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bk7794 wrote: ↑Sun Jul 31, 2022 11:19 pm So I had a 2000 Civic. Great car...but I noticed something. When driving an indicated 65 mph, I got great gas mileage. 37-40. I then got a scangauge and realized when my speedometer indicated 65, the computer was reading about 61 mph...

That could very well be. I try to stay at 65 mph, but is it really 65. When I do 70, I'm pretty much keeping up with traffic. But that may really be 65 mph.

Get one of those scanners. See what speed you're really going. I've driven one Toyota and noticed a 2mph discrepancy. But it was an Auto and a completely different model. So YMMV

All cars will have a slight discrepancy in the speed displayed versus your actual speed. You'll always be going slightly slower than the speedometer. It's a liability thing for manufacturers.

The exception is law enforcement vehicles, which historically have CERTIFIED displayed on the speedo somewhere.

The scangauge isn't necessarily any more accurate than the speedometer. The only potential inaccuracy it takes out of the system is the input and display elements of the speedometer (whatever form those take in any given vehicle), and that inaccuracy is as likely to be in the opposite direction of the other inaccuracies in the system as in the same direction.

If you really want to know, it takes maintaining a specified speedometer reading over a known distance. If your area has section line roads laid out at exactly one mile intervals, checking average speed across several of those would work. Failing that, an average GPS speed over an extended distance should tend to wash out its inherent error.

WASH OUT - Traffic is stopped up ahead!