Could suspicious tachometer readings be caused by a change in wheel or tire size?

Dear Car Talk

Dear Car Talk | Mar 01, 1995

Dear Tom and Ray:

I enjoy your column, but I think you goofed recently. You answered a letter from "Ken," who noticed that his tachometer was reading about 100 rpm higher than it used to at 70 miles an hour. Based on his information, you correctly ruled out a slipping clutch and told him the gauge had gone off slightly over the years and not to worry about it. What you neglected to consider is that engine rpm is sensitive to wheel diameter. The diameter of the wheel on my Ford Escort is about 57 centimeters, which gives it a circumference of about 179 Cm. If the diameter of the wheel were reduced to about 55 Cm, that would change the tach by 100 rpm at the 3,000 rpm range. Tire wear wouldn't account for that much of a change. Assuming Ken didn't get cheap tires which were smaller, my guess is his tires are low on pressure!

TOM: Gee, Bob, we hate to pick on you. But of all the letters we got suggesting "different wheel size" as a possible explanation, your letter had the best handwriting! The problem is, tire and wheel size does not affect the relationship between the tachometer and speedometer.

RAY: The tachometer simply reads the engine speed, so it's not affected at all by the wheel diameter. And the speedometer reads the vehicle speed--but not from the wheels--- from the output of the transmission. So no matter what size wheels he's got on there, Ken's tachometer speed should be the same for a given speedometer reading.

TOM: In fact, if the car were up on blocks, and it had NO wheels on it, the tachometer and speedometer would still have the same relationship.

RAY: The speedometer wouldn't be RIGHT, of course. If Ken were using smaller tires, his actual SPEED would be lower when the speedometer read 70 mph (he might actually be going 67 or 62 or whatever), but he'd never know that from looking at the gauges on the dashboard.

TOM: So we have to stick by our original, that the gauge had simply gotten off calibration over the years.

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