This site will give you a good idea of what your new speed will be compared to your old speed based on tire size:

https://tiresize.com/speedometer-calibration/

 

I just drive thinking to myself 60 = 65MPH. If I wanna be dead accurate = GPS problem solved.

 

Thanks Billfromthehill, I know its not cheap, but I'm gonna do it just because of my tire size. Thanks again!

 

I would do it ( I did), your speedo correction is the least of the advantages it brings. ( you get your low end grunt back, better mpg, and easier on drivetrain components!) Just match your gear ratio to your tire size, Ashcroft has a great calculator for this. Me I run 255/85r16 and Ashcroft 4:11 gears, which puts me right back into stock powerband and returns speedo to a more correct reading.

 

This is for a Defender, but I'm sure someone smart could get it to work on a Discovery:

https://bearmach.com/speedo-correction-kit-ba-5250

 

from my limited research, unfortunately not true as Disco 2 doesn’t use a mechanical cable but instead the speedometer and odometer get pulses sent to them. So you need a tool that can take that pulse before it gets to the speedometer and adjust it to be more accurate. There used to be a thing called a Superlift TruSpeed Speed Senior Calibrator, but no longer produced.

 

There are plenty of these devices that work with the electronic pulse style of speedometer, but non are per-programmed for LR:
https://www.summitracing.com/search/...ration-devices

Maybe one of the electronic engineers here could figure out if one of these could be modified to work for a Discovery.

 

I have a similar issue, my speedo seems to be off by about 5mph at 55mph and my tires are the same size as the pic TimInNova posted above. I'm running 265/65-18 and 59.77 divided by 55 yields a factor of 1.0727.

TireRack states the "Revs Per Mile" for my 265/65 Wrangler Duratrac is 663, where as a set of BFG AllTerrain T/As in 255/55 is 717, a factor of 1.0799.

Therefore an indicated 5k miles will actually be 5363.5 miles according the calculator, or 5,399.5 miles per the TireRack specifications. A difference of 36 miles or <1% between them, which is not a big deal really, and a 7% jump over a stock setup.

However when doing the math on what the my odometer reports versus google maps, my indicated 24.3 mile trip Monday was actually 28.2 google miles (my exact route), or a factor of 1.1604. The same 5k miles traveled on the odo would actually be 5,802 miles which is about a 14% jump over indicated, or an additional 7% over what the calculator and TireRack would predict.

So when calculating mileage for maintenance and MPG, whIch formula would be appropriate for me to use?