No..not talking about a race between the two..lol

If the speedometer is off by 5 MPH, would the odometer be off too? Case in point, my '72 Supreme actually travels 5 mph slower that what the speedo reads. I know this because I checked it with GPS. So, would that mean that distance traveled on the odo would read less than actual miles traveled?

 

If you Speedo is off it’s because the drive/driven gears do not match your rear end ratio/rear tire height. It would be off a percentage not just an even 5 mph across the board. Your odometer would be off by the same percentage.

 

Yes, the odometer would be off by the same amount (see below) as the speedometer.

The speedometer error is a percentage, so at zero MPH there is zero difference, and as the speed increases, the difference also increases. For instance, with a 10% error, at 20 MPH the speedo is off by 2 MPH, at 60 MPH it is off by 6 MPH, and at 100 MPH it is off by 10 MPH.

So at what speed is your speedometer off by 5 MPH? Using that, you can calculate the percentage error, and apply that to the odometer reading to achieve a corrected number.

Years ago when I swapped from the original 2.56 rear to a 3.55 rear I printed out an Excel chart with indicated and actual speeds in 10 MPH increments. I taped that to the dash near the speedo so at a glance I knew how fast I was really going.

 

It always cracks me up when I hear people say the speedo is off by whatever figure. If it’s off at 60mph, it’s off at 30, just be a lesser percentage. it’s also off at 80mph, just by a higher percentage.

The amount off the speedo is also effects the calculated mpg. If the speedo reads 10% slow, the odometer is also 10% off. Just to keep the math simple, the calculated 10 mpg is actually 11.

 

Ok..so that makes sense. I didn't check the error at all speeds, but I do know when I read 60 on the speedo, I'm actually going 55. Does that help? Also, when I'm on the highway, I keep my speedo as close to 70 as possible, so what would the actual speed be, and how would I translate that to actual distance traveled?

 

Simple math.
Indicated = 60
Actual = 55

Your speedo reads 5 MPH high at actual 55 MPH, so the offset is 9.09% as in the speedo reads 9.09% higher than the actual speed or miles on the odometer. So to get the correct values and make the calculations easy, round to 9.1% and multiply the speedometer or odometer value by 0.91, as in 60 MPH indicated x 0.91 = 54.6 MPH actual.

As I said above, back in the 80s I printed out an actual vs indicated table using the above formula.

 

And to get back to the original question, there's only one cable from the trans to the speedo. That same cable drives both the speedometer and the odometer. The speedo head is designed for a specific number of cable revolutions per mile traveled - typically 1,000 revs per mile. When the rear axle ratio or tire size is changed from stock, the cable revs per mile changes, which effects both speedo and odo.

 

Y = mX + b
Basic equation for a straight line. m = slope error and b = offset error. X = indicated MPH, Y = actual MPH. Ideally, m = 1 and b = 0. Most folks talk about slope error but there is also the possibility of an offset error (aka: y-intercept). My speedo reads 4 MPH low (WRT GPS) at 40 MPH, 4 MPH low at 50 MPH, 4 MPH low at 60 MPH, and 4 MPH low at 70 MPH. Do the math (fit a line to the data points), and you'll find that for my speedo, m = 1 and b = -4, which means the ratio between indicated and actual MPH is = 1.0 (got that speedo gear in the trans dead nuts **), but there is a -4 MPH offset that is constant regardless of how fast it is going. Essentially, the needle position on the speedo shaft is off by 4 MPH. It only reads 0 at rest because the needle hits a pin and won't go any lower, but until I'm rolling 4 MPH or faster, the needle sits at 0.

** EDIT: The true measure if you've got the speedo-gear selected correctly for your axle ratio and tire size is to use a GPS and drive like 100.0 miles and then check the ODO to see how far off you are. If you have that correct and the speedo is still off, then that is either an offset error like I described above (the needle is rotated on the speedo shaft), or either the spring constant of the spring that is resisting the torque induced by the spinning magnet has changed OR the strength of the spinning magnet has changed (either can induce a slope error).

 

I think I'm just gonna take it out for a drive and check speed indicated vs actual speed and distance indicated vs actual distance traveled. That way I'll have a true indication of what I've got.

 

I would not even worry about 5 mph, just compensate for it in your driving.

 

How much is it effected if your driving into reverse?

 

Dunno what you mean 🤔

 

You will lose miles going in reverse, it will drive backwards. This is how the car dealers would back the speedo off with a drill. However, the amount of miles a car is driven in reverse is probably less than 10 over its life.

 

This is also how you borrow your parents car without them finding out. Back in the day, several of us (especially my cousins & myself) (prior to gaining our licenses and some even after gaining their licenses) would 'borrow' our parents car(s) and take them for a little spin around the block (when our parents were away). We'd write down the mileage, take it for a spin, immediately following our return we'd jack up the rear end, put it in reverse & roll back the odometer to the beginning mileage. These were not long trips - essentially (1) to prove we could do it & (2) it was fun to get away with something.

 

That’s the same principle we used when drinking our parents booze, we would fill the bottle with water to bring it up to the line they marked on the bottle. They were either too stupid or too drunk to tell the difference

 

Just don’t kill the car!!!!