Proper Tire for Speedo

[SAWBA]

Was the stock tire size for the 1970 914's 155SR15, and the option tire size 165SR15?
Would the stock tire size be used when calculating a new tire size? I hear people use 205/60R15 and 205/50R15.

I currently have 205/55R15 on my 1970 914 mostly because the tire specialist said the stock tire was 155SR15, and 205/55 is what his chart in the back shows to best match the speedo.

Can anybody confirm if I do have the proper size tire? If I don't how far off will my speedo be?

Does anybody know what brand the original tires were? My spare is a Continental 165SR15 mounted on the original steel wheel.

Thanks in advance.

[derek_sf]

use this page to do your calculations...

http://www.paspeedo.com/calculator.htm

[Dave at Pelican Parts]

You have the original sizes correct. 155SR15 was standard, and 165SR15 was optional. That's on the early cars. I'm not sure how late that goes, though, and I think many or most cars were delivered with the 165 tires. Not sure how many of those may have been dealer-installed. There is a different part number for the speedometer that goes with 155 vs. 165; you can check the back of your speedo. I'll try to dig out the part numbers when I get home.

The practical difference to your final tire size calculations is probably fairly small.

--DD

[pbanders]

Huh. Here's something interesting from Dunlop

http://www.dunloptire.com/tt101_sizing.html

They claim on this page that all the old sized tires like the "SR" tires had an aspect ratio of 82%. This would mean that a 165SR15 would have a sidewall height of 135.3 mm - so a 205 width tire would need to have an aspect ratio of about 65 to be correct, not 55 as your tire guy said.

Brad Anders

[DDS]

http://toy4two.home.mindspring.com/tire.html

I've been using this one and came up with 634mm dia. The link to tirerack is handy.

Dave

[This message has been edited by DDS (edited 06-24-2001).]

[SAWBA]

Brad,

Keep in mind the tire specialist came up with 205/55R15 based on the original tire size of 155SR15, not the 165SR15.

[John Rogers]

The most accurate way to see what is happening when you change tires is to mark the side of the tire and roll it one revolution on the car. Measure the distance to the nearest fraction of an inch. Then download the Gears.exe program I wrote from the PPP site hear and enter the trans ratios, final drive ration and tire circumference ( or dia or revs/mile). A slider will show RPMs of each gear related to speed in MPH. Good luck.

[pbanders]

Originally posted by SAWBA:
Brad,

Keep in mind the tire specialist came up with 205/55R15 based on the original tire size of 155SR15, not the 165SR15.

Whoops, you're right. But, it just makes his suggestion worse. A 155 at an 82% aspect ratio has a sidewall height of 127.1 mm. For a 55% aspect ratio, this works out to a tread width of a bit more than 231 mm. A 205 would need an aspect ratio of a bit more than 60% (62%, actually) to be right.

Brad Anders

[Rob-O]

Or...and this is infinitely more fun than measuring to the nearest fraction of an inch, then multiplying by your final drive ratio, etc.

Hop into the 914. Grab a friend with a stop watch. Find the nearest open freeway. Accelerate up to 60 m.p.h. As you pass a mile marker start the stopwatch. Hold that speed steady!! As you pass the very next mile marker, the stop watch should read 60 seconds. If it reads anywhere above, like 62 seconds, You're travelling slower than the speedometer is reading. If it reads lower, like 58 seconds, you're travelling faster than the speedo says. You may want to keep this up for, oh, say, forty to fifty miles just to be sure you're getting an accurate read!

[SAWBA]

Dave,

Have you been able to determine the part numbers for the 155 and 165 speedos?

If my speedo is calibrated to a 165 tire, I calculated my actual speed to be 92.8 if my speedo read 100. Based on 155 calibration, I was be doing 95.2 on a speedo reading of 100.

Based on everybodies input, it looks like a 155 tire converts to a 205/60R15. A 165 tire converts to a 205/65R15. A few tire specialists still insist on a 205/50R15 and 205/55R15, although the numbers don't add up.

Since I currently have 205/55R15's that fit with no problem, should the other tires fit without any modification?

Thanks.

[bowlsby]

Most 914 owners seem to prefer either the 205/50's or the 195/60's on 5.5" rims

Some like the 195/60's because the 205's seem too wide for the rims with lots of sidewall flex, but there is more tread area on the pavement with the 205's, and I believe that the 205's are nearly a perfect match for stock tire diameter for speedo accuracy.

[Bleyseng]

hmmm, there is no way a 205x50 is the same as stock tire of 165SR15. I just went and measured the old spare 165SR15 and it's 25 inches across and 79 1/2" around. The 205x50x15's I am running are 22 1/2" across and 73 1/2" around. I know my speedo is way off since I switched from 195x60's. My guess is about 10 mph at 60mph, so it read 70mph.
Geoff

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76 914 2.0L

[DDS]

Base size for comparison is 165/80-15, dia around 640mm
Source is in my post above.

Tire Size Side Wall Height Tire Diameter True MPH when speedo reads 70 Percent Error Tire Revs per Mile
155/80-15 124 629 68.26 2.48 814.44
155/75-15 116.25 613.5 66.58 4.88 835.02
155/70-15 108.5 598 64.9 7.29 856.66
155/65-15 100.75 582.5 63.22 9.69 879.46
155/60-15 93 567 61.53 12.09 903.5
165/75-15 123.75 628.5 68.21 2.56 815.09
165/70-15 115.5 612 66.42 5.12 837.07
165/65-15 107.25 595.5 64.63 7.67 860.26
165/60-15 99 579 62.84 10.23 884.78
165/55-15 90.75 562.5 61.05 12.79 910.73
195/65-15 126.75 634.5 68.86 1.63 807.38
195/60-15 117 615 66.74 4.65 832.98
195/55-15 107.25 595.5 64.63 7.67 860.26
195/50-15 97.5 576 62.51 10.7 889.38
195/45-15 87.75 556.5 60.4 13.72 920.55
205/60-15 123 627 68.05 2.79 817.04
205/55-15 112.75 606.5 65.82 5.97 844.66
205/50-15 102.5 586 63.6 9.15 874.21
205/45-15 92.25 565.5 61.37 12.33 905.9



[This message has been edited by DDS (edited 06-25-2001).]

[Dave at Pelican Parts]

SAWBA, thanks for the reminder!

These are from the factory parts manual. They are reproduced pretty much exactly as listed; when there is no tire size specified in the list below it's because there isn't one specified for that part number in the book.

911.641.501.00 km/h speedo for 914-6 w/165x15
911.641.506.00 MPH speedo for 914-6
914.641.502.10 km/h speedo for 70-72 1.7 w/155x15
914.641.503.11 MPH speedo for 70-72 1.7 w/155x15
914.641.504.10 km/h speedo for 70-73 1.7 w/165x15
914.641.504.20 km/h speedo for 74+ 1.8 & 2.0
914.641.504.30 km/h speedo for 74+ 1.8 & 2.0, not for Japan
914.641.507.00 km/h speedo for 74+ 1.8 & 2.0, for Japan only
914.641.505.20 MPH speedo for 70-72 1.7 w/165x15
914.641.505.20 MPH speedo for 70-73 1.7 & 2.0
914.641.505.20 MPH speedo for 74+ 1.8 & 2.0

--DD


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Pelican Parts 914 Tech Support

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