adjusting speedo

I have a VDO (0-150mph) Speedometer driven off the driver's side front wheel. Stock VW tire is 165/80-15. My front tire is P205/50-15. So using a calculator on the web, I think our speedometer is 9.1% fast, which feels about right.
Does anyone know how I can recalibrate the Speedo?

Drive 9.1% slower. Seriously, I'd like to know too, as my speedo reads about 10 mph faster at 70mph.

Go to the Palo Alto speedometer site and they explain a procedure to get a basis for speedo calibration. Something like removing the cable from the back of the speedo and counting the cable revolutions as you push the car some number of feet. 57 feet? I dont recall. You then send the speedo to them and they do the necessary changes. I believe that Hollywood Speedometer does the same thing, I hear for a good quantity less money. I have not done mine yet, I GPSed my speed and found mine was off 10% over the entire speed range I checked. Up to 100 mph indicated. 100mph indicated was GPS'd as 90mph, and 50mph indicated was GPS'd at 45mph, etc. You could use a GPS and put a piece thin tape at the speeds you need to keep track of such as the freeway speed limit in your area. Or do like Ron suggested and drive 9.1% slower!

Or could you drive 90.9% faster?

To do this you need to know the rolling radius of the tire. The speedometer turns once for every revolution of the wheel. So now you can use a cordless drill to turn the speedo at a known speed (I used an optical tach to confirm the drill speed). You then convert the drill RPM into road speed and figure out which way to bend the little brass spring. You have about 2 or three tries at bending it before you break it. I don

PS: With the smaller diameter wheels you don

You have to get a speedo shop to change a little gear in the speedo. I think if you tell them what tire size you have they can calculate the ratio you need. You can find a neat tire size calculator here:
www.miata.net/garage/tirecalc.html

BD

Patrick, I like the way you think. Dale,using a GPS sounds a lot cheaper than spending my speedo away. As soon as I find someone with a GPS I'll try it.
Ron

My girlfriend ran track in college. She was able to run up to 12 miles per hour. I will have her push the Speedster as fast as she can and check the speedo for accuracy.
In addition, I

Maybe she could help Ron O get his bus started.
ed

The only problem with GPS is accuracy. At best they are accurate to within 3 meters and most waver in accuracy between 3 and 7 meters. It only takes a couple of inches to really throw off your speed reading. The inaccuracy can get really bad from time to time. Hence the reason the FAA built the WAAS system for commercial flights. It uses a satellite based system, but has ground transcievers that broadcast time correction.

In addition, you always have the chance that the US government is deliberately diminishing the accuracy, because of a military operation on the other side of the world. Of course they promised not to do this anymore. :)

-=theron

Ed, you've hurt my wife (and my single cab) deeply. Just the other day, when my wife was pushing the pickup down the drive way, I commented on much stronger she seems to be getting. She didn't take it as a compliment...women...go figure.
Ron
Actually, she starts every time (the single cab, not my wife). Fixed the bad ground on the CD box, had new bushings put in the starter and adjusted the fuel pump.

Ron.O when are you going to post some pics of the porsche pick up truck? It's been getting more press then your speedster.
DS

Here's a picture of the beast. She needs a little body work (very little rust) and the paint is very faded (Florida sun). Rear flared fenders have serious rust. All the painted script was put on by the auto repair shop that owned the truck and installed the 2.7 motor. My wife says that it's 'butt ugly'. I told her to wait until I get a new paint job and lower the suspension. She said that the only thing that will improve the looks of the truck is a car cover.
Ron

Hey dude..Whats that laying on the floor behind your speedster in the garage? And why is it there?

This picture was taken during the summer, when my engine builder test drove my car and seized the new motor. It was about to be put back in when the picture was taken. I did the re-and-re and he made good on the rebuild (seized rear main bearing).
Ron

Hey Ron, I have a hand held GPS, but haven't used it to check the speedo yet. I'll do it first thing this spring.

Theron, If I remember the white paper details correctly, the inaccuracy of a GPS's fixed location doesn't effect the ability of a GPS unit to accurately determine speed. Also, SA (the USMil's interference with accurate positioning) does not fluctuate fast enough to effect a speed reading, especially with the newer 12 channel units.

Ron, hopefully I can drive in this spring and we can go for a drive and check out your GPS with my speedo. Also, if you need a hand puttng your motor back in give me a email.
Ron

Ron L -

The problem with using a GPS to determine speed is that you're not getting a "real time" measurement.
It's delayed several seconds.

Dave

Hi Guys,
I gave up on the Speedo and trying to convert KPH to MPH and adding a fudge factor for tire size. I installed one that

That is true David, but if you can hold a speed, or rpm, for a few seconds, you can get a pretty good reading. We use GPS to set the speedos on our tow boat, and then check them by time in the slalom course - they are right on every time.

I may take you up on the helpp doing my engine install. I know how much experience you have putting yours in and out. Of course I know the real reason is you want to help set the red line....

Greg, that is amazing. I would not have thought that the bike unit would be able to get a reading after the speed really got up there.