken care of pretty soon as my mileage is also going to be off by the same percentage.

Thanks in advance!!!

I thought that it was pretty common knowledge that many automobile speedometers are optimistic? Once in awhile you'll see the car magazines mention it - I seem to recall BMWs being mentioned several times.
Whatever, the manufacturer will certainly say that there is an acceptable range for accuracy. I've seen numbers for Porsche, for example, that put the range at 10% over to 1% under actual speed. BBC's "Top Gear" program tested 3 cars in response to a threat of a zero tolerance approach to speeders from British police and discovered that all three were inaccurate. BTW, I've also seen reference to 10% as an acceptable variance in government standards for speedometers in some countries.



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The NSX Model List Page

 

[lemansnsx]

PS: I have not verified this but the Federal standard is supposedly +/- 5%.

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The NSX Model List Page

 

[DanO]

Good morning John!

When you did you testing on the highway, did you notice whether the odometer was reading fast too? Seems if the odometer and the speedometer were in agreement, then the speed sensor would be suspect. But if the odometer and speedometer disagreed, then maybe the speedometer gauge itself could be off.

Here’s the relevant service manual pages and words:

Pg: 23-123
“Indicates 60 mdh [60 km/h] at 1.026 [637] rpm of the speed sensor”

Pg: 23-131 (Troubleshooting)
“Speedometer operates, but deflection error is great”
Check: “Printed circuit panel A”
Check: “Speed sensor is not installed correctly”

See pg: 23-133 for speed sensor test:

I’ll look at the electrical manual later on today to see if there is any additional info there.

good luck,
DanO

[This message has been edited by DanO (edited 29 October 2001).]

 

[Bryan]

I believe the federal and European standards for accuracy are asymmetrical. The speedometer can't read low, but can read high within tolerances (10%?). This leads to the annoying practice of the average speed reading in a new car being 2-5% high.

[This message has been edited by Bryan (edited 29 October 2001).]

 

[nsxtasy]

I would expect the average speed reading to be high, INTENTIONALLY. There can be severe consequences of a speedometer that reads low, but not when it reads high. So I would expect (and hope) that the manufacturers would design it to read no lower than actual speed.

 

[smoothf1]

I remember reading somewhere you can adjust your speedometer by adjusting some "pots" on the backside of the instrument cluster. (you can do the same for the tach too)

nick

 

[W]

I think that my tach may be off a little because it went up to around 8200 on two seperate occasions while accelerating onto the highway in second gear. Tonight, I bounced off the limiter (by accident) and swear that I saw 8500 on the tach. Later in the night I was able to slowly climb to 8500rpms (twice) without the rev limiter cutting in. The car was at around 84-85mph at 8500rpms in second gear. I have a '92 with 255 40 17 rear tires.

Anyone have any ideas on how to determine how correct the rpms are? I'll probably leave it as is because it makes it easier to double clutch down to second because you just look at the speedo and rev to 100 times that (ie. 50mph = 5000rpms).

 

[smoothf1]

I'm not sure how much this will help, but when I got my car inspected for registration. They held an instrument over the engine compartment that picked up the revs. Maybe an autorepair shop has something similar.

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'91 Black NSX
'87 Jumbo Jet