today when i was on the highway i noticed that my speedometer said 80mph but the tachometer said 2500rpms which is usually wheat i do at 70mph. so when i got home a friend of mine got up to 70 and i was right next to him and my speedometer said 80mph. does anyone know whats wrong? is there a way i could recalibrate it? kinda like set it back 10 mph. i noticed on top of the speedometer theres a screw. does that do anything?

thanks

 

Leave it like it is. Next time you get pulled over you will be going 10mph less than you intend.

 

a different tire size can also do that to your speed reading...dont know if u have different tire size.

 

You might have to go to the dealership to get it recalibrated.

 

Could be your speed sensor on the transmission.

 

If you changed tire size or wheel size you can calculate how much your speedo will be off by using this calculator http://www.miata.net/garage/tirecalc.html

Brian

 

A different tire size would affect both the speed and the tach. In other words, no matter how fast the car is going the ratio between RPM and MPH should not change.

 

How can that be true?

If you are travelling at 10mph on the road, you are doing like 1200rpm, then you give it some gas and it climbs from 1200 to 5000rpm, then shifts to another gear, the rpm monitors the engine revolutions, and the mph is your average wheel speed.

There is some large equation that will give you the avg. speed from rpm, but that is usually based on a single gear, like on a boat that is how fast you can tell you are going, since there is only one set of gears. So linearly Revolution per minute is related to nautical miles per hour, but since we have gear ratios to deal with rpm isnt directly proportional to wheel speed.

Brian

 

I have a I have 215/55/15 on my Max with aftermarket rims, When my speedo says I'm going 100, I'm really going like 93. I used one of those online calculator thingies, it said I am off by 6.3%. The rims/tires were on the car when I bought it, so I didn't know if was off until I was following a friend on the 101 and when we got there I was like "damn, I didn't know you were such a fast driver, 90 MPH in a 70?" and she was like "what? I was only doing 85 or so....."
It's good when you get pulled over though, I was so scared once because I thought I was doing 105 (which is a felony charge, over 100, at least in Cali) but the ticket was only for 95. Phew!

 

Vehicle speed IS directly proportional to RPM when your torque convertor is locked up at highway speed which is what CanadianMAxima was referring to in his post. My Max revs 2300 at 60MPH indicated all the time no matter what size tires I have on it.

 

Do different years have different gear ratios? My 92 GXE turns 1900 RPM @60 MPH with the torque convertor locked up. The car is stock.

 

Your AutoTragics are probably way different than our sticks