1995 Honda Civic h/b tire size puzzle
 

Background: Recently swapped a CX tranny into my DX. Before the swap, odometer was registering 2.5% too many miles. Per Firestone specs, my F380 tires (size 175/70/13) spun 919 revolutions per mile. Tires only have 9,000 miles. After the swap, odometer registered 2.6% fewer miles than actual with the same tires.

I was looking to buy new LRR tires (Sumitomo HTR T4) b/c the Firestones are not LRR and are better saved for the winter due to their great traction. I figured I could correct the odometer error by purchasing a tire that spins about 943 revs per mile (919 x 1.026) and was going to buy size 165/70/13 which is 949 RPMs per Sumitomo specs. I decided to purchase size 175/70/13 at 927 RPMs instead because they were on sale for $34.99 each versus the 165s at $48 each. I estimated these tires should cause the odometer to read about 1.6% fewer miles than actual.

When I checked the odometer for accuracy w/ my GPS, I discovered the new tires are causing the odometer to register 4% fewer miles now instead of 2.6% fewer miles with the old tires!! I measured the diameter of each tire, and they are within .002% of each other (old is 71.25 inches, new is 71.4375 inches). I can't figure out why the discrepancy got so much worse except perhaps the new tires are less flexible and therefore turn fewer RPMs than the old tires.

Any insight??

https://www.miata.net/garage/tirecalcold.html

:) this is the best i can do. also dont forget that tire pressure can account for a lot more than .002%

The tire size calculator in that link doesn't help in this case because both the old and new tires are the same size: 175/70R13. And as I mentioned before, I measured the diameter of each with a tape measure and they're within .2% of each other. Same tire pressure in both tires. So, unless the old tires are much more flexible (which would cause them to turn more RPMs than the new ones???), I can't figure out why the odometer accuracy has gone from 2.6% less than actual to 4% less than actual.

ok so you swapped trannys now your speedo/odometers off...its either the electronic speed sensor is different between the 2 trannys or if thier a cable driven spedo then the little gear running it is different.

if it is electronic then one might have been on a tolerance level one end of th spectrum and the other on the other end. (usually if its within 2mph then its within tolerance)

then you said it yourself. i cant think of any other explanation, other than tire flex while the cars weight is on it.:(

If it's really bugging you, you could put them back on the car, measure the rolling diameter while in use, and see if you agree with the manufacturer as far as revs per mile.

Ooo, ooo, I figured it out, maybe. The spec is for a tire at full tread depth. you were comparing worn old tires to full tread new tires. That makes a pretty stout difference, esp in a small tire like yours.

I don't think that would make a difference. The tread is steel belted... It's not going to get shorter or longer, and the tread isn't going to gradually drift around the tire relative to the bead and rim.
Think about a tank tread. Each segment of the tread covers a fixed distance when it's layed down under the tank wheels. There's a fixed number of segments in a given tread. Now imagine marking a preticular segment, and counting each time it passes as a revolution. Since there's a set number of segments in the tread, and thus per revolution, and each segment covers a fixed distance (assuming the tread doesn't slip on the ground), the tread will cover a fixed distance per revolution.
Now immagine taking that same tread off the tank, and erecting it into a perfect circle, like the tread of a tire. The number of segments or the distance they each cover hasn't changed, only how many of them are touching the ground at once. So, the distance per revolution must still be the same.

thats why im saying the sensor is within a different tolerance... i swapped a spedo out of a 83 chevette into my 80, same tranny same wheel sizes and it reads 3 high going 55...its just the little clock spring on the needle shaft is a little bit weaker than the old one...

same thing is happening to your car. so if its relaly a huge concern for you swap in a new speed sensor and i guarentee its probably going to read different from your other 2.

btw you should have done several test runs with your gps. i have a handheld one and cloudy days it isnt as accurate...(still close but off a tad)

my point is, car tires are not tank treads, a tanks treads are driven by a gear, a cars treads are driven by the sidewall, which flexes and varies from tires to tire. all tires allow slippage in the treads, especially during acceleration and deceleration and different tire designs allow different amounts of slippage.

i am not too sure how to quantify it, i just know it happens. i also know that the shorter the tread, the less slippage is permitted, depending on the compound.

"If it's really bugging you, you could put them back on the car, measure the rolling diameter while in use, and see if you agree with the manufacturer as far as revs per mile.

Ooo, ooo, I figured it out, maybe. The spec is for a tire at full tread depth. you were comparing worn old tires to full tread new tires. That makes a pretty stout difference, esp in a small tire like yours."

Bill, the old tires (Firestone) only had about 9000 miles on them, so tread depth shouldn't make a huge difference. Besides, I measured the circumference of the old and new tires, and they were within .2% of each other. This is why I'm perplexed about why the odometer is now registering 4% low w/ the new tires when it was only registering 2.6% low with the old tires. It must be the flex in the old tires causing more RPMs.

"ok so you swapped trannys now your speedo/odometers off...."

Vette, the speedo is actually precise now, whereas before the tranny swap it was about 2 - 3 mph high at 60 mph. The odometer changed from being 1.6% high to 2.6% low. I figured the new tires w/ a claimed 927 RPMs versus old tires w/ a claimed 919 RPMs would make the odometer a little more accurate...like closer to only 1.7% low instead of 2.6% low.

I did not swap my speed sensor from the DX tranny to the CX tranny because the CX tranny had a broken bolt on it, and I didn't want to mess around with it. I may swap them now to see if it makes a difference. It certainly can't get any worse than 4%!