So, compared to GPS, my Speedo is consistently 5mph too optimistic. My tires are 195/65-15. I’ve read in other threads that has shown to be a good size for correcting error.

So, not really wanting to go taller still, is there any other way to calibrate the Speedo?

Thanks,
Van

Bump, similar issue on the 1975.

The reason your speedometer is off is because the drive cable is spinning faster due to the smaller tires.

To correct this, you can open the speedometer up and adjust the spring tension on the needle to correct the calibration. I don't think any of us have a speedo calibration bench, so you will have to get some help to do it.

Have someone drive using a GPS speedometer app on their phone while you sit in the passenger seat and adjust the spring tension. Do that until the speedo matches as closely as possible to the GPS speedo readings. You will want to compare at different speeds to make sure it is as accurate as possible.


Or...

You could put in a GPS driven speedometer.

GPS Speedometer Thread

I have been running the GPS speedo for 7 years now and it has always been accurate and reliable.

Clay

Or drive the stock speedo with a GPS/mechanical drive:

https://www.speedhut.com/ecommerce/product/...Speed-Converter

I keep threatening to do that...or maybe I'll install one of Speedhut's GPS-driven speedos to match their tach I installed...

https://www.speedhut.com/gauge/GR338-GPS-01...edometer-120mph

Who was it that was selling the speedo drive aluminum block offs? I bought one some years ago for the race car...

One of my "must try that one day" projects is sourcing a decent GPS speedo as a core and adapting it to a 911/914 speedo housing. Much like the tachs I modify using the Bosch cores to get a more modern movement, different RPM range / fascia, etc.

The only "non period" aspect would be that all of the GPS units I've seen use an LCD display for the odometer - I could live with it though......

- Tony

As an Air Force guy, I don't want to be any more reliant on GPS than what we already are ... tunnels, cloudy days, wrong area for sat coverage and your needle will develop ED real quick lol...

Clay,
I do have a couple questions for you though. Are you saying 195/65-15 is still too small compared to the mostly undefined 165HR15 OE tire size? Also, is there anymore info on adjusting this tensioner I can look at?

-Van

Van,

A 165/80R15 (165SR15) tire has a rolling diameter of 25.4 inches, and turns 796 revolutions per mile. So if you want an accurate speedometer, any tire you put on the car has to have a rolling diameter really close to 25.4 inches.


A 195/65/R15 has a rolling diameter of 25.0 inches and turns 809 revolutions per mile. It has the lesser circumference for 31 mm (1.22 in), which is 1.5% lesser than the 165/80R15. The actual speed will be 1.5% lesser than the speedometer reading.In the relation to the 165/80R15., the 195/65R16 has the lesser overall tire diameter for 10 mm (0.39 in).

A 205/50R15 has a rolling diameter of 23.07 inches and turns 877 revolutions per mile. In the relation to the 165/80R15, a 205/50R15 has the lesser circumference for 185 mm (7.28 in), which is 9.1% lesser than the 165/80R15. The actual speed will be 9.1% lesser than the speedometer reading. In the relation to the 165/80R15, the 205/50R15 has the lesser overall tire diameter for 59 mm (2.32 in).

I know for a fact that makes a 914 speedometer off by more than 5 miles an hour.


All of the tire size diameter comparisons came from here: https://www.tyresizecalculator.com/tyre-whe...tire-dimensions


I would make a suggestion for you. Instead of opening the speedo and attempting to re-calibrate it, just figure out the difference between what it is reading and the actual speed of the car. I use a GPS speedo app on my phone to verify the difference. Changing the speedometer spring tension only fixes the calibration of the speedometer, not the odometer. Unless you change out the speedo gears in the transmission, your odometer will be off no matter what you do. And I have never heard of different speedometer gears for the 914/early 911. Porsche built all their cars with the same rolling diameter tires for years. That's why the 944 turbo has the same diameter tire as the 914 and early 911.


I hope this helps.

Clay

Dan

After a while you get a feel for what RPM you are turning in a particular gear for a particular speed. So if my GPS driven speedo stops working, I can get buy close enough to not get a ticket.

But when it is working, it is deadly accurate. And with the Cayman 6 speed transmission, there is no place to drive a speedometer on my car.

Clay

Copy all Clay, I've done these calculations. But I wanted to be sure it was my only real option before I replace four relatively new tires for four new tires, ya know?

