Actually, there are gears in most american cars to alter the spedometer and odometer performance for the use of different tires/wheel combinations. They still won’t reverse. I had a 1960 Austin A40 that would back off the odometer while in reverse. Very shortly after that it became very common for it not to work.
Tris
As a slight tangent/hijack/rant, the newer mechanical odometers were also virtually tamper-proof. In addition to not rolling backwards, if you opened them up and messed with the numbers, it would disturb the clockwork on them and the numbers wouldn’t line up any more. The only way you could change the reading was to actually turn the whole thing and, of course, it would only move forward so to roll one back you’d have to roll all the way forward, which would take quite a long time on a 6 digit odometer!
But then they started using the digital ones, in part because they were supposedly even more tamper-proof because they required exclusive dealer-only tools to change the reading on. Except that now there’s cheap generic versions of these tools that anyone can buy off the internet, so rolling back an odometer is easier than ever!
I think the only reason why there hasn’t been an epidemic of roll-back fraud is that mileage really doesn’t affect the value of a vehicle as decisively as it once did, so there’s less of a motive, and you’re a lot more likely to get caught because of things like CarFax.
If you’ve been in a vehcle on ice, you notice that spinning the tires cause the speedometer to go up even if you’re barely moving. I noticed this in a cab and wondered if frequently doing this upped the cabbies’ take. SO the pickup is between the transmission and the drive tires. Probably not on one whel to allow for differential…
A friend many years ago bought a clunky old truck (late 60’s); after fixing it up, we went on a road trip. He was amazed how smooth and quiet the ride was, until we stopped for gas 90 miles later and I calculated we had been doing 55mph not 70. I think the previous owner had among other things swapped the tires and rims for smaller ones.
Speedometer rollbacks have been a common complaint about used car dealers since the 60’s. I assume the requirements to make spedometers tamper-proof and difficult to roll back date back to those days.
Yes, there are several scan tools on the market that you can use to not only read your own OBD II engine trouble codes but to also adjust for changes in tire diameter, change the RPMs at which your automatic transmition will shift, and many other neat things.
Unless you are a car person it probably wouldn’t pay to buy one of these tools for one time use. Take it to a shop and they will plug into your OBD II data port, usually located under the dashboard about where your gas pedal is, and set the new ratio. The OBD II data port looks a bit like the printer port on a computer, it is usually located on the driver’s side of the car under the dash, because part of the sequence of doing the scan and clearing the codes, etc, involves turning the ignition on and off. Stick your head under the dash and look, it’s there.
If the shop starts talking about expensive alterations and new speedo gears, go someplace else. This is something that the local tire shop should be able to do without any problem when you get bigger wheels.