speedometer is off, effect on fuel econ?

[California98Civic]

Originally Posted by Old Tele man

...the MPH equation for tires:

MPH = [ 60 / (G × A) ]×[ RPM / rpm ]

...where:

MPH = vehicle speed, miles-per-hour
RPM = engine speed, revolutions-per-minute
rpm = tire speed, revolutions-per-mile
60 = conversion constant, minutes-per-hour
G = gear ratio (typically highest gear)
A = axle ratio

So the VSS is the source of the data for both and the ECU is doing the calculation you describe. What is "axle ratio"? Same as final driven gear?

[gone-ot]

...yes, axle ratio is same as final driven gear.

[California98Civic]

So then if I am at 2000 RPM and tire rack says my 13" Michelin Harmonys have 923 revolutions per mile and my fifth gear ratio is .702 and my final drive is 4.058, I should see the speedo at about 45.7 MPH (rounded). But the tire size calculator Ford Man posted says my tires should only turn 891 times per mile (not the 923 tirerack claims). And I have over inflated the tires anyway and therefore have probably reduced revs per mile somewhat. So I still am curious enough to measure as convenient. This formula will be really useful though, thanks!

[gone-ot]

FWIW - tire companies "measure" their published tire revolutions-per-mile (rpm) at 45 mph speed, with the tires specified "load" rating applied.

EPA does their N/V measurements at 50 mph.

[Ford Man]

Originally Posted by California98Civic

Thanks Ford Man, but the calculator is not quite precise enough, since inflation pressure can change the diameter a little and I run my tires pumped-up high. And anyway, the real question I am asking is about the disparity between the 8% or 9% speed increase and the 1.9% distance increase that my measurements so far suggest. Does the ECU is process data from the VSS in two different ways? Or does the VSS not supply the data for distance? I didn't find the answer in the service manual. Thanks in advance... james

From cars I now own or that I've owned in the past it seems that maybe the manufacturer factors in a small buffer on the speedometer, maybe this is done intentionally to keep from being tied up in law suits because of speedometers that are reading too low, therefore causing people to get ticketed for speeding. As far as I know all modern cars speedometer/odometer's are calibrated through the VSS.

[California98Civic]

Reviving a somewhat old thread: my speedometer is now almost exactly accurate, according to roadside radar readouts, but my odo is about 5% pessimistic, according to comparisons along specific routes plotted using GPS. The reason? Not totally sure, since the fifth generation CX transmission I just installed should have the same speedo gear as my sixth generation DX and since the VSS sensors are supposedly the same for both trannys. I am running 14" wheels with a no-name generic tires. Perhaps the speedo gear and VSS were designed with the fifth gen tranny in mind, with 13" wheels. And therefore, perhaps, all sixth gen, non-Si Civics actually under count distance quite significantly. Or perhaps Honda made a tire selection that closed the gap somewhat between the 175/70-R13s and the 185/65-R14s.

[Whitey]

I think our Hondas' gauges generally over-report speed (not undercount distance).

My 1998 Civic LX speedometer reads about 5% too fast while the odometer is correct. My ScanGaugeII reads the correct speed, so I think that means the VSS is correct. Radar confirms what ScanGaugeII says, and I've analyzed many miles on the turnpike. It really is the Honda speedometer that's off (in my case, anyway).

Perhaps your 14" wheels (instead of the expected 13" wheels) have made up for the error in the speedometer at the expense of the odometer's accuracy? Of course, if your gauges were accurate before the transmission swap, something else is happening.

-Whitey

[California98Civic]

Originally Posted by Whitey

I think our Hondas' gauges generally over-report speed (not undercount distance).

My 1998 Civic LX speedometer reads about 5% too fast while the odometer is correct. My ScanGaugeII reads the correct speed, so I think that means the VSS is correct. Radar confirms what ScanGaugeII says, and I've analyzed many miles on the turnpike. It really is the Honda speedometer that's off (in my case, anyway).

Perhaps your 14" wheels (instead of the expected 13" wheels) have made up for the error in the speedometer at the expense of the odometer's accuracy? Of course, if your gauges were accurate before the transmission swap, something else is happening.

-Whitey

I read and tested further (and better) and created a new thread on this: http://ecomodder.com/forum/showthrea...ion-25435.html

Take a look. A question would be what tires you are using.