Anyway, I think I'll test the waters on these new vredestein classic tires. Their 185-15 tire looks like the right one to bring my speedo to w/in 1-2mph of actual.

There is that. I took a 10-hour round-trip in the 914 last weekend and found myself using the front-and-center tach for speed (for my car a needle-width above 3000 was ~71-72 per Waze. Of course, then there's the Waze display...)

I'm personally not too worried about GPS taking a dive. I'm certainly not minimizing the concerns - overall society is gonna be in a heap 'o bind if the military is forced to degrade or restrict the signals - but I can work around quick/short-term losses of signal. - GA

"A 195/65/R15 has a rolling diameter of 25.0 inches and turns 809 revolutions per mile. It has the lesser circumference for 31 mm (1.22 in), which is 1.5% lesser than the 165/80R15. The actual speed will be 1.5% lesser than the speedometer reading.In the relation to the 165/80R15., the 195/65R16 has the lesser overall tire diameter for 10 mm (0.39 in)."

...based upon the above #'s and my fuzzy math, the 65 series tires should only cause the speedo to be off by less than 1 mph at 55 and only about 1.2 mph off at 75. I only bring this up because I would say, only anecdotally, that many folks here prefer a tire between a 55 and 65 series, many times based upon performance and availability of 15 inch tires, and with 65 series only being off by that much, it seems almost inconsequential. Granted the discrepancy gets worse as you head down to 60, 55, and 50 series

...perhaps my calculations are off, ...sometimes I do have to take my shoes off to count to #'s greater than ten.

Use the link that Clay provided above. Or try this one.

https://www.wheel-size.com/calc/

I prefer not thinking any harder than I have to...

My Porsche 996 was optimistic with OEM tires by about the same amount. Which meant, just like Clay said, for every mile rolled across the earth, my odometer logged an extra .02.

I’ll go with what I knew and you guys confirmed: get a tire that fits, but is also the correct circumference.

I swapped to a 944 speedo (replaced the cone shaped plastic cover with a flat glass cover).

My 225/50 x 15 wheels and tires are a perfect match for that speedo.

Of course, we are making a basic assumption:

That 50 years ago the speedometer were GPS accurate to begin with. They never were..plus the well known practice of having slightly optimistic gauges vs 100% accurate to account for period correct tire size availability.

If I.. maybe I've just settled fir being 10% high with my 195/60 14 vs 185-14s!

I currently have 195/65R15 tires on my 74 and the speedometer is off by 5mph. So when I am going about 80mph on the speedometer, I am actually going 75. 70 is 65 and so fourth. I may end up going with vreds in 165/80R15 at some point as my tires are due to be replaced by age.

Exactly the same on my 74 1.8L
I’m going to try the vredestein 185VR15 I posted above. A 165 is just too skinny for my taste.

FWIW, they’re made in Holland/Nederland, not China…

Running 225-50x16 and the heights is as close as I could get to stock, actually a little taller I believe.

My speedo also reads 5 mph faster than the GPS readings. on the highway is the only time I really worry about it. and since I already know it ... No problem.

Taking a trip I usually have the phone on, running WAZE, and that gives me a GPS speed reading and warning of Police, debris on road, traffic etc.

I only know of a few options correct your speedometer reading.

- Get tires with a rolling diameter that gives the reading you want.
- Get a cable-driven speedometer that gives you the reading you want.
- Use a GPS-based speedometer.
- Get ahold of an an old-fashioned rally odometer gearbox. You can change the number of rotations in versus the number of rotations out using different gears in the box. Change them until you get the reading you want.
- Have the speedometer recalibrated.

I don't think I would want to try recalibrating the speedo on my own. The speedo works by having the input spin a magnet, which induces eddy currents and therefore magnetism in an aluminum cup that is attached to the spring-loaded needle on the gauge. You either have to change how the magnet and cup interact (possibly changing the position or size of either or both) or change the tension of the clock-spring on the needle. Those are waaaaaay too fiddly for me to mess with, so I would probably take my gauge to Palo Alto Speedometer (who are local to me) and have them deal with it. Which would probably be expensive.

--DD

I've been running Michelin 205x65x15 on original 5.5" steel wheels for several years, and at 75MPH my speedo is only about 2MPH different than my GPS readings. Of course I'm not sure how accurate the GPS readings actually are?