Speedometer accuracy - or inaccuracy

[gsmaclean]

I have seen a few people mention their engine RPM at 65 mph is right around 2000. On my 94 Limited, my tach is closer to 2300 at 65 mph, so I thought one of three things was happening:

• I had a lower ratio rear end
• My speedo was out
• My tach was out

This weekend I borrowed a friend's GPS and mounted it on the dash to figure out which it was. I now know it's the speedo that's out.

The odometer is fairly close. For every 10 miles driven, it shows 9.8 miles. I have brand new tires, so this isn't unreasonable.

My speedometer is dead on accurate up until 40 mph. After that, it's:

Indicated Actual
30.........30
35.........35
40.........40
45.........46
50.........52
55.........58
60.........64
65.........70
70.........76

So it would seem that for every 5 mph over 40 mph, it adds another mph. Good to know when going through speed traps - if it's a 60 zone, I need to drive so my speedometer shows 56 in order to show up as 60 on radar.

Anyone else have a GPS and can check out their truck to see if it's the same...or is it just mine?

 

[Mud Bug 94]

what size tire and what gear ratio?

 

[gsmaclean]

what size tire and what gear ratio?

Standard 235's with a D4 3.73 limited slip back end.

 

[ActionJackson]

speedometer errors increase as speed increases so the faster you go the bigger the error. its just the nature of the beast

 

[gsmaclean]

speedometer errors increase as speed increases so the faster you go the bigger the error. its just the nature of the beast

Except that mine is dead on accurate to 40 mph, after which it starts diverging, showing approx. 1 mph slow for every 5 mph beyond that.

As far as what I'm showing for various RPM's...

 

[Tony H]

My speedometer is all over the place depending on Outside Temps.

On a cold morning, My Speedo Pins itself and I'm only doing 55ish. No noise from the cable or the Speedo itself but boy is it way off the chart. It calms down a bit after driving many miles but it's not right.

 

[R.J.]

i think my dd is doing something like this. sometimes i think it works fine and others it seems like its mybe 5 miles off. i should try your gps idea. good things to know

 

[Anime]

The factory Ford Repair Manual says the accuracy of these speedometers is +/- 10%. Mine always seems fairly accurate though, at least going by the speed limit radar warning signs. You might try replacing the speedo cable, sometimes they wear out and a new one gives more accurate results.

 

[Mud Bug 94]

i just have to say you got balls to take these pics...and how the hell are you in park?

 

[gsmaclean]

i just have to say you got balls to take these pics...and how the hell are you in park?

Broken shift indicator cable. It's on my "to fix list" when I take the dash apart to replace some burned out lights.

And it wasn't hard to do - just sitting alone on the highway, shoved my phone up there, stabilized the speed, and took the picture.

Now THIS was hard to do. It was in my 1991 MR2 Turbo, in 5th gear, wide open, just about 140 mph - to prove I could do it, of course. Notice how I started drifting into the opposite lane as I was concentrating on taking the picture.

 

[Mic_]

Except that mine is dead on accurate to 40 mph, after which it starts diverging, showing approx. 1 mph slow for every 5 mph beyond that.

As far as what I'm showing for various RPM's...

How'd you get to 65 in PARK !!!

 

[RedRuninProject]

Just a thought to throw on the pile, when I had a U-joint replaced the stock was too small, had to use one from a full size truck. The mechanic said in the factory they have used full size truck parts if they were short on regular Explorer parts. Has the speedometer sensor been replaced before? Mine does the same thing and I'm considering picking one up out of a full size to see if I match the flow of traffic instead of blowing by everybody.

 

[gsmaclean]

How'd you get to 65 in PARK !!!

You should see what I can do in overdrive!!

As long as it's not a huge PITA, I'm going to fix the shifter cable (which I assume to be broken, a known 1st Gen problem) when I have the dash apart. Otherwise I'll just leave it as is and live with it.

 

[Mud Bug 94]

ur low on gas....

 

[mikeinri]

"ur low on gas..."

You beat me to that one. Probably the only way he could get that Japanese POS to 135...

Hope you're saving all that money you're not spending on the good vehicle for a better life insurance policy!

And yes, if you couldn't guess, I'm jealous!

Mike

 

[EMG7895]

I think that its safe to say that as a whole the mechanical speedometers and odometers are not the most accurate, I was pulled over once by a cop for speeding and then let go because he also had an explorer and said that his speedo was nowhere near accurate.

 

[Mud Bug 94]

Now THIS was hard to do. It was in my 1991 MR2 Turbo, in 5th gear, wide open, just about 140 mph - to prove I could do it, of course. Notice how I started drifting into the opposite lane as I was concentrating on taking the picture.

ur only at 6000rpm not 7000rpm(the redline) i think ur lieing about the wide open part....jerk

 

[Phager]

Well top speed could be limited by aerodynamic drag or progammed in to the computer with a speed limiter

 

[Mud Bug 94]

good point but hes still a jerk. lol

 

[gsmaclean]

"ur low on gas..."
You beat me to that one. Probably the only way he could get that Japanese POS to 135...
Mike

It only had 200 HP, even at full boost - in top gear, that's as fast as it would go.

"ur low on gas..."
Hope you're saving all that money you're not spending on the good vehicle for a better life insurance policy!

And yes, if you couldn't guess, I'm jealous!

Mike

No need...that car met its end with a rather ignomious high-speed introduction to a concrete barrier after running over debris that took out a tie rod. I walked away from it.