I have a 1967 Chrysler Newport. 68,000 miles according to dash. My speedometer is too fast. If it’s reading 65 mph you’re really going 53 (according to my phone). I understand this is a more primitive setup than modern cars, so I was wondering if there’s a way to slow it down other than tire size or rear end gear changes. Thoughts? Unsure if it was inaccurate before as the tires are brand new, never drove on old tires. Also, thoughts on leaded gas substitute additive? Can’t get a definitive answer as to yes or no to leaded or unleaded.
The speedometer is cable-driven by a gear inside the transmission. In cars of that era typically the gear would be sized differently depending on the rear-end ratio.
Those speedometers were not all that accurate but should not be that far off. Possibly at some point the rear axle ratio was changed. The tires would have to be sized way different from stock to make that much of a difference.
There's been a lot of controversy over whether a lead additive is needed for older cars. The general consensus seems to be that it's not necessary in limited use vehicles not subject to strenuous service. If you do a lot of driving, go in for racing, or haul heavy loads then a lead substitute additive is probably a good idea.
I have not used such an additive in any of my cars that hail from the leaded gas era and not had any valve problems, but those vehicles are not driven much.
