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Discussion Starter · #1 ·
Okay, I have seen many posts on here about people who say.."I went 145 in my stock IS" (like I have in the past)....then someone will respond "BS you couldn't have, the car's mechanical limits with the rev limiter is 143.7".

There is actually a simple explanation for this. The truth is that both parties can be right (in a way). I was on vacation and driving with a GPS device hooked up. I noticed that consistantly the GPS device reported my speed 3 mph less then what my speedo said. Also I remember a friend of mine's uncle (who was a state highway patrol officer) told him that almost all imports have the speedo overstating the car's actual speed. I am working under the assumption that my car on stock 17" wheels is a good sample of all stock IS, in that they all have the speedo overstating the car's actual speed approx. 3 mph. Thus, someone could go 145 (on the speedo) without running out of RPMs. This is because if the speedo says 145, the person is actually going 142 (well within the mechanical limits of the car). Further we can say that the max the speedo can get to before the rev limiter cuts in is 143.7+3= 146.7 ~ 147 mph.

Just thought I would try to clear up some of the confusion!
 

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My 03 IS300 overstates the speed by 2 MPH.
I have a GPS installed and I monitor it frequently.
The same "overstatement" applies to most vehicles on the road.
I read Car-and-Driver review on it and almost all of the cars they tested overstated the actual speed. It turned out that the sports cars oversted it the most. I think the only one that actually understated the speed was a super compact car.
 

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Yes Gosha you are right... Toyota/Lexus did have the lowest percentage of error. I believe it was in a Road & Track mag a couple months back, If I remember correctly Mercedes was right behind Toyota/Lexus but BMW and Jaguar were near the bottom.
 

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pmergler said:
Okay, I have seen many posts on here about people who say.."I went 145 in my stock IS" (like I have in the past)....then someone will respond "BS you couldn't have, the car's mechanical limits with the rev limiter is 143.7".

There is actually a simple explanation for this. The truth is that both parties can be right (in a way). I was on vacation and driving with a GPS device hooked up. I noticed that consistantly the GPS device reported my speed 3 mph less then what my speedo said. Also I remember a friend of mine's uncle (who was a state highway patrol officer) told him that almost all imports have the speedo overstating the car's actual speed. I am working under the assumption that my car on stock 17" wheels is a good sample of all stock IS, in that they all have the speedo overstating the car's actual speed approx. 3 mph. Thus, someone could go 145 (on the speedo) without running out of RPMs. This is because if the speedo says 145, the person is actually going 142 (well within the mechanical limits of the car). Further we can say that the max the speedo can get to before the rev limiter cuts in is 143.7+3= 146.7 ~ 147 mph.

Just thought I would try to clear up some of the confusion!
Good input and you are on the right track, your formula may not be quite correct.

Usually the speedos have a percent error. What this means is that if you speedo is off by 5% (a common error percentage) then your absolute speed at 70 MPH will be off by 70 x 0.05, or 3.5 MPH. At twice the speed, 140 MPH however, with the same 5% error, your speedometer will now be off by 7 MPH, twice the absolute error that it had at 70 MPH. Bottom line is that the absolute error may be dependent on your speed at the time you are looking at the gauge.

If the error were a simple 3 MPH offset, your speedo would show 3 MPH when you were stopped at a light!
 

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Discussion Starter · #8 ·
wjbertrand said:
pmergler said:
Okay, I have seen many posts on here about people who say.."I went 145 in my stock IS" (like I have in the past)....then someone will respond "BS you couldn't have, the car's mechanical limits with the rev limiter is 143.7".

There is actually a simple explanation for this. The truth is that both parties can be right (in a way). I was on vacation and driving with a GPS device hooked up. I noticed that consistantly the GPS device reported my speed 3 mph less then what my speedo said. Also I remember a friend of mine's uncle (who was a state highway patrol officer) told him that almost all imports have the speedo overstating the car's actual speed. I am working under the assumption that my car on stock 17" wheels is a good sample of all stock IS, in that they all have the speedo overstating the car's actual speed approx. 3 mph. Thus, someone could go 145 (on the speedo) without running out of RPMs. This is because if the speedo says 145, the person is actually going 142 (well within the mechanical limits of the car). Further we can say that the max the speedo can get to before the rev limiter cuts in is 143.7+3= 146.7 ~ 147 mph.

Just thought I would try to clear up some of the confusion!
Good input and you are on the right track, your formula may not be quite correct.

Usually the speedos have a percent error. What this means is that if you speedo is off by 5% (a common error percentage) then your absolute speed at 70 MPH will be off by 70 x 0.05, or 3.5 MPH. At twice the speed, 140 MPH however, with the same 5% error, your speedometer will now be off by 7 MPH, twice the absolute error that it had at 70 MPH. Bottom line is that the absolute error may be dependent on your speed at the time you are looking at the gauge.

If the error were a simple 3 MPH offset, your speedo would show 3 MPH when you were stopped at a light!
Good call dude. I didn't think about that. So did that article say what error % our IS has? I recall that I was roughly 3 mph off at an average cruising speed of 85. = 3.5% but that is going from memory (and not a good one at that). It would be cool to know the actual percentage error on our stock IS. (Then there would be variations based on wheel size of course)
 
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