Caption This for 3/4/16

Re comments about over-pressure or under-pressure, or tyre wear. How does this affect the speedo or mileage.

I can guess, but I have not seen this mentioned in driver manuals or service manuals.

You would have to measure the difference in circumference first. Then its a simple equation....

Furthermore, all speedos are allowed to show a higher speed than the actual, but not a lower.

I have checked many cars speed against my GPS, its rare than one is correct, they usually show as being up to 5% faster at almost any speed.

As the tyres wear, the difference increases.

The GPS, as long as the road is fairly straight, will be between + - 1 KMH accurate according to Tom Tom.

I hope this helps!

Yes thanks. Andy. Satnav speeds in my car equate to the speedo being about 10% slow. ie, 70mph satnav is reached when the speedo shows 77mph. As you say, speedos read low, and logically lower when running soft, or when worn out.

Measuring the circumference of a flat tyre itself is not easy. I think it would be easier to measure the ground distance the car travels for one rev of the wheel.

Considering speeding is a crime, you would think more attention would be paid to speedo accuracy.

OK, we see that differently, I see that as the speedo reading "high", but I think we are both on the same page I feel.

The law states in the UK (paraphrasing) that + up to a percentage (20% when I was a kid, but 10% nowadays I believe as they got better and the EU stuck their oar in!!), but "-" nothing!

Your method would be better way to measure the circumference, I actually did that when calibrating my bike speedo......its also the simplest!! Thanks for the reminder!!

By the way, you can get car speedos re-calibrated, here the AA equivalent do it for very little money....though its been many years since I had one done. It may not be so cheap anymore!!

Is it really a crime over there, or more of just a moving violation. Over here we think of crime as something a criminal would do, just as disturbing a pet behind a fence on purpose, or breaking and entering.

I always thought speed limits were just advisory, certainly seems that way when driving here.

I speak as a driver - not a lawyer.

Speeding is a crime here in the UK, inasmuch that you can get fined or banned, but not prison, unless linked to serious consequences like dangerous driving resulting in damage and injury or death. And you have a criminal record.

Well, pardner, it does affect my speedos at all, since I never wear them while driving. Only when swimming, and I want to (not) impress a girlfriend.

LOL!!

My 1963 Ford Falcon station wagon said to put 22 pounds in the fronts and 32 in the rear. I placed a slightly smaller tire on the front and ran them at 24. The car had a rear weight bias and got real good traction in extreme conditions.

I had heard that bicycle tires could blow off the rims from heating up while riding the brakes going down mountain passes. I thought it was a rare event until it happened to me.

But there were some extenuating circumstances. The wheel had come out of tune causing extra friction. The wheel had some damage which might have allowed the tire to slip off. And I had higher pressure tires than previous descents.

Previously I had lower pressure tires which may have been able to handle the extra heat. Other times I was riding an older bike which had heavier rims with more mass to absorb the heat.

I also found that I was descending wrong. I was streamlining to cause as little drag as possible. It's suggested to maintain an upright position and let air drag do more of the work.

Recently I was did a slow descent on a hill and checked the rim temperature. They were almost too hot to touch. When I did this on an older bike they were warm, but not hot.

Then I did this on a mountain bike coming down a mountain trail. I ahd to stop several times to keep my hands from cramping from riding the brakes and I would check the rims. They got fairly warm but not too hot. But then the mountain bike rims have quite a bit of mass for strength.

Good point, well controlled.

Sounds like a good reason to have disks on your bike......they have been available for some years, but whether they will fit on any wheel, I have no idea at all....I suspect that new wheels may be needed.....

I read a story, can't find it now, about a guy who built a bicycle in the way it would have been made for the 1st Tour De France. That involved fixed gearing, wooden tyres & brake blocks made from wine corks. He then rode one of the downhill routes, scary stuff.

Talk about scary, I read a narrative about riding a penny farthing without brakes down Mt Washington in New Hampshire. The operator had to run into the brush to stop!

I just discovered one of my van tires is down to 25 pounds. Looks like it's time for a workout.

Redneck repairs on a redneck truck.

There may be some mandatory overtime occasionally.

Then there was the night I "blew up" city hall. I was attending a Bicycle Advisory Board meeting. Several of us bring our bikes into the meeting room It was Winter. About halfway through the meeting there was this loud "BANG". We were startled ar first but quickly realized what had happened. We looked around at our bicycles. It was was my antique roadster that had the flat. The tire had slid off the rim. We attributed it to coming into a heated space from frigid temperatures.