Disco experts,

I’m trying to figure out if my speedometer is accurate. On the highway the speedometer says 70 mph, the Ultragauge says 64 mph, and my phone’s GPS says 66 mph. I’m surprised the speedometer and Ultragauge give different values because they’re both getting their info from the vehicle. I’m on standard 235/70R16 tires - exactly what’s in the manual.

Question for the community - where do the speedometer and Ultragauge derive their speed data? Which would you trust?

 

I think your phone GPS is probably the closest

 

Maybe use a stop watch to time 1 mile intervals, since 60 mph = 1 mile per minute (straight road with little to no traffic). Time more than 1 mile for best accuracy. Agree with Harv about the phone GPS for accuracy.

 

Most vehicles read a few miles faster than actual speed to discourage speeding tickets, I'd trust the GPS more than the speedo or the Ultragauge. The most accurate would be using a cruise on a flat level road, over a marked distance and a stopwatch, tough to do but I'd use about a 5 mile distance to take out any inconsistencies. I have compared my speed to those police units parked by the road that display the speed and my speedo is within about 2 mph (slower) than what the sign says. close enough for my needs.

Maybe tire pressure or tire wear could make a difference but it shouldn't be much

 

my D1 with stock everything runs about 4 mph over based on millions of different road radar signs - always the same regardless of 49 state radar signs

 

Thanks for the comments. I timed highway markers for five miles and confirmed the speedometer is wrong and GPS and Ultragauge are correct. When the speedometer reads 60mph, the vehicle is actually at 55mph. For anyone else who notices a mismatch in their vehicle, your phone’s GPS likely has the correct value. I wonder how my Ultragauge is correctly getting speed data from the vehicle if the speedometer is wrong. I’ll look in the workshop manual to see if there’s a way to calibrate the speedometer. Thanks for the help!