Speedo out by miles.. - Ford Forums - Mustang Forum, Ford Trucks, Ford Focus and Ford Cars

Axeman82 · 06-23-2003, 04:47

I was driving home the other day (bout 3 hrs of highway cruising) and my gf was following me in her VS. Anyway we pulled up to a servo and she said "Damn u drive slow". Well I was going 115-120 clicks and she reckons hers was only on 100ish. Saying that mine is more likely to be out, what could be the cause of this? I've had another vehicle tested and I was reading 100 and he was reading 90.

EA's have electronic speedos so could the vehicle speed sensor be reporting incorrectly? Tyres are 215/65/14's so that shouldn't be an issue. The only other concern is that I had a diff replaced two years ago and the new one doesn't have a diff tag on it. Could it be that a 2.92 or 3.08 was put in instead of the 2.77?

Or some other problem?

Dan..

cookie_thief23 · 06-23-2003, 04:56

i think that speedos can have a +/-10% reading..so being 10kph out at 100kph is still legal for the speedo..but it wont get u off a speeding ticket using that as an excuse.maybe you were doing 110kph..your speedo might be +10 and hers -10 out.of course diff ratios, tyres and worn out speedo could be causing this..could even be the VS commo..its a common problem with modified cars having speedos out of whack

Axeman82 · 06-23-2003, 05:19

both our cars are stock driveline wise.. hers has a bodykit but i don't think anything was changed diff wise with the kit.. both have tyres at same tread level, etc..

My speedo tends to bounce a little (0.5 mm) or so, so maybe my vehicle speed sensor is dodgy.. or just the speedo dial itself..

Dan..

Axeman82 · 06-23-2003, 05:22

It's just a little annoying not knowing your actual speed - i like to keep up to traffic but i don't want to speed if i don't know i am doing so... i like to know when i am breaking the law..

Dan..

WBT56 · 06-23-2003, 14:22

If the diff ratio is a factory choice, you will be able to get a gear to fix it. Drop out your speed sensor and count the teeth on the gear. Get an accurate speed check, calculate the tooth count variation you need and get a gear to suit.

LTDHO · 06-23-2003, 16:36

Whenever you change diff gear ratio the speedo gear has to be changed to keep it accurate. Changing tyre size also changes speedo reading but usually not by much. Ford speedo gears are all colour coded so be sure you get the right colour! :)

Axeman82 · 06-23-2003, 16:38

I'll have to jack up the rear end and spin the tailshaft and do a physical measurement of the diff ratio I think..

Dan..

Dellboy999 · 06-23-2003, 17:19

Speedo Sender/Diff/Wheel Calculations

Check this thread, might help you decide on the diff being the problem or not.

Dellboy999

rossi · 06-23-2003, 17:35

Yeh my ef had the same problem. Only found out when the missus got done for 120km/hr when the cruise control was set for 100.

The guy I bought it off, put and xr6 motor and gearbox into it. Diff ratio for xr6 was 3.45 and the fairmont was 323 (by memory).

For every 5km/hr over 100, you reduce the gear in the speed sensor by 1. I had a #23 in the gearbox and reduced it to #17 to make sure, and now it's fine.
Best to be under by 5km/hr then over.

Another misconception is that if you have standard 15' wheels, and upgrade to 16' that this throws the speed out. Not necessarily as going to a larger wheel usually means you have a smaller profile, therfor diameter of the tyre stays the same.