I just got my first traffic ticket ever in the Disco of all cars, 77 in a 55. I had cruise control on and the speedo was between 65 and 70. I have 33" tires and 4.37 Ashcroft gears.The gear ratio is actually higher than factory, I went with 4.37s because I'm going to 35s soon. If anything, the actual speed I was going was lower than the speedo? I've never even seen the speedo on the Disco go over about 73-75, that appears to be its top speed. Very curious if I'm mistaken about the gearing, this doesn't make much sense.

 

Gearing unfortunately has no effect on the speedo in a D2. D2 reads speed off of the ABS sensors in the hubs.

 

It's correct that gearing has no effect on actual speed vs. the speedometer reading, but tire size does.

The stock 255/65R16 tire has a diameter of 29.1 inches. That's a circumference of 91.4 inches. So to travel a mile, the ABS sensor is seeing the wheel rotate about 693 times.

The 33 inch tires on your truck have a circumference of 103.7 inches. That means in 693 rotations your truck is traveling 5,991 feet, or 13.4% farther than stock. That means that if the speedometer is showing 55 mph the truck is really traveling at 62.4 mph.

So if you got clocked at 77 mph with 33s, that means your speedometer was reading 68 mph.

And, 35s have a circumference of 110 inches, which is 20.3% more than stock. That means that if the speedo is showing 55 mph the truck will really be going 66 mph.

One nice feature of the WAZE app is that it displays actual speed calculated real-time (or close to it) using GPS data. Other apps may do the same.

 

Yep once I got into modifying my LR's for serious off road use = Speedo is no longer accurate and I use GPS to figure out my speed.

 

I have a dash mount for my phone so I can run the GPS speedometer when I'm driving. With my oversize I'm off about 5 miles an hour - the faster I go the more it's off.

 

I know that my scangauge allows you to compensate the speed display if you know what percentage you’re off by. Any number of tire comparison calculators online will give you all the information you need. I assume that an ultraguage would allow you to do the same thing if you already have one of those installed.