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Tire Math

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JFS

JFS

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My TR3 speedometer has been calibrated via a ratio box to read fairly accurately on 165 HR15 tires having a diameter of 25.6" and which rotate at 817 revolutions per mile. I am thinking of replacing them with 175 65 HR15 tires having a diameter of 24" and which rotate at 868 revolutions per mile. Does this mean that at 100 mph my speedometer would read 106 mph (868 divided by 817=106) and 53 mph at 50? Seems to me that the difference should be greater. The car would also stand 3/4" lower, which isn"t that bad, but I'm not sure how they would look in the wheel well.
 
Sorry I don't have a link but I think the Miata forum has a tire size calculator that really help me out when I had a similar question.....I am sure one of the Miata guys will pipe in with a link
 
I was under the impression that no math would be required for this course.

Actually, I think your calculations are sound. If you're not too particular about the accuracy of your odometer, that much speedo error can probably be adjusted out by relocating the needle.

In my experience these speedos' accuracy (or I should say inaccuracy) isn't particularly linear so all you can sometimes do is get them accurate for a particular range of speed that is important to you.
 
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...165 HR15 tires having a diameter of 25.6" and which rotate at 817 revolutions per mile...

[/ QUOTE ]

Okay, so I tried the math:

(5280 * 12)/(25.6 * pi) yields 788 revolution per mile
 
Raymond, thanks for the sites. The first one calculates to a 5.7 differential and speedometer reading of 64 mph at 60 mph. The second site only goes up to an 80 profile.

Geo, I can also change the gear in my speedo ratio box, but I don't know if it would be worth the expense (I put the ratio box in to compensate for my overdrive conversion).
 
Better keep a small old tire as a spare that will fit in the spare wheel well. It's a tight fit with a 165 R 15 tire.

You didn't say why you want to go to a 175 tire ???
 
I've found it easier to just check the speedo against a GPS at several speeds, figure out a factor, and then multiply by that. You'll quickly learn the numbers. For example, I have a motorcycle calibrated in kilometers, but also with an oversized tire. It's just a matter of multiplying any speed by 0.7 to get a very close reading of the actual.
 
[ QUOTE ]
I've found it easier to just check the speedo against a GPS at several speeds... It's just a matter of multiplying any speed by 0.7 to get a very close reading of the actual.

[/ QUOTE ]

A TR3 speedo (or most any mechanical speedo) can be calibrated in a couple of minutes w/o even getting out of the drivers seat. I use a GPS to determine the error, pull over to the side of the road and adjust, pop it back in and confirm or further adjust. IMO it's worth learning how to do this simple task.

Links on this page detail the repair, maintenance and calibration of Jaeger sppedos and tachs:

https://mywebpages.comcast.net/rhodes/
 
Don, I am considering the 175 tire because it is a modern tubeless design, H rated, significantly lighter and cheaper than the Michlin 165, and the smaller diameter will work fine with my overdrive.
Geo, thanks for the great link to the Jaeger speedos -- I should be able to recalibrate the speedo myself rather than paying the shop to change the ratio box gears.
I still have to decide whether to use the cheaper Khumo 165 SR or the Perelli 175/65 HR. I'm thinking Perelli at this point.
/ubbthreads/images/graemlins/smile.gif I admit to modernizing some aspects of my TR, but am having difficulty putting GPS and TR together in the same sentence /ubbthreads/images/graemlins/smile.gif
 
Geo, the math does not agree with rpm figures given in Tire Rack charts and the link calculator posted by Raymond. The calculator will not work without including the ratio aspect, but the aspect should have nothing to do with it. The math (63360/75.36) says the 175 should turn at 841 rpm, but Tire Rack specs says it turns at 868 rpm. Using math (788/841=106.7%) I get within 1% of my two previous calculations; therefore, the 175s should probably give an acceptable rpm for an overdrive TR3. /ubbthreads/images/graemlins/smile.gif I wonder if increased accelleration and reduced gas mileage will be noticable.
 
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