Skip to main content

My speedo indicates I'm going about 10 mph faster than I actually am.  This is at a real 65 mph (so it incorrectly reports 75 mph).  I'll have to confirm, but I'm assuming this ~15% proportion of error is constant across all speeds.  

This is a safety issue, and that should be reason enough to address it.  But also, I find myself thinking I'm going the speed limit (65 mph here in Cali) but actually poking along at ~55 mph.  Good thing our cars are pretty, so no one gave me too much attitude for my 2 hr drive on the highway home after buying it.  But even more, the odometer must be accruing ~15% more mileage than is actually true, that's not fair!

Anyway, I'm thinking to install a speedo / odo ratio adapter.  They can be made custom to the proportion of speed up/down that you need.  There are also various offerings on Amazon / eBay / etc with preset adjustment percentages.  However, apparently their claims about compatibility can't be trusted much, so you have to know exactly what you're ordering.  

I know I have Porsche 914 VDO gauges- whether Chinese knock off I don't know.  Nobody seems to know what ratio adapter fits the speedo (after calling around to some of the usual suspects, without naming names).  I'm assuming it's metric, but I don't know the mm.  I'm not even sure what shape it should be!  Looks like a square rod with rounded corners.  The attached photos are of the back of my speedo gauge, and the cable coming from the transmission that fastens to the gauge.  Hopefully the adapter can be installed at this end as opposed to down at the tranny (but I can lift the car if need be).

Pls excuse the quality of the photos, I can take much better shots in a few days.

IMG_3949IMG_3948IMG_3951IMG_3950

(Ps, just for anyone who lands on this topic in the future, there are other solutions to this problem.  I believe the gauge itself can be re-worked to adjust for the difference.  Or changes nearer the transmission can be made, I believe.  Or you can use a GPS-based sensor to report the speed.  But the adapter route seems to be the quickest/easiest/cheapest).

(Double ps, for posterity & completeness' sake, one way to measure speed error is have a modern car with an accurate speedo go X mph, sync your speed with them, read your own gauge.  This is what I did while driving the car home on the first drive.  But a smarter way is to simply download one of many free speedo smart phone apps.  Or, of course, a GPS device if you have one.)

Attachments

Images (4)
  • Speedo gauge back
  • Speedo gauge back
  • Speedo/transmission cable
  • Speedo/transmission cable
Original Post

I've not heard of folks using a gear adaptor on a VW speedometer. It would be cool if it were easy. 

So you know, Sean, the Speedo in your car takes its signal from the left front wheel, not the transaxle. It's very common for them to be off for two important reasons: 1., the reproduction Speedster gauges used until recently came from China and are of marginal quality (they are not VDO/Porsche 914 gauges). And, 2, because almost nobody (except me) runs the VW Bug-spec 165/80-15 tire, preferring instead these days the beefier 185/60. That takes the sidewall from 5.2 inches to about 4.3, and the rolling height from about 25.4 to 23.6, and thus the distance covered per revolution from almost 80 inches to 74—a bit more than an 8 percent reduction. 

You've probably got a combination of both issues—inaccurate gauge and "wrong" tire size. 

The fix for this I see most often in people mounting a GPS or their phone on the dash and reading speed from that.

Second most common is to ship the gauges to North Hollywood Speedometer or Palo Alto Speedometer, both specialists in the marque. They will rebuild the gauges to VDO or better quality and calibrate them to your preferred tire size. This process can take a month and run into money, but I never hear people complain about the service those two venders provide. 

I did have North Hollywood rebuild/reface the 914 gauges I sourced for my Spyder. They're like jewelry.

Before (these are Chinese repops):

After (914 gauges, surgically altered):

IMG_6320

Attachments

Images (1)
  • IMG_6320

Once upon a time, there used to be more than several companies that offered mechanical speedo ratio adapters For VW and Porsche.  (sorry, Ed  -  They might have been before your time 😉   .   These were made for the rally set to accommodate their different tire sizes depending on rally route, competition or weather.  They’re probably still out there, but the demand has evaporated in the current GPS-based world.  

The “old way” we determined what we needed was to run the car on a “measured mile” either at speed (to see the distance in 60 seconds) or as a measured distance against the odometer and then tell the adapter supplier that you needed an “X” ratio adapter offset to get you to where you want to be.  

I have used adapters that connect directly to the speedo OR little adapters that connected to the speedo cable and then had a short-ish cable from the adapter to the speedo gauge.  THAT always seemed more logical than an adapter attached at the wheel (or transmission, if you had a different Euro car, like an Alpha).  

So, if you have enough discrepancy Between your speedo and your tire circumference to worry you, then go for the adapter, but find one that mounts at the speedo gauge because it’s definitely un-cool to mount it on the wheel end of the cable.

