"Correct" Tires Sizes

[red-beard]

OK. With all of this talk...

What are the tire sizes which will produce correct speedo readings?

I've also heard that the "original" tire size was 165/80R15. Was this true for all years and Models? I've heard that some of the later years with the smaller engines came with 155/80. I'm interested, because I'm about to buy 205/65R15 for my 1970 914-6. I'm tired of having the speedo read wrong!

My 1973 came with 225/50! These are off by over 6%
The 1970 came with 195/60 and these are off by almost 5%

James

[Jim Chambers]

165x15 is 25.4" in diameter, 6.3" cross section on 4.5" wheel (stock is 5.5" wide but couldn't find that combo).
205x65 will be perfect for speedo, 25.5" diameter but I think too wide at 8.2" cross section on 6" wheel.
195x65 will be closest low profile at 25.0" diameter and 7.9" width. I have these on stock 5.5" wheels and they rub in front at full lock. Best way to research is to go to www.tirerack.com and click on "tires", enter size you want to check out and when tires come up click on "specs". When you see diameter of 50 and 60 series tires you will see why your speedo was so far off.

[Dave at Pelican Parts]

On early cars, I believe that 5" wide rims were the "stock" ones, and the 5.5" ones were optional. (And very very few 914s drove off the dealer lot with those wheels--they were purely to keep the base price down, and all the other wheels were spelled "extra profit".)

On those wheels, I believe that the stock tire size was 155HR15. There is a factory part number listed for a speedometer calibrated for 155HR15 tires. The rest came with 165HR15, except for the 914-6es that came with the optional 14" Fuchs wheels. I don't know what size those came with; probably 165HR14.

--DD

[red-beard]

"Ya made me look"

I do have the 14 inch Fuchs. So I can't use any of the 15 tires I own on this thing....Sigh...

Maybe I'll know in a couple of days, as I'm getting an original owners manual.

What did they do at the dealer ships? Keep a giant stock of Differently calibrated Speedo's?.

James

[red-beard]

Opinions

I have 195/60R14 on the car now and I'm off by 5%, so _anything_ will be an improvement.

205/65R14 - Almost perfect for the speedo , but very wide.

195/70R14 - Reads about 1.5% "low"

195/65R14 - Reads about 1.7% "high"

185/70R14 - Reads about 0.8% "high"

This car is going to be a daily driver, driven a little aggressively.

Opinions

James

[Jim Chambers]

Is your car a 6 or a 4 converted to 5 lug wheels?
Regardless, if you want stock and accurate speedo you need overall diameter of about 25.5". 185's are available but not in high performance tire. Primarily a small pick-up size now. Several brands of high performance 205/70x14 are available and they are about 25.5" in diameter. Most 914's will take a 205 but no wider.
914 six with 5.5x14 Fuchs came with 185x14 tires, diameter 25.6".
I believe 911's with 6" Fuchs came with 185/70x15 which had same diameter as 165x15. Tire Rack does not stock 185/70 so I can't check diameter for sure.

[bowlsby]

As good as analog German speedo/odos must be, I doubt they are truly accurate to anything closer than a few percentile of a laboratory calibrated standard, given all perfect variables for tire wear/over inflation, and gearing. Especially with 30+ years of wear and tear and dirt and friction build up.

If a tire size effects the actual displayed value by an extreme of 5% plus or minus (probably 1-2% as noted above), depending if the tire is larger or smaller than the OEM spec, it would mean:

A real 65mph would indicate 62-68 mph on the speedo.

A real 30mph would indicate 28-32 mph on the speedo.

A real 10,000 miles would read as 9,500-10,500.

Trivial under most circumstances.

[red-beard]

I have a 1970 factory 6 with 14" Fuchs.

Started looking at Tirerack: Uggh. Nothing in the 205/65R14.

James

[Dave at Pelican Parts]

I have 195/60R14 on the car now and I'm off by 5%, so _anything_ will be an improvement.

