Speedo Brain teaser

[stan909]

I passed four radar signs showing my speed yesterday on highway 126 from Valencia Ca. to Santa Paula.Each showed 51 when I was driving 55 according to my speedo.When I got to Ojai from San Bernardino my trip meter showed 137 miles.Is my odometer correct or did I travel less than that?Does my van really have 109,000 miles or less? I went to San Diego a few weeks ago and got 18.3333333 mpg,filling before leaving and filling upon arrival to calculate,but if my odometer is off what did I really get? I'm not sure I want to know but please chime in.

[edm3rd]

I have a ScanGauge II, and was most concerned with it having as little odometer error as possible (can live with speedometer being off) in order to get accurate miles per gallon. It needs to be re-calibrated to adjust for tire wear, say every 10-15K miles. Same thing happens to vehicle odometer and speedometer, but cannot be adjusted. Have no idea what manufacturers use for accuracy, ie brand new tires, half-worn tires, etc. Used the mile marker comparison (do at least 50 miles). Going from stock tires to new (same size, diameter, and revs/mile) required about a 9% adjustment to get zero odometer error on ScanGauge. Truck odometer went from 1% under to 3% over with new tires. Truck speedometer is about 2 mph off. If you're serious about knowing your true mpg you must account for odometer error. From new to 3/32 tread remaining tire lost about an inch in diameter.

[Davydd]

A TomTom or Garmin GPS will give you a more accurate mph readout than your speedometer.

[stan909]

Will do the speedo check. If I come across one on the highway. I would commonly do that with my Bounder on long trips but have not seen any where I've been with the Van

[topless]

wsfurrie wrote:
Try driving exactly 60 mph on the highway and watch the mileage signs. It should take exactly one minute to go a mile.When you have adjusted your speed to get to a minute you will know how far off the speedo is.
Wayne

This method will also let you check the odometer. The mile marker signs are exactly 1 mile apart.
Years ago, the cops used to paint lines on the road, usually by a down ramp, (they could sit out of sight on the back of an overpass) and use a stop watch to time a car traveling 528 feet, multiply the number by 10 and that's your speed. They used small aircraft to do the same thing out in rural areas.

I still use this method to check speedos and odometers on vehicles I drive. GPS is great, but this is also something to do when on long trip. I like to know what speed I'm driving, speedometers are all different and most are off.

[wsfurrie]

Try driving exactly 60 mph on the highway and watch the mileage signs. It should take exactly one minute to go a mile.When you have adjusted your speed to get to a minute you will know how far off the speedo is.
Wayne

[gotsmart]

BudJ wrote:

2oldman wrote:
The answer is the cube root of 8.

No it is not, it's the square of the speed!

stann909, simply use your smart phone or navigator to check the speed vs speedometer.

Bud

No. You're both wrong. The answer is: 42

🙂

[ScottG]

Not unusual for the speedo to be that far off.

[stan909]

I have no gps or phone with those capabilities. I only have every radar I've ever passed. van is a 97 Dodge. Tire size is what is on the door according to Dodge.

[booster]

stan909 wrote:
I passed four radar signs showing my speed yesterday on highway 126 from Valencia Ca. to Santa Paula.Each showed 51 when I was driving 55 according to my speedo.When I got to Ojai from San Bernardino my trip meter showed 137 miles.Is my odometer correct or did I travel less than that?Does my van really have 109,000 miles or less? I went to San Diego a few weeks ago and got 18.3333333 mpg,filling before leaving and filling upon arrival to calculate,but if my odometer is off what did I really get? I'm not sure I want to know but please chime in.

I don't know how old your van is, but the newer speedometer/odometer seem to separate the two functions. We have 4 vehicles and all, with original tires, read high on the speed vs actual, and high on the miles vs actual. But, they were way off compared to each other. The speedometers always seemed to be about 2mph off, not a % of speed. 2mph at 30, 2mph at 75. The odometers were a fixed % of actual distance off, usually about 1-2%.

If yours is similar, you probably have less miles than indicated. Four miles an hour off is a lot to be off. Time to check it with a gps for speed and distance to know for sure.

[BudJ]

2oldman wrote:
The answer is the cube root of 8.

No it is not, it's the square of the speed!

stann909, simply use your smart phone or navigator to check the speed vs speedometer.

Bud

[2oldman]

The answer is the cube root of 8.