I’ve seen this a lot in older vehicles. After 10-15 years, the speedometer reads abou 10-15% faster than the car is actually going. If it’s going to be inaccurate, I guess it’s better to read that way because it won’t be earning you any speeding tickets, but why does it happen?
At first I thought the speedometer cable was just wearing out or getting gummed up and sticky, but that would result in giving a slower reading, not faster. So what’s up?
Tire wear?
I thought all speedometers are calibrated to be about 10% over. Are you sure it’s an age thing?
I haven’t done math in 23 years (apart from the bit where I subtracted 18 from 41 just now). If a car uses a 16 inch tire and has 1/2 inch of useful tread on the tire, what is the speedometer difference between new & bald?
The car that prompted the question is a MR2. It’s a sportscar. I’d assume the speedometer for such a gizmo would be calibrated to be as close to “dead on balls accurate” as possible. It’s got the right tires with good treadwear, but it tells me 90 mph when I’m really doing 75 (according to a new car’s speedometer–closed course, professional driver, etc.).
Circumference is proportional to radius, so the ratio of 16/15.5 gives a 3% overspeed.
The 16-inch figure probably represents the wheel size, as this would be remarkably small for the actual diameter, and really large for the radius.
Googling suggests that the MR2 can use 185/45R16 tires. This site explains how to calculate tire diameter, which in this case works out to around 23.7". If we assume a half-inch of usable tread (probably a bit optimistic) the ratio would be 11.85/11.35. which represents around a 4.4% overspeed.
How is it you dudes put a heavy car on a pneumatic tire on a road and the tire doesn’t deform? Wouldn’t that have an effect on your calculation? What’s the radius after that?
And how can you spin a wheel and not consider how it expands?
Um…the centrifugal expansion of the tire compensates nearly for the deformation of the curb weight of the vehicle against the average radius of the tire. Maybe. 
But there’s still a lot of inaccuracy in the speedometer that the treadwear isn’t accounting for.
Neato. How conveeenient. 
Me don’t think the speedo is a purely cable-driven device in the MR2.
If it were, it’d be hard to squeeze out that kind of inaccuracy. It can’t be purely digital/electronic obviously, since there is an analog needle…
…so maybe some sort of motor and electronics run it?
More than a steady drift up or down, I have seen many old cars with dancing needles that just shake around 20-30 times per minute in a 5-10 mph range. What’s up with that?
As for tire wear, the OP never said that he was still using the same tires after 10-15 years.
Worn/skipping gears in the speedo assembly, Sapo.
The stranded metal speedometer cable starts to wear and fray and the frayed parts hang up on the sheath that surrounds the cable. This usually results in a broken cable after a while. It’s usually worse at lower speeds for some reason.
I have a 35 year old car that started to do that. One day, I pulled the speedometer out and shot some silicone spray lubricant down the inside of the sheath and haven’t had a problem with it since (if you try this, wear some sort of safety glasses…on my first attempt, the spray reversed direction and came right back in my face).
Sorta related question: I had to get my car’s check engine light bulb replaced for it to pass inspection. When I got the car back, the speedometer and odometer were non-functional. I took it back again, and when I got it back the second time, the speedometer was about 5mph too slow and the odometer was off by 750-1000 RPM, or something along those lines. I didn’t feel like trusting that garage with my car a third time, so it just stayed like that.
Anyone know what they might have screwed up?