speedometer is off, effect on fuel econ?

[Execut1ve]

I dug out the paper from when the speedometer was calibrated by radar, here are the results. looks like I'm off by closer to 7%

[mekanic]

Originally Posted by Old Mechanic

I would still calibrate the odometer by using mile markers on the Interstate for 50 miles. Your solution is old school when they had cable drives and you could change the gear, or go into the speedo head and change the spring tension.

The odometer is a revolution counter and could be right on even when the speedometer is way off. Easiest solution would be to confirm its accuracy. Say it reads 49 miles in 50 measured by mile markers, then you know its off by 2%.

Fairly sure the OPs 2000 Honda has no speedo cable, or gear.

regards
Mech

I should have looked closer to the left side of his post.

[SentraSE-R]

My Tall tire test found the benefits of taller tires largely offset the negatives of heavier weight, less aerodynamic wheels, and wider tread. My mpg went up over 3000 miles of highway driving with new tall tires, despite those negatives. The keys to mpg benefits from taller tires are their diameter and their weight. The taller tires dropped my engine revs @60 mph down 200 rpm. Their 12 lb. extra weight per tire/wheel unsprung weight had negative mpg consequences. I later mounted 5% taller tires on the OEM wheels without the weight and aero penalties, and confirmed significant benefits even under repeated acceleration (P&G) conditions.

As everyone else noted, speedometer accuracy isn't very important. We use it to avoid speeding tickets, to estimate trip times, and to battle wind resistance.

What really matters in relation to fuel economy is odometer accuracy. and while your speedometer may be 7% optimistic (high), your odometer may be 7% optimistic, 3% optimistic, completely accurate, 5% pessimistic (low), or anywhere else in the ballpark.

My Scion xB's speedometer was 5.6% optimistic with the OEM tires, while the odometer was 3.3% pessimistic (100 miles indicated = 103.3 miles actually traveled). I went with taller tires that are speedometer accurate, but now my odometer readings are 9.9% pessimistic. I checked them on a long trip against my GPS over 60, 80, and 100 mile distances. When my odometer says I've traveled 100 miles, I've actually traveled 109.9 miles. That's a major difference in distance traveled, and therefore in fuel economy.

[Execut1ve]

you are referring to your setup where you had stock wheels/tires on one hand and taller tires/larger wheels on the other, correct? and you found that the taller tires more than made up for the extra weight of the larger rims, if I remember rightly

when stepping up to the taller/wider tires did you have any troubles with rubbing with the steering wheel at full lock? or tire hitting the inner fender after a big bump?

[SentraSE-R]

Yes, Executive. See the link in my previous post in this thread.

My 20 mm. wider, 1.3" taller tires rub the inner fender liners on full lock turns, but they don't rub on bumps. I don't remember whether my 10 mm. wider, 1.2" taller tires rub there, or not, but I don't think they did.

[user removed]

Originally Posted by SentraSE-R

My Tall tire test found the benefits of taller tires largely offset the negatives of heavier weight, less aerodynamic wheels, and wider tread. My mpg went up over 3000 miles of highway driving with new tall tires, despite those negatives. The keys to mpg benefits from taller tires are their diameter and their weight. The taller tires dropped my engine revs @60 mph down 200 rpm. Their 12 lb. extra weight per tire/wheel unsprung weight had negative mpg consequences. I later mounted 5% taller tires on the OEM wheels without the weight and aero penalties, and confirmed significant benefits even under repeated acceleration (P&G) conditions.

As everyone else noted, speedometer accuracy isn't very important. We use it to avoid speeding tickets, to estimate trip times, and to battle wind resistance.

What really matters in relation to fuel economy is odometer accuracy. and while your speedometer may be 7% optimistic (high), your odometer may be 7% optimistic, 3% optimistic, completely accurate, 5% pessimistic (low), or anywhere else in the ballpark.

My Scion xB's speedometer was 5.6% optimistic with the OEM tires, while the odometer was 3.3% pessimistic (100 miles indicated = 103.3 miles actually traveled). I went with taller tires that are speedometer accurate, but now my odometer readings are 9.9% pessimistic. I checked them on a long trip against my GPS over 60, 80, and 100 mile distances. When my odometer says I've traveled 100 miles, I've actually traveled 109.9 miles. That's a major difference in distance traveled, and therefore in fuel economy.

LOL, but it would keep your warranty in effect for an extra few thousand miles on a new car .

regards
Mech

[California98Civic]

[EDIT: I am reposting this after taking it down for some corrections.]

I think my Civic’s ECU uses its Vehicle Speed Sensor for both speed and distance measurements. With slightly smaller than stock wheels/tires on, I have been meaning to test for mileage over-count. I have recently had convenient trips that offered good chances for test, so here are my current figures:

Distance: 13" VX wheels with 175/70-13s tested against a google maps route of 64.4 miles, mostly flat terrain, 1.9% over-count by my ODO. To account for the small rolling terrain sections, I might reduce that to 1% or 1.5%, but I’m not sure yet. I want to do more testing.

Speed: using roadside radar digital readouts I compared my speedometer readings. When my dial showed 40mph, the radar showed 36mph. When the dial showed 25mph, the radar showed 23mph. I did five passes at each speed. That seems to mean about an 8% or 9% over-count, I think.

I will do more tests as convenient, but any thoughts on sources of error or why are the percentages for distance and speed seem to vary differently? I expected the percentage would be the same for speed and distance. It’s important to me because ultimately I want a good number to adjust my FE results in my fuel log.

james

[Ford Man]

Originally Posted by California98Civic

[EDIT: I am reposting this after taking it down for some corrections.]

I think my Civic’s ECU uses its Vehicle Speed Sensor for both speed and distance measurements. With slightly smaller than stock wheels/tires on, I have been meaning to test for mileage over-count. I have recently had convenient trips that offered good chances for test, so here are my current figures:

Distance: 13" VX wheels with 175/70-13s tested against a google maps route of 64.4 miles, mostly flat terrain, 1.9% over-count by my ODO. To account for the small rolling terrain sections, I might reduce that to 1% or 1.5%, but I’m not sure yet. I want to do more testing.

Speed: using roadside radar digital readouts I compared my speedometer readings. When my dial showed 40mph, the radar showed 36mph. When the dial showed 25mph, the radar showed 23mph. I did five passes at each speed. That seems to mean about an 8% or 9% over-count, I think.

I will do more tests as convenient, but any thoughts on sources of error or why are the percentages for distance and speed seem to vary differently? I expected the percentage would be the same for speed and distance. It’s important to me because ultimately I want a good number to adjust my FE results in my fuel log.

james

Tire size calculator

[California98Civic]

Originally Posted by Ford Man

Tire size calculator

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

[gone-ot]

...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