Tach compared to Speedo...

[D1A3]

Hi guys,

I just took a chance and ran my '76 2.0 from Atlanta to Chatanooga for a weekend trip. First of all, it was the first trip in the car in the last year, and it ran GREAT! Only used about 3/4 of a tank, up, back and around while we were up there too. Seems changing the spark plugs made a HUGE difference (used Bosch Platinum)!

Anyone in the SE looking for a great getaway, head to Chatanooga area and check out the twisty's on and around Lookout Mountain!

Now my question... Coming back today, traffic was moving at a good clip on I-75 and I noticed at 4,000 RPM in 5th, I'm doing 90 MPH? Seems my speedo is off a bit, but I'm wondering in a stock setup what the corresponding speed should read at 4,000 RPM?

Funny thing is, no matter how hard I pushed her, she always stayed smooth, just purring along. It woudl be something if she really ran 90MPH for 2 hours on the highway, huh? Temp ran at 50% on the gauge the whole way as well.

Anybody that can can let me know what they run at 4K RPM in a 2.0 woudl be much appreciated.

Thanks,
Jason

[AaronM]

Actually, that doesn't seem too far off. These cars were geared pretty low. At 65-70 I was over 3,000 RPM in mine.

Aaron

[Acox914]

4k is 90 mph

[D1A3]

Thanks Guys! It just seemed like we were not going that fast. Nice to know the car runs so smooth at high speed!

All the best,
Jason

[SGB]

My speedo is a bit generous. Indicated 90 is about 78. I've got 50 series tires though (205/50 is a smaller rolling circumfrence than stock 175/70). But I've always thought the 914 spoeedo was whacked, anyway. It STARTS at 10 mph.

[AaronM]

Originally posted by SGB
My speedo is a bit generous. Indicated 90 is about 78. I've got 50 series tires though (205/50 is a smaller rolling circumfrence than stock 175/70). But I've always thought the 914 spoeedo was whacked, anyway. It STARTS at 10 mph.

The speedo in my Mark VIII starts at 10 mph.
The speedo in my dad's Explorer starts at 10 mph.
The speedo in my old Honda Accord starts at 10 mph.
The speedo in my 944 starts at 10 mph.

That's pretty normal.

Aaron

[D1A3]

I have stock size 165's on mine now. Guess I will have to get somebody to pace me out on the highway...

Thanks for all of the feedback!

-Jason

[John Rogers]

Two very accurate ways to get the real speed. First is to use a GPS, if you don't have one they are easy to find and borrow. The are accurate to within 0.1 MPH. Or you can download the GEARS.EXE program I wrote and available on the site here and follow the directions. Basically you measure the tire roll-out, get gear ratios and final ratio and enter the numbers and move the slider in the window. It is exact and I even checked it with my Garmin Street Pilot. Good luck

[SGB]

What I meant was it STARTS at 10 mph. Ok, maybe the needle rests just below 10, at about 9.5 mph when the car is motionless! That is not exactly accurate, eh? I gotta look at some other cars, maybe this is common, but I did not think so.

[schosh]

I agree with John..use a GPS to calibrate your speedo. I have an Etrex mounted on the dash . It is particularly useful on longer trips as I try to maintain a predetermined moving average.
It is also very helpful if you enjoy backcountry touring.
On my car the speedo and the gps agree closely. I am not so sure about the tach accuracy. I have two seperate tune up/dwell tachs. Neither of them agree with the dash unit or each other . The discrepency is rougly 500rpm +-.( Probably only important when setting the timing.) I guess I'll use Johns program to determine which unit is accurate.
Bill

[BigD9146gt]

You don't need the GPS, just use the mile markers and a stop watch. It doesn't have to be exact, but you can figure out your error easily, and cheap.

60mph: 60miles/1hour = 60miles/60minutes = 1mile/1minute.

90mph: 90miles/1hour = 90miles/60minutes = 3miles/2minutes = 3miles/120sec = 1mile/40sec.

Just hold the car steady at what ever speed you can without getting a ticket, and then figure the above as a linear rate. If you get 50 sec on the stop watch, then your going 75mph.

Correct me if i'm wrong, but i'm pretty sure the math is right

[AaronM]

Originally posted by SGB
What I meant was it STARTS at 10 mph. Ok, maybe the needle rests just below 10, at about 9.5 mph when the car is motionless! That is not exactly accurate, eh? I gotta look at some other cars, maybe this is common, but I did not think so.

The principle on which a mechanical speedometer works makes it difficult to calibrate spring tension to read well at both very low speeds and at normal road speeds. Most automotive speedometers are not accurate below 10 mph since it's not necessary to have accuracy in the sub 10 mph range on an automotive speedometer, they just don't typically go to the extra trouble of making it work at very low speed.

What I mean is, it takes a certain amount of magnetic field generated in order to overcome the static resistance of the hairspring in the speedometer, and since the magnetic field produced inside the speedometer is non-linear in strength with regards to rotational speed, the forces at low speed are disproportionately small, making it hard to get a reading for the first few mph.

Aaron

[SGB]

Aaron-
I had not ever thought about the actual electrical and mechanical processes for a speedometer. That does make sense, especially for older vehicles. I guess some manufacturors choose to include a larger area below ten MPH to at least imply that there is some measure in that range. The limit in the parking lot here where I work is 5 MPH, and my Mazda (actually Ford) truck at least has a spot on the dial I can aim for...

[AaronM]

Yeah, it varies within a manufacturer too. Dad's Explorers (also Fords) never had markings below 10 mph and the resting point for the needle was where 5 mph would have been. Same with my Lincoln (also a Ford product).

Just all kinda depends on how things are set up from the manufacturer where they put the stopper peg.

Aaron

[schosh]

Stopwatch? Math? Thinking?
BIGD that is sooooo 20th century!

Just kidding with ya. I've spent many hours of highway driving working out moving averages of time vs mile markers or something similar. I'm actually old enough to have run a TSD ( Time/Speed/Distance) rally using an analog stopwatch for timing along with a Big Chief and graphite cyphering system for mathamatical computing.

Bill