My speedometer has been acting up lately and I'm not sure how accurate it is.

On cold mornngs it makes a ratcheting sound and the speedo bounces around alot until the interior gets warmed up. So I think the gears need to be lubricated and may be getting stripped. I'll pull it out during the holidays when I get a chance to work on it.

In the meantime, I'd like to find out if it's calibration is off. So what I want to do is calculate my actual speed by RPM and gear ratio.

Does anyone know what the formula is, considering I have stock rims and tires, tranny, rear end, and gear ratios for a 1980 RX7?

Thanks.

 

Your tach is probably not accurate enough to make more than a rough guess at speed, even if you have the formula worked out accurately and measure your tires' actual current diameter (they shrink as they wear, remember.)

The best way to calibrate speed is to measure time over a marked distance. Get on a highway that has mile markers, and time yourself as you go a measured distance at as close to a constant indicated speed as you can.

Velocity = distance divided by time (V=d/t), so if it takes you 60 seconds to go 1 mile measured distance at a constant speed, you convert your time to hours (3600 seconds in one hour) and then divide the distance by that number:

60 seconds/3600=0.01666666666667 hours
1 mile / 0.01666666666667 hours = 60mph

As to your speedo; no gears in there as I recall. It's a magnetic coupled meter movement (an eddy ring). If it's making scraping noises that bounce the needle when it's cold, it's maybe dirty inside or perhaps the bearings are starting to go.

Speedo movements are very sensitive; work with caution.

It could also be the cable, rather than the speedo, that could use some lube.

 

Whups, I forgot... the speedo uses no gears itself, but it DOES use gears to drive the odometer and trip meter off the same cable.

If they were dragging too much, could cause your problem.

 

Affirmative.

My Odometer is stuck and the speedo floored itself beyond 130 when I was only going about 70mph.

I just want to get a relative idea of speed and not exact. In other words I just want to see if I'm with 5 mph of actual speed, or whether the speedo is farther off.

Friday night I was doing about 5300 RPM in 5th gear, but the speedo was bouncing so much I couldn't determine my speed.

If I could calculate my approximate speed based upon RPM and gear ratio, then I could test accuracy of my speedo by determining the appropriate speed per gear and RPM, then driving under those conditions and seeing what the speedo indicates.

This should be a simple mathmatical equation, and hopefully there's some mathmeticians out there to help me.

Thanks.

Then I'll know if it's close, and if not, how far off it is by each gear.

 

It's easiest to calculate in 4th gear, because the trans ratio is 1:1 in 4th.

The formula is RPM/60, Times trans ratio, times diff ratio, times tire diameter in inches. That gets you inches per second, which can easily be converted to miles per hour based on 3600 seconds to the hour, and 63360 inches to the mile.

 

Use a good calculator. They're all over the web. Try this one:

http://4wheeldrive.about.com/gi/dyna....com/auto.html

I used something like this and made grid of 3, 4, and 5 gear on one axis and rpm across the other from 2,3,4, and 5. I calculated t he 12 values and posted a sticky where I could easily see it. The I would always know exactly what my speed was.

 

Buy or borrow a gps. Even the cheapest ones will show your speed and they are fairly accurate.

 

Found a good calculator at this web site:

http://www.car-videos.net/tools/spee...B1=Recalculate

BLUF: I used the following data for my 1980 RX7:

Tire Size: 195/75/13
Differential Drive Ratio: 3.909
Gear Ratios
1: 3.374
2: 2.217
3: 1.432
4: 1.000
5: 0.825
RPM Redline: 7000

I attached an Doc file with the calculations generated by gear ratio for every 500 RPM.

 

75 series tires... Snows?

 

Good work. I like the calculators that allow you to put in the actual tire diameter because you can actually measure it on the car and make a small correction for tire wear and so on. Those numbers look pretty close. I know at 4000 rpm with my tires I'm at about 80 mph.

 

Yeah. I checked my speed on the way home from work today and found that my speedo is withing 2-3 mph of the chart...as long as it's heated up and not slipping.

The tires aren't snows. I've got Eagle GTs on the rear and a matching set on the front.

When I went to work this AM at about 20 degrees F, it squeeled and read 80 mph when I was only doing about 35. So I've got a problem with the speedo and need to pull it during the holidays.

I hope this info helps others.

 

After erratic behavior as previously mentioned, my speedo quit working as well. I made a chart but found that it's only accurate in 5th gear.