speedometer is off, effect on fuel econ?

[Execut1ve]

I don't know why this didn't occur to me sooner, but my speedometer is off by a pretty significant amount (reads about 5 mph fast at 65 mph). I have a sheet of paper somewhere from when I had it calibrated with a radar gun years ago, I'll dig that up and post the actual numbers...

But what would be the effect of my speedometer inaccuracy on fuel econ? Could I make up for it by moving up to a slightly larger tire? currently running 185/65 R14

[Kodak]

Well, the speedo is just telling YOU what to do. So let's say for sake of argument that your best target speed is 60. But you're actually doing 60 when it reads 65. Gets confusing. Surely it's equally inaccurate at other speeds, right?

What will throw off FE calculations is when the odometer reads incorrectly. Depends which way. If it thinks you're covering more distance than you really are (like the speedo reading faster than real speed), then you have a somewhat artificially high 'miles' in the miles per gallon ratio.

Tire size - height actually - does in fact affect speedo readings. Since it counts revolutions and the formula must assume stock tire size, a taller tire would simulate a taller final drive gear. It'll bring down your revs at a highway cruising speed. I've read a few threads about it on here. If I recall correctly, the general consensus was that the lower rpm might save fuel (despite the increased frontal area) as long as you're vehicle has enough power to prevent constantly downshifting for small hills. Read: underpowered vehicles might lose FE with tall tires if it results in more downshifting. I'm sure you can find out better info than my brief synopsis around here somewhere.


To get really nitpicky, a nearly bald tire will be a bit different from a brand new tire of the same size, make and model. Fun, huh?

[Execut1ve]

it's a manual trans, so I don't have to worry about a computer deciding to shift when I don't want to, for what that's worth

[Ford Man]

Most cars I've noticed do show you are driving slightly faster than you actually are. I expect this is something the car manufactures do to allow the drive a small buffer without having to worry about speeding tickets. A good way to check the accuracy of your odometer would be on a long interstate trip by checking the actually odometer reading against the interstate mile markers. By checking it over a 100 mile distance this would give you the actual percentage you are off and would be more accurate than trying to determine the exact amount by estimating after only 5-10 miles.

[Whitey]

I just started driving a Honda Civic recently, and I noticed right off that the speedo was reading fast. I timed several miles on the Mass Pike and determined that the speedo displayed 105% of actual speed [edit: corrected %]--but the odometer seemed to be accurate. I was able to confirm my speed using the radar near a local school.

I purchased a ScanGaugeII [edit: fixed typo] soon afterwards. Interestingly, SGII displays the correct speed. SGII also displays slightly lower RPM than does the Civic's tach.

Of course (as already mentioned), the odometer needs to be accurate for true MPG figures.

[Execut1ve]

a scangauge is another purchase I'd like to make for this vehicle. I might just step up to some slightly taller tires to make up the difference... speedo reads about 8% fast at highway speeds if I remember rightly. I'm just worried I won't be able to find taller tires that aren't also wider which might offset any gains

[gone-ot]

...since: MPG = MILES / GALS

...when you don't know the MILES accurately, you sure-as-heck can't expect the MPG numbers to be correct.

...FWIW, most GM cars intentionally indicate 2 MPH faster than your actual MPH...they're programmed from the factory to do that.

[user removed]

Check you odometer accuracy.

regards
Mech

[mekanic]

Originally Posted by Old Mechanic

Check you odometer accuracy.
regards
Mech

And the speedo should be off as a percentage ,...as long as it is on zero when the vehicle is stopped.
So the faster you go the more it is off/ the slower you go the less it is off.
at least that is the way that 99.9% of speedos that are off behave.
If it in cable driven you can usually buy a different gear for it.
If my calculations are correct you should be about the same amount (8% high) in your MPG figures IF the odometer is off by the same amount.
Look for a speedo gear with 8% more teeth to correct the speedo.

[user removed]

Originally Posted by mekanic

And the speedo should be off as a percentage ,...as long as it is on zero when the vehicle is stopped.
So the faster you go the more it is off/ the slower you go the less it is off.
at least that is the way that 99.9% of speedos that are off behave.
If it in cable driven you can usually buy a different gear for it.
If my calculations are correct you should be about the same amount (8% high) in your MPG figures IF the odometer is off by the same amount.
Look for a speedo gear with 8% more teeth to correct the speedo.

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