Tire sizes and Speedo Correction
 

I'm redoing my speedometer and would like to replace the gear/s so that I get the right reading with my new tires.

The only info I can find on the OEM equipment is that the wheels were supplied for 165 SR 15's - which is fine BUT what was the tire height?

In order to calculate the differential between the original wheels and my new ones - I need to know the overall tire circumference. (I can calculate it ok but I need to know the height)

Does anyone know either the OEM wheel height or the original tire circumference?

I'm putting new 195 65 R15's on in a few weeks. (bye bye 165 rubber bands!)

Thanks

55 was the stock tire height I believe.

I always ran 205/50/15's on my old 914. I had North Hollywood Speedometer re-calibrate my speedo for the tires. However i think with that tire size, it will read about 5MPH off.

After I put the 205/60/15's on my car, I did something a bit different with the speedo change - I don't look at it. Fixed the whole problem.

I figure my tach is more important anyway! ;)

Don

The aspect ratio for the 165 is 78 or 80.

195/65/15 is quite close to the original diameter.

The original speedometers were never very accurate, so you are close enough to be in the original range of error.

there are several tire size calculators on the web that can directly compare two tires, and tell you what exactly the size differences are. I think there is one that even gets into gearing and speedometer differences too. just do a quick google search.

I read somewhere that the stock 155 tire profile was 82, translating to a 25" tire diameter.

The easiest thing to do is measure the roll-out of the rear tire and then use the GEARS.EXE program I wrote that is on the PP site here to see what happens to RPMs and speed. Mark the sidewall where it is close to the ground and the ground, then push the car to roll the wheel one rev till the mark is in the same location and mark the ground. The distance is tire circumference, very accurate.

the stock outside diameter was a little over 25 inches.

On a 4 cylinder: 195x65x15 will not get you the right reading. Nor will 205x60x15. I have tried both of these combinations and measured them against my GPS unit. The speedo will read high.

Anyway, the 195x70x15 will give about the best correct speedo reading. 205x65x15 is also very close.

The speedo-drive drive-ratio on the 914/6 is different from the 4 cylinder. The little worm gear in the tranny has a different part number.

James

James, I got news for ya--155R15 and 165R15 tires will also result in a high speedometer reading. Porsche speedos are almost always optimistic.

--DD

You can download a tire size calculator program from here
http://members.rennlist.com/chuxter/TireCalcAd.html . The aspect ratio of the stock 165/15 tires is listed as 82 there but I don't know how accurate that is.

Alfred

Before I saw this post this morning I was reading a back issue article from R&T on the 914. Part of the article was on the speedo error with 165SR/15 tires.
They state...
At 30 mph speedo indicated actual 30 mph
40=39.8
60=59.2
70=68.3
80=77
100=94.7
Odometer stated 10 miles actual was 9.8
bruce

Stock tire on the 914-4 seems to have been 165-82-15.

Assuming this is correct then the stock tire was as follows:

25.65” diameter
80.541” circumference
801.37 revolutions / mile

Because variances in tire pressure, tread height, diameter from different tire manufacturers and especially when the actual accuracy of the speedometer is questioned – this is not an exact science.

However I have extrapolated the following information from http://www.miata.net/garage/tirecalc.html

Tire Size (TS)
Diameter (D)
Speedometer Reading % ERROR (%E)
Actual speed when Speedometer reads 60 mph (AS)


(TS) (D) (%E) (AS)
165-82-15 25.65” 0 % 60 mph
165-60-15 22.8” 12.3 % fast 52.6 mph
165-65-15 23.4” 9.8 % fast 54.1 mph
185-60-15 23.7” 8.7 % fast 54.8 mph
185-65-15 24.5” 5.9 % fast 56.5 mph
195-50-15 22.7” 9.9 % fast 54.1 mph
195-60-15 24.21” 6.9 % fast 55.9 mph
195-65-15 24.98” 3.9 % fast 57.6 mph
205-60-15 24.69” 5.1 % fast 57 mph
205-65-15 25.49” 2 % fast 58.8 mph
205-50-15 23.1” 11.3 % fast 53.2 mph

Cheers,

Rob

Depending on which tires and who you listen to, the original aspect ratio was either 78 or 82. Most people just assume 80 as "close enough".

For more specific data, you can check the tech data for specific tires in a given size. For instance,
Firestone F-560 --
Size: 165/0TR15 OD: 25.4" Rev/mi: 816
Kumho Euro Metric 771 --
Size: 165/0TR15 OD: 25.4" Rev/mi: 818

Those were the only two 165R15 tires that the Tire Rack showed me today...

Personally, I run on 195/50-15, and live with the speedometer being very optimistic. I will probably change over to 195/65-15 when I get dedicated street tires, mostly for fuel economy... And I will live with however the speedo reads.

--DD