Speedometer and Odometer ERRORS

[RussBarnes]

I have about 2,000 miles on my smart and have made several modifications since January. During that time I broke it in carefully for maximum piston ring sealing to get my best mpg. I got that up over 45 mpg calculated with everything stock at about 1,600 miles (70/30% highway/city w/o A.C.), but something bugged me. When I drove by the portable radar boards and down the interstate I could see the errors in the speedometer, normal in all of the 75 cars I have owned since I was about 16 yrs old. I didn't measure how far off they were on the smart since I knew I would make changes and this is the first car I have been such a geek about to care about accurate mpg. Usually I cared about 0-60 mph times!

Now that I have installed four wider and taller tires (185/60-15) I know that the readings are even more off so I did several tests and here are the findings. Sorry I don't have the data from before the tire swap but these tests are easy for those of you reading this to duplicate and we all may be interested if you do the same and post your results.

With a 4% larger diameter tire like I knew the speedo should read wrong, but it DIDN'T! I knew in the past when I drove with traffic I could tell that mine was off, just not how much. So with the new tires I took our portable GPS and now find the the speedometer is EXACT. That means that it wasn't right before. I backed up this reading with a portable radar board last night and when the speedo says 45 mph - so does the ScanGauge and the radar. When it says 65 mph, so does the GPS, so I know that now what I read is what I am running. (BTW, the Scangauge has been readjusted to match the in-dash speedo).

The odometer may have been right, I don't know, but today I took two interstate runs of 20 miles and 25 miles each. Using the mileposts I calculated that my odometer reads 5.6% low. So when I calculate my mpg I now multiply the odometer miles times 1.056 and get the real miles. So with the new bigger tires, Doug Thorley exhaust and easy driving I got 44.9 mpg today. That was one week highway, Interstae and city driving, premimum fuel, with 80% A.C. and usually set at 62 deg. I expect it will go back over 45 mpg without A.C. and shows me that the wider front tires at 26 psi aren't causing much drag, but sure give a better ride and I think superior highway handling.

I also have found by watching the Scangauge that driving back roads at 45-55 mph the instant mpg is much higher than my interstate driving at 65. BTW, at 65 mph I am being run over as here in TN we are posted at 70 mph and in spite of my trying to save fuel EVERYONE else is using their share at 80 mph. I feel confident that if I could drive state highways at 50-55 mph, in flat areas, without A.C., I could easily be in the 50 mpg area.

So if you have access to a GPS, you might want to report here what you are running and if you run an interstate and can go at least 10 miles you may want to let us know what your odometer error is.

All of this may have an effect on the average MPG that some are reporting, but it should give all of the rest of the geek squad more information to mull over.

[mfoster]

My scangauge and GPS agree. Speedo is off by 4% (high). Factory tires.

[sheureka]

Originally Posted by mfoster

My scangauge and GPS agree. Speedo is off by 4% (high). Factory tires.

I've used a gps for years, in 3 different cars plus about 50 rentals. I have yet to see a speedometer that's less than 3% (high) off.

[tivoboy]

I just did this today, with two external GPS units. At 60, the GPS were at 58.8. At 65, it was 63.5. So, bout, uh, 2.5% or so. I think the odometer is also a bit jabberwonky and I'll check that one tomorrow.

[Kermit]

Here's even a better one. My father and I drove two Smarts back from central Florida. Both cars went the same exact distance. When we got to the gas station my odometer read 295 miles while his read 293. I didn't have my trip meter on my GPS set so no reading there. Just odd one car read 2 miles different than the other.