All speedometers are designed to display a couple MPH off, this is normal. So if you go 65 by your speedometer, you're going 63 on the police radar lol.

 

That's fine. The only reason why I care is that it skews the calculated MPG's. Not by a lot obviously, but it does. Just want to make sure that mine is skewed just like everyone else.

 

Mine is closer to 3mph higher than the radar signs I pass around town...so @ 10 over 60 and 70 limits, I've got a nice safety margin.

 

Speedo could be off but odometer could be right. My trip meters on both cars are accurate exactly verified by mile markers while my speedo is slightly low reading.

 

^ This. Both of our Toyotas (Highlander and Tacoma) have speedos that are off, but the odo is accurate. And it's not surprising that it's off the same amount at all speeds. The Tacoma is off by a percentage, so the error increases with speed (acceptable per Toyota's chart). But our HL is off by 3 mph no matter what speed we're going (although haven't checked it at 5 mph.

 

Thanks, this is reassuring. Next I'll test it against the mile markers on the highway.

 

^^^ the "error" is built into the needles, so even though the proper exact speed is inputted, it can display whatever you want. I actually fixed mine on the Corolla by taking the needle off and reorienting it according to my GPS.

 

That would make sense seeing as how mine was 2mph over the whole range. When I changed tires on my mustang, I was up to 8mph off around 70mph (car would say I was doing 78mph), and far less off at lower speeds. I used GPS and my programmer to adjust the tire revs per mile until they matched.

Also, I trust my phone GPS, I've clocked it against stationary radars and mile markers.

 

v = wR, where w is the angular velocity. If a car assumes a fixed R (wheel diameter), our v will be off. When v is off, s = vt will also be off, which means the odo shows incorrectly. Those quantities are related.
Is that right?

 

All depends on why the speedo is actually off. If its off because of tire size or gear ratio change, then mpg and odometer will be off. This would be the case if the amount the speedo is off grows as the car goes faster. If the needle is always off by the same amount no matter the speed, then its just the speedo (which is probably what my issue is).

Like I said with my mustang, its definitely off the bad way with smaller than stock tires. The speedo is off more and more the faster the car goes AND the mpgs are overly high due to the odometer also being off. The person that buys my car will think its way faster than it is, hopefully that keeps them from getting tickets. I ran it up to 155mph last week and the gps only said 139mph. When I had it corrected before returning the computer to stock, 155mph said closer to 152mph on the gps.

 

Odo is a digital input which should be accurate. If it weren't accurate, it would cost Toyota money. If it were reading too few miles, then your warranty coverage would be longer due to the under recording of miles. On the flip side, it's very costly to have an Odo record more miles than actually traveled. Just ask Honda if there's a cost with that. Now converting that digital signal to analog can incorporate error, either by design or just because of quality control. Here's Toyota's chart on acceptable speedo errors (you won't find a chart on Odo error because any error is unacceptable).

 

Thanks, that helps a lot. This logic would leave me to believe that the calculated MPG should have no relationship to the inaccurate speedo.

Also, back when I was modding my mustang I had a bad realization. Once you're in the computer like I was, you could have the car thinking it was traveling almost half the distance it really was by changing the rear axle ratio and/or tire revs per mile. Allowing you to keep the miles low and make more money when you sell it.

With an auto, screwing with the axle ratio would cause shifting issues. But with a manual, you could set that rear axle ratio to anything within the believable spectrum and the car would run fine outside of the speedo and odometer.

 

Has anyone checked if the trip meter clocks up when in reverse, and if it ticks up 1/10 mile after you have come to a complete stop?

 

If it didn't work in Ferris Bueller's Day Off, its not going to work now. Its going to go up.

 

My 2011 camry runs 2 mph slower real time by GPS and police speed signs with stock size tires (215/60r16)..switching to 215/65r16 should fix that

 

Good thing you know that.