I want to know how to fix that. I when to the 1/4 track and notice it was off. Then I checked with a radar and it 5mpg off. It's a 88T2 with stock rims tire size is 225/50/16. the engine and tranny is from a jspec, but dont know if that should make a different since it's the same.

 

you speedometer is 5 miles per gallon off?

 

at what speed was it 5mph off? the stock t2 rims are 15's i believe, not 16's. so at the 1/4 mile, doing 100mph, it being 5mph off is a possiblity.

 

lo...lol..lol... sorry guys..lol..mph..
I have the stock t2 rims they are 16inch

 

Speedos can be calibrated.

 

Get it calibrated, my old Avalon was 4mph off, got me a ticket even.

 

i believe there is a POT on the back of the cluster to calibrate it.

 

Thanks guys

 

I don't think so.
Typically, calibration involves (after putting the car on rollers to see exactly how far off it is) interposing a small gearbox between the gauge and the cable.
It's a fairly pricey operation too.

 

Damn then never mind, thought it was a quick fix. But thanks anyways guys

 

To the OP...

Stock tire size is not 205-50-16. It is 205-55-16.

This is your problem.

 

Stock T2 wheels are 16x7.

 

maybe i'm thinking of the tach then.

 

Bingo.
My car is actually 5 off at 60 (going 55 when speedo says 60) but I have stock sized tires. Oh well.

 

Probably.
Being electronic, it would make sense the tach could be adjusted via a pot...the speedo, being mechanical, would need a gearing change.

 

5 mph off isn't a big deal as long as you're going slower than it says you are.

Just get a GPS and use that as your speedo.

Mines about 4 MPH off because I've got 225/50/15s on my NA.

 

Maybe for some cars, but not for RX-7's. I just had mine calibrated to get out of a speeding ticket, and they just replaced the speedo gear inside the tranny with one that had more teeth or was smaller or something...

 

How much was it?

 

Actually, we're both right.
Mazda- like every other manufacturer- makes a range of speedo drive gears (in this case, 17-23- see here) which are interchangeable and accommodate different wheel size/tranny/differential combinations.
If you are lucky, a simple drive gear change will get you in the ballpark/close enough.

This is not the same as calibrating the speedo though.
In the real world, speedometers are at best pretty inaccurate. There is usually one speed they are dead right at and increasingly wrong both above and below that one area. The additional gearbox method will widen the range at which the speedo is "pretty close", but due to the nature of the beast, it'll never be perfect all through the range.

In my misspent youth I worked for a company that installed the special equipment for State Patrol pursuit vehicles and the speedo gearbox was part of the package. Of course, with the widespread use of radar, cops no longer rely so much on their speedometers, so this may not be done anymore.

For the average guy, a few passes on the Interstate with a stopwatch using the mile markers will give a fairly close idea of where your speedo sits, accuracy-wise.
Make multiple passes at the same speed and try a few different speeds (say, 50, 70 and 90mph) and you'll end up with a passable notion of what your speedo says compared to the real speed.

In the end however, if a cop wants you, he'll get you, and in most cases there ain't a lot you can do about it.

 

Bout $200.

So clokker, is my speedo not actually technically "Legally" calibrated?

 

I'm not sure what "legally calibrated" is.
Last time I had a speedo done- probably going on eight years now- I got a certificate showing the correction that had been applied and a graph that displayed the accuracy from 30 to 90mph.
I never had occasion to use it in a court and frankly, I don't think it would have done much good anyway.
When you've been charged, it's your classic "He said, she said" standoff and courts are predisposed to believe the officer.
Especially if the officer has radar evidence.

You can always try the "Let's see the radar's certification/calibration" ploy but your best hope is the cop doesn't show up at all.

Far better than a "calibrated" speedo would be some GPS proof, a method that I'll bet will become more popular in the future.

 

Yeah that's the same sheet of paper I got, the same one that (partially) got me out of a speeding ticket. But I don't think I ever had that cable adjuster thingy installed that you were talking about. Does this mean I got shortchanged? Or not as excused as I should have been?

 

The "cable adjuster thingy" is actually just a small gearbox that goes between the cable and the speedometer itself. It is a further, more precise adjustment to the gearing (remember, the speedometer is a completely mechanical device, not terribly unlike a clock) that drives the unit.
Obviously, your people did not feel it was necessary (or possible).

I don't know how a "calibrated" speedo would get you out of/excused from a speeding ticket- it's main function is to KEEP you from getting caught in the first place.

 

Hee, hee. Giggle, giggle.

 

The problem with relying on GPS as your speedo is it is a lagging indicator depending on how fast your GPS updates your positions. Most low end GPS systems update about once every 45 to 60 seconds.