This car is going to be a daily driver, driven a little aggressively.

Opinions???

Opinions--I got a lotta those.

In my opinion, the 195/60-15 is a very good tire, and there ain't nothing wrong with using it. 195/65-15 is also a very good tire, and if you can find a better tire in that size go for that.

Shorter sidewalls give you:
Lower gearing for better acceleration (good)
Shorter stiffer sidewalls for more crisp response, better feedback, and better lateral grip (good)
Lower gearing so you have to shift more often (bad)
Speedo and odometer will read high (bad-ish)
Shorter stiffer sidewalls which ride worse (bad)

All in all, I don't think the downsides are very big. Then again, I'm the idjit that runs 195/50-15 tires on stock wheels... (Though when I go to separate street & race tires, the street tires will be taller.)

--DD

[Jim Chambers]

If you are going to stay with 14" you must use 195/70 (23.8") or 205/70 (25.4") if you are going to stay close to stock diameter. Several brands available at Tire Rack.

[red-beard]

Well, the first thing I'm going to do is "calibrate" the speedo: Use a known good speedo vs mine. Then I'm going to measure the tire and calculate the size needed to get the "correct" speeds.

If I go with the 195/70's, I think I've settled on the Yokohama AVIDs.

Anyone have a slightly used pair of 15 inch Fuchs?

James

[piperpilotduane]

I've had several 914's. I dont think I've ever had one with an accurate speedometer. Try a GPS.

[red-beard]

I took the 914-6 out yesterday and "played" with the guy next door with has 1986 911. My speedo is reading 10% high. I have a feeling I have a 911 speedo which is setup for the 15 inch wheels. I'm still going to meaure the tires and then find something that will make the speedo accurate.

Or find some 15 inch wheels. No one makes performance tires in for 14 inch wheels....

James

[SummerSledSix]

When you guys are out tooling around on the highway, keep an eye out for a "measured mile" sign. That is just what it says, an accurately measured mile. Stay at 60 mph and time yourself between signs. A 60 second trip represents a 100% accurate speedo, you can calculate error from there. Of course, if that semi behind you is bent on doing 80, you might want to come back and try it later...

[JP Noonan]

I put a VDO "Cockpit" series electric speedo in my 75 with the 901. Had to rig a GM VDO sender to the 914 output on the trans, but I have an absolute AND adjustable speedo. If I change tires I can either enter the rotations per mile, or use a measured mile, or put in the frequency. Very easy.

GO into the archives, I posted the whole proceedure about 2 years ago. I've got close to 7000 miles on the speedo with no problems.

[red-beard]

I measured my tires. I checked the diameter: 22.9

Using the Palo Alto site, I figured that the Speedo in my car must reference the the original 165/R15's. One mechanic I talked to said that the 914/6 came with a 75 series tire (Maybe a 185/75R14).

No matter.

_IF_ I can get a hold of a set of 15 inch Fuchs, I'm going to go with the 205/65R15's. Or I can always steal the set of 215/60R15's on the 1973...If they will fit under the wheel well!

James

[Jim Chambers]

Stock 914/6 tire on 5.5x14 Fuchs is 185x14.
Stock 911 of same era on 6x15 Fuchs was 185-70x15. This size no
longer available. Closest is 195-65x15. If you want speedo accuracy 205- 65 will be too big (and might not fit) and 215 is way to big. You want 25" diameter, or real close to it, and 8" or less cross section.

[red-beard]

The 205/65R15 is 25.49 diameter.

Stock tires were 165/80R15 which is 25.39 - Is this correct?

If the 914/6 was 185/70R14 it would be 24.92

I'm running 205/60R15's on my 1971 with Pedrini wheels. They just barely fit in the wheel well.

195/65R15 would be 24.98 - Almost perfect.

Hmmmm.

195/65R15 looks like it would be perfect! And they have the Yokohama AVS db in my size. Now all I need is the 15 inch Fuchs!

James