 

Last edited by Gordon Nichols
@Sean Seena posted:

(Double ps, for posterity & completeness' sake, one way to measure speed error is have a modern car with an accurate speedo go X mph, sync your speed with them, read your own gauge.  This is what I did while driving the car home on the first drive.  But a smarter way is to simply download one of many free speedo smart phone apps.  Or, of course, a GPS device if you have one.)

Or, you could do it the old fashioned way: with a tachymeter. 

Head down your favorite road, spot a mile marker, when it crosses the plane of the windshield support, hit the button, hold your speed steady then hit it again at the next mile marker. Checking at 60 mph (100Kph) is the easiest, because it's a "mile a minute." A tachymeter face has a scale to confirm your measurement at other speeds.

image

 

I'm guessing that my speedo has been recalibrated, because although I don't know how it was reading with the 165/80's on it, after I cleaned the 30 year old grease out if it and put a new needle on it, it reads 100Kph at 59mph with the 185/65's.

Attachments

Images (1)
  • image
Last edited by dlearl476

Interesting Dave, since the 185/65 is almost an inch shorter(.9). 

2.9" shorter per revolution. 96% of the taller tire.

To add to my previous post, I believe Carey also sells a VDO GPS speedo. The Speedhut speedo is close to the usual repro gauges, but you need the whole set for them to match perfectly.

If you have the typical VDO gauges, the speedo from Carey is your best bet.

The VDO and Speedhut both have an LCD odometer, so if that bothers you and you need the analog/mechanical version, you won't be opting for the GPS version.....

Last edited by DannyP
@DannyP posted:

Interesting Dave, since the 185/65 is almost an inch shorter(.9). 

2.9" shorter per revolution. 96% of the taller tire.

To add to my previous post, I believe Carey also sells a VDO GPS speedo. The Speedhut speedo is close to the usual repro gauges, but you need the whole set for them to match perfectly.

If you have the typical VDO gauges, the speedo from Carey is your best bet.

The VDO and Speedhut both have an LCD odometer, so if that bothers you and you need the analog/mechanical version, you won't be opting for the GPS version.....

I got a complete set of gauges from Carey and haven't looked back.

@DannyP posted:

Interesting Dave, since the 185/65 is almost an inch shorter(.9). 

2.9" shorter per revolution. 96% of the taller tire.

To add to my previous post, I believe Carey also sells a VDO GPS speedo. The Speedhut speedo is close to the usual repro gauges, but you need the whole set for them to match perfectly.

If you have the typical VDO gauges, the speedo from Carey is your best bet.

The VDO and Speedhut both have an LCD odometer, so if that bothers you and you need the analog/mechanical version, you won't be opting for the GPS version.....

I plucked that size out of my (fading) memory. Is that the size Carey recommends?  IIRC, it's 205/60-15 rears and 185/65's up front.

My car was originally built in the 90's and it has Brazillian VDO's.  

Last edited by dlearl476

Halda, makers of trip pilot and speed pilot rally instruments, also produced The adapters to adjust for tire sizes, but I messed with that stuff over 45 years years ago so nothing I know would be current, for sure.

Might be a lot easier to get your gauge calibrated to your chosen tire size or just get a new, GPS driven speedo.

I wish I'd have bought a set of those before they became unobtainium. About the only thing I'd consider adding to my dash. 

Thanks all for all the great feedback.  @edsnova those gauges are a dream 😍 (don't know if regular emojis work here but those are heart eyes!).

I might've misunderstood, but it sounds like most of these solutions require a new gauge, which would be pretty pricey and probably come with new problems to solve, unfortunately.  Or rebuilding the existing gauge, which is also pricey and not worth it on my likely Chinese made gauge, again unfortunately.  Ie, it sounds like the only solution where my gauge stays put would be a ratio adapter?  Which evidently, is hard to find these days?  Arghhh

@Gordon Nichols I'm surprised there aren't 1-2 companies still out there, offering these for the trickle market that still exists.  In fact, I'm surprised there isn't a market just for bad guys trying to doctor their total mileage for resale (although I guess those people could just disconnect the gauge entirely and drive off of feel).

 

Yes, the readily available solutions are mostly expensive.

It all comes down to whether you want the speedo to work as it should or if you just want to know how fast you're going.

Most of us have found (depending on tire size, transaxle gearing, and tach accuracy) that you can pretty closely 'read' your speed out in top gear by doubling the tach reading and adding just a little.

At 3000 rpm, I'm doing about 63 mph, etc.

My speedo error turns out to be about 10 per cent high. It's not off by a fixed amount up and down the dial.

So, under about 40, the speedo is sort of close. By the time I'm at 50, I'm in top gear and just do the double-the-tach trick. This is nonsense that would be inexcusable in a modern car, but hey, no one ever claimed these cars are normal.

