[pnyxps66]

I have reviewed the forums on speedometer issues and still have a question from a different angle. The setup is a 1988 LX coupe 2.3L NA engine 5 speed borg warner Tranny. It was replaced in kind several years ago, and the car has seen an abundance of idle time (not running) in its lifetime.

I also changed the factory 195/75R14 tires (height 25.52") with 225/60R15 tires (height 25.63") off of a 1986 mercury capri GT.

I have noticed a 10 mile per hour lag in my speedometer, and it hops until about 25 miles per hour. This was occurring before I changed out the tire sizes.

I just replaced the original speedometer and the speed sensor. The gearing in the Tranny is both black on the drive and driven gears. No damage was observed on either gear when rotated. The rear axle is the factory standard, I have the ratio written down in my manual.

I was wondering if it would be possible that the speedometer itself is damaged and causing this issue.

Any ideas? Thanks for your help and insight.

[mg man 75]

This may help decide the correct gear for the cable into trans. Speedometer Gear Selector

[DBT4220]

different tires also cause speedo problems. like sizes too big or too small.

[pnyxps66]

Thanks folks for the info. One note though, the factory specs for the tires are either a 195/70R14 or a 205/60R15 for the V8 mustangs. With the information I have, the actual tire heights for these tires are similar, and within the range of the tires I have on the car. I made sure to keep the overall size of the tire within the factory scope precisely to avoid this problem.

Tire size is irrelevant if the overall diameter of the tire does not deviate from the factory specifications. I checked with Ford and for my setup - LX model with T-5 transmission, the gears are the same whether it has the 14" or 15" wheels on it.

Additionally, the needle on the speedometer oscillates while travelling from zero to 15 then again from 20 to 25 mph then smooths out. I am wondering if the speedometer itself could be the problem.

I removed the factory speedometer cable and speed sensor from the car, so I know that is 20 years old in itself. The gear on the sensor is not damaged at all, and neither is the gear in the transmission.

Any ideas/suggestions on checking or repairing the speedometer itself?

Thanks again for your input.