dicktahoe

And the Odometer reads 2.1% low. In other words if I go 100 miles the odometer will show only 97.9miles. Looks like my warrantee will be good for 51,000 actual miles.

Cyncris

That also means that when you figure your MPG manually, your figures are off.
You may want to check your speedo as it is likely off too. Is there some chance that you changed wheels or at least tires since the car was new?

Usually if anything the speedo will read about 2mph higher than actual speed @ 70ish

Richard Solomon

Isn't that what he said?

.021 * 70 = 1.47 so he actually travels 68.53 miles in one hour but his speedo shows 70 mph, <STRIKE>or when his speedo thinks he's travelled 70 miles in an hour, he's actually travelled 71.50 miles (70 / (1-.021)), so almost the 2mph high you expected.</STRIKE>

Edit: boy I screwed that up "or when he's actually travelled 70 miles in an hour, his speedo thinks he's travelled 71.50 miles (70 / (1-.021)), so almost the 2mph high you expected.</STRIKE>"

dicktahoe

I installed Michelin AS3 to replace the Pirellis, but exactly the same size as Original. And yes, i checked the Speedometer digital readout with my GPS and at 75mph. The Digital Speedometer reads about 1 1/2 mph higher than the GPS. And you are right, my MPG will be about 0.5MPG higher than I calculated, and closer to what the Computer reads, which is about 2MPG higher than calculated based on Tank fill-up.

Richard Solomon

Sadly tires are pretty inexact - if you look around you may be able to find mfg-quoted rotations per mile and you'll see they're different even within the "same" tire size

dicktahoe

There was a time when odometers were not so tightly controlled, and often they would read higher than the actual mileage driven--which of course would negate the mileage warrantee sooner. I think the government stepped in and corrected that abuse by the manufacturers. Having an odometer read a little low, keeps the manufacturer on the safe side of the law.

Grapeking

How did you do this? One of those 'measured mile' segments?

gatorguy7

Or use a gps, on your phone or otherwise.

jr'sAudiA6

I determined this error about 2 years ago on a trip from California to Virginia. In my case I came up with 2.5% error on the low side if using the stock tire size (255/35/20). When I use my 275/35/20 tires the error is just about 4.5% but my speedo error now is about 0.

dicktahoe

Yes, with a "measured mile segment." Actually the segments along the highway I used were calibrated in 2/10ths of a mile and I went for a little over 5-miles. Sometime I'll try to check it over a somewhat longer distance.