Speedometer Accuracy in Question

[tweetdriver63]

I had my elan out for a drive today, about 160 miles each way, going to my kid's high school basketball game. Here's my problem: I got a new GPS, and I had it with me. The GPS has a speed readout, and GPS information is very accurate. I can attest to this as a military pilot. When my speedometer was showing 70 mph, the GPS was telling me 62. The GPS was always showing slower than the speedo, and the faster I was going, the greater the difference...pretty much explains why I haven't gotten any speeding tickets when I've thought several times I deserved one.

Either the GPS is wrong, or the speedo is. I did a little timing trick pilots do* and it tended to agree with the GPS speed. The car is acting like it might have the wrong size tires, but the odometer seems to be correct. It's only the speed that's in question, so I don't think tire size could be the problem since I think that would affect both the speedo and the odo readings, right? BTW, the tires are 205-45/16.

What can make the odometer read accurately, while the speedometer is reading wrong? Bad VSS???

Any ideas would be appreciated.

*time for 36 seconds, and your speed is 100xdistance covered (36 seconds=1/100 of an hour). Timing this way in the car, based on the odo, isn't that accurate, because of the scale (1/10 mile increments), but the speedo was indicating 70 mph, and I went 6/10 mile in 36 secs, indicating 60 mph. The GPS was reading 61 mph at the time.

[GeoffSmith]

What is the rev counter reading at '70'?

Have you compared it to ElanScan data?

[ailwyn]

The markings on the speedo gauge are notoriously inaccurate.

Lockwood sell aftermarket dials that read 160mph on the same arc(?) which is much closer to actual speed.

[paulwatkins]

They are supposed to be inaccurate! In your favour!

THIS LINK indicates that the accuracy of speedomters has to be in the range -0->+10%.

That would be within the range that you are experiencing Tweety.

I experience similar differences

[Specky]

There's firms out there that'll recalibrate your speedo for about £45..

TTFN

[Paul.P]

The markings on the speedo gauge are notoriously inaccurate.
Lockwood sell aftermarket dials that read 160mph on the same arc(?) which is much closer to actual speed.

Several of us have proved that going to S2 wheels & with the Lockwood 160mph dials, the speedo is far more accurate. I have done this upto 120mph, someone reading the GPS whilst i was reading speedo, & it was 3mph out all the way !.
By the way, all standard equipement speedo's will read over what you are actually doing, if it we're other way around there would be loads of car manufacturers being sued !

[GeoffSmith]

By the way, all standard equipement speedo's will read over what you are actually doing, if it we're other way around there would be loads of car manufacturers being sued !

We got that by reading Paul's link!

ElanScan report the pulses from the speed sensor which is a hell of a lot more accurate than the speedo.

[Paul.P]

We got that by reading Paul's link!

ElanScan report the pulses from the speed sensor which is a hell of a lot more accurate than the speedo.

Just confirming Pauls link

Its fine elanscan being more accurate, but its a pity them pulses can't work an electric speedo to give an accurate reading, can't you sort this Geoff, your an electronic wizard ?

[ailwyn]

Paul,

You need a speedo drive that makes the VSS signal.

The only reason you can't swap (directly) the original cable driven speedo for a VSS one is the VSS is part of the speedo, so if you remove the speedo assy you are left with the cable and no VSS.

LGM has fitted a speedo drive that produces the VSS directly.

Hope that makes sense

[tweetdriver63]

Thanks everybody. I didn't expect so many replies so fast. I was aware that speedometers err in our favor by design, but the amount led me to think something's wrong. I watched a story on one of the TV news shows a couple of years ago, 60 Minutes maybe. I remember them finding that BMWs had the most accurate ones, and that most read the speed faster than you're really going, by only about 1-3 mph at highway speed (65 mph.

My speedo reading about 75 mph when the car is actually only going 65 mph is an error of around 15%, which I think is a bit out-to-lunch. I could look for somebody who could re-calibrate it, or I could just live with it. I could also put bigger tires on the next time I need them. How much would it affect the handling if I did that? Going from the stock 205/45-16 tires to 215/50-16 would make a 5.2% difference in the right direction (and I guess over the long haul, I'd be putting fewer miles on the odometer).

[dermotbell]

I have found that most cars I have owned over read by about 5%, except for Lotus.

Both mine and my dads overead by 10% - performance enhancing

I use my Road Angel for an accurate read out.

PS are you at Columbus, seem to remember being through there on the VC10 (the other classic i drove, the machmeter over read on her as well )

[tweetdriver63]

Yes. I'm in Columbus. I've been here for about 15 of the last 18 years. I'm retiring from the Air Force at the end of April, after 22 1/2 years. I haven't flown anything measured in mach in a long time.

[dermotbell]

Training baby pilots? Can't quite match 22 1/2, only 12 1/2 yrs in RAF.

Just moved to Sentry, long days of groundschool

[tweetdriver63]

Training baby pilots?

I've pretty much made a career of it - 3,000 IP hours in the Tweet and T-34C (Navy) over 17 years, after 3 years in the KC-135.

I did have the pleasure of spending 6 weeks TDY at Mildenhall back in 1988 (Ah..the Bird in Hand...), passing gas to many of your brethren in the F-111 and a bunch of NATO AWACS weenies. One of your F-111 guys blinded me for a few minutes with his lightshow following a night refueling. That was very cool.

[dermotbell]

I was a Tanker Wanker for 7 yrs, loved every minute of it.

Many happy dets in Bahrain, one very good one in Eglin (stayed in Destin) in 2004 for JCIETs.

The 10 is now 40 yrs old in RAF service - just another 10 before she finally is put to bed for the last time. I still tell people that as Concorde has retired she is now the fastest airline in the world.

[Specky]

Ah the two winged master race...

I also was on 10's a few years ago..

Found out Skydrol burns evrything you touch the hard way..

TTFN

[dermotbell]

I left in 2004, how bout you?

[Paul.P]

Bloody hell, you lot would qualify as EM members at this rate

[Monty]

Thanks everybody. I didn't expect so many replies so fast. I was aware that speedometers err in our favor by design, but the amount led me to think something's wrong. I watched a story on one of the TV news shows a couple of years ago, 60 Minutes maybe. I remember them finding that BMWs had the most accurate ones, and that most read the speed faster than you're really going, by only about 1-3 mph at highway speed (65 mph.

My speedo reading about 75 mph when the car is actually only going 65 mph is an error of around 15%, which I think is a bit out-to-lunch. I could look for somebody who could re-calibrate it, or I could just live with it. I could also put bigger tires on the next time I need them. How much would it affect the handling if I did that? Going from the stock 205/45-16 tires to 215/50-16 would make a 5.2% difference in the right direction (and I guess over the long haul, I'd be putting fewer miles on the odometer).

I wouldn`t change size of tyres for the sake of a speedo

Yours does seem to be out a little more than the norm, in pinky at 110 is showing 100 on the Sat Nav and thats with lockwood dials and 16" S2 wheels.

I`ve heard some of the older type Elan owners use IIRC one of those speedo`s you get for a push bike and stick it to the dash, but you need to calibrate it

[The Ref]

I was in the army for six WEEKS before they kicked me out for standing up to a bully