speedo may be off, experts welcome.

Drove my car to college and it seems when im doing 50, its not doing 50! it feels like 50 mph when im doing 70 mph

Trans is the same, the speedo cable is from a 6 cylinder (original to car)

Rear gears are 9 inch 3.25 trac loc with traction bars

Thanks for the input,

its probably because i havent driven the car in awhile but when i go into turns i dont get the same feel, seems to be a better handling car or im just going slower

 

whats the tire size? If its taller than the stock tires the speedo will not mark correctly. The stock ones were around 25"



http://www.miata.net/garage/tirecalc.html

there's a difference calculator^

and here's where to get the gears:
http://www.transmissioncenter.net/speedometer_calibration_______va.htm#ford

 

215/70/14 on front
255/75/15 on back a couple weeks ago it was reading almost dead on.

 

did you just change the rear to the 3.25 gear ratio?
if so the speedo will be off. as you put lower gear ratios in, it takes more rpms to go the same speed. you need to change the gear on the end of the speedo cable to one with more teeth to recalibrate your speedo.

 

Assuming your car came with C78-14's the tired height would be 24.2"
Your 255/75/15 are 30.1".

so if we plug in those numbers=

Your speedometer reading with non-stock tires is 24.2% too slow.
When your Speedo read 60mph, your actually going 74.5mph.

:Handshake

but that doesn't account for the gear swap. (like bryant mentioned)

 

yes i did, i went from a 2.79 8 inch to a 3.25 9 inch. Would my speed by higher or lower?

Could you point me in the direction of a new speedo gear?


as for the tire situation, i was 10 mph slower at 70 with 14 inch tires

 

Thanks for the site alex.

 

Read this also, before your buy a gear:

http://1bad6t.com/gear_ratio__mph.html

 

3.25 from 2.79 will cause the speedo to read high. Driveshaft is spinning faster to one turn of the tire.
Speedo should be out by about 16 percent. 55mph your speedo should read about 64 mph assuming you have the same tires as with the 2.79


Anyhow never mind all the math its out cause of the gears and needs a different gear like Bryant said. It wont be perfect because of tire size.

 

Buy a cheap gps, or a cell phone with gps built in.

I keep a cheap TomTom in the Maverick since the speedo is so far off, even with a speedo gear swap.

 

When I saw the title SPEEDO MAY BE OFF , I was little hesitant to open it, for fear there might have been a photo attached.

 

Thats how I checked mine. Used the GPS out of my boat. I now have a Magellan and my speedo reads about 5mph over, at 60mph on the GPS I'm going 65mph on the speedo. Pretty good for having 4:11's. I run the white speedo gear.

 

I didn't go for the white one, even though it was supposed to be a closer match than what I have (brown maybe? I don't remember, maybe orange or red).

I have heard that the teeth on the white are so sharply angled that they wear out quickly.

So I stick with the electric GPS.

Mine reads about 70 mph at 60.

 

Oh, that just put a picture in my mind, I'd like to forget.....

 

But those tall assed 255/75's would have cancelled out the gear change. 255/75's? Never seen that size before. I wonder if that's the correct size of the tires ?