Do you trust your speedometer/trip odometer?
 

My 87 Civic's factory odometer reads 3.45% higher than it should, even with the stock sized tires on it. Thus there is always a bit of disappointment when make that correction when calculating MPG :(

I noticed that when I drive by the stationary highway radar stations that say "your speed is __ mph" that they always said I was going a few MPH less than my spedometer read.

My "test" assumes that the green milemarkers on the side of the interstate are dead on. I reset the trip exactly on a marker and then see how many miles it takes to for my trip to be 1 mile off. In my case, it took 29 mile markers for my trip odometer to read 1 mile too high.

I recall an episode of Car talk on NPR a few months ago where an engineer was commenting on how his brand new car's odometer was reading about 5% too high (doing the same sort of highway mile marker test) and thus his gas mileage appeared about 5% higher than it really was. The car talk guys said they had heard many ppl describing the same thing and jokingly suggested it could have been inaccurate mile marker placement by DOT workers :D

I don't want to say its a conspiracy, but this does make consumers believe they are getting better MPG (and maybe saves them a few speeding tickets:) ).

I know most of you are really concerned about accuracy and I wanted to see if any of you noticed simular discrepencies on your cars when running on the stock tire size.

If that's the case, you're not only calculating your MPG higher than it should be, you're reaching service limits (100,000 mile warranty...) sooner too.

A cheap/easy way to check your odometer, if you don't have a GPS:

https://www.metrompg.com/posts/odomet...ogle-earth.htm

It was talked about here:

https://www.metrompg.com/posts/odomet...ogle-earth.htm

Yes, I have noticed. With my previous transmission, my speedometer/odometer was about 3% high. When I changed transmissions, I was suddenly about 2% low. This was with the same stock sized tires. I can only assume that other Honda's (and probably other makes) are experiencing similar inaccuracies.

-- Scott

I have an 89 Honda. It runs on the high side about 5%. I figured they might have averaged for 14" wheel's and mine has 13", stock.

I have had a vehicle or two in the past that the odometer read on the high side right off the lot. I used to wonder if manufacturers didn't do this on purpose.;) The county roads where I live are 1 mile apart and I ususally check every vehicle I get soon after I purchase it. Also, if I take it on a road trip, I use the mile markers to see how accurate the odometer is. The odometer on my current ride seems to be right on.

Well I finally did the metroMPG Google Earth method yesterday...it's very cool.

I plotted a course that was 36.87 miles according to Google earth...I was very detailed, zooming in as much as I could trying to get the lines in the exact lane I would be in and trying to get the curves as non-linear as possible.

My trip, according to my odometer, was actually only 36.2 miles. This means I'm acutally going 1.85% further than my odometer is actually reporting.

Hello free FE gains!!!!

I'll have to try it, but will google earth compensate for the added distance due to hills and valleys?

Does the odometer keep working in A 94 metro while EOCing with the key in any position? I'll have to try checking that too.

According to mapquest, mine is off by a third of a percent. But then again, mapquest may be off, so who knows?

I demand you adjust your mileage logs and drop below 40mpg instantly! ;)

Smart question! I don't think so. Better do your testing on a flat route, if possible.

Yes. Speedo/odo are mechanical and keep working regardless.

For evidence: see the video of the Forkenswift's motor revving up & down (in gear) - there isn't a key in the ignition or even a 12v battery connected to the car's wiring, but the odo & speedo are spinning: https://www.youtube.com/watch?v=2Ins29dqbac