There are usually more serious issues that need fixing with these cars, but If this is the worst you've got, maybe it's worth springing for having the speedo rebuilt at one of the reliable shops.

 

If I'm using my phone with Waze or Google for navigation I get the GPS speed, too.  Usually I look at the "GPS" speed, then look at my speedo and think, "Hmmm...   That's interesting" and keep driving.  Then again, I have a speedo in Kilometers and that's why I do the math in my head as Lane mentioned, so already there is a built-in error factor ('cuz I never was great at Math).  

I've used the "doubling the tach reading" in fourth, from time to time, but mostly I just look at my speedo (in kilometers, for me), multiply what it says times 6 and extrapolate what the more-or-less speed is.  I'm far too lazy to use the correction of 6.2 instead of 6 to increase accuracy by 2 mph, and I'm totally OK with the indicated speed being just a suggestion.  I just drive carefully for the conditions and keep up with traffic.

If my speedo read a more-or-less consistent 15% high I would just factor that into the equation.  Indicated speed is 75 MPH?  OK 75 X .85 = 64 mph.  There's your correction factor for highway driving - mentally subtract 10 mph.  Got it.  The gap will probably be less as you go slower.  Use your phone for "true" speed at 45 and 35mph just so you'll know what the correction factor is down there and drive it.  That's one helluva lot easier than messing with adapters or getting new guts in the speedo for something no one else will ever notice, and if you're worried about the odometer being off, NO ONE CARES unless you're looking at a car for sale with under 2000 miles on it (and wonder why it wasn't driven more).  

I honestly can't remember, after 20 years, whether my speedo is running high or low to the true speed, but OTOH, I really don't care a whole lot.  As long as I'm keeping up with traffic or not over-speeding down a back road ('cuz you'll never know when someone will be walking their dog or stroller out there), I'm fine.  

Just drive it.

Last edited by Gordon Nichols

After calling around a few more places, I think I'm surrendering to the same conclusion:  Just live with it.  Every solution is pricey or involved or both.  I thought a little adapter module could be had for say ~$100 with easy install.  No such luck.

The math approach works, but my brain likes a look-up table corresponding to notable speed limits:  

  • Shows "75" so it's really -> 65mph
  • Shows "40" so it's really -> 35mph
  • Shows "30" so it's really -> 25mph
  • Etc.

Whatever works.  And for total miles on the odo, oh well, will just have to explain and demonstrate it to any future buyer.  

 

Here are my gauges.  

@Michael McKelvey from the lighting I can't tell if the bezels for yours are entirely black, or if the inner edge are metallic while the outside is black?  For mine, the black was bothering me and I wanted chrome bezels.  Even though VDO and others sell chrome bezels, I couldn't easily find them for the two 914 bezel sizes, and messing with that bezel ring is cumbersome.  So I hit them with a bench grinder in an inconspicuous spot and was surprised to find beautiful brushed metal underneath.  Therefore, I buffed them all.  I can't tell but they *might* be stainless steel, and if so, I'm not sure of the grade.  So I clear coated them, which took away from the brilliance, but at least I sleep better at night knowing that rust is less likely to say hello.  Not chrome alas but I'm still really happy with it, I'd do it again.  It was a long/tedious/diligent process (at least the way I did it), so I'm happy to elaborate if you think you might try yourself.

IMG_4170IMG_4171

Attachments

Images (2)
  • Gauges
  • Gauges
Last edited by Sean Seena

Thanks Ray for the kind words. The paint guy who did my body work and paint really worked his tail off on my car. I paid dearly but it was worth it. It was the only thing that I did not do on the restoration. The car was down to the pan and body. If I didnt replace a item I rebuilt it or made a better part. It was worth the effort, because I can jump in the car and go anywhere. I can count on a few fingers the issues that I have had (small ones) in the 11k I have driven it.  Back to the subject at hand. Palo Alto or North Hollywood Speedo both do great work.I have had serveal gauges repaired by both with great success. 

Last edited by Tab Tanner

Well don't be too bezeldazzled yet, I have to admit that it's only been 1-2 months in warm CA weather that I did that, so time will tell.  

At first I thought to apply a thin coat of something like motor oil or garage door silicone lubricant, but would have to remember to keep reapplying that from time to time.  The beauty of the metal really glowed when there was simply an oil coat.  

But then I gave in and sprayed with clear-coat gloss-finish spray paint, which takes away from the brilliance but I'm assuming more or less hermetically seals it.  I don't know, maybe there's a known way to clear-seal raw metal but I couldn't find any knowledge online.  

Maybe the metal's already stainless steel but I didn't feel like finding out.

Add Reply

Post Content
×
×
×
×
Link copied to your clipboard.
×
×