275/65R18 would be 32.1" diameter; 295/55R20 at 32.8".

That's 2.2% off on the meters. 70mph on originals, 71.5 on the bigger tires without calibration.

If I ever go to bigger tires, I want it right on the meters.

That's why I asked in follow-up to your post.

 

Note that by design, the speedo is going to be a bit high when properly calibrated. Actual 70 should read about 72.

 

Thanks fellas. I think I'll grab a calibrator and get it dialed in.

 

For that, I wouldn't bother. Depends how much the calibration tool is. But if you want it exact, go for it! Truck looks really good!

And everything MGD said are great points. I don't always think about the warranty since it's been so long since mine's had one . . .

Looks like both together is about $200-300+. But definitely worth retaining warranty, though you can just calibrate the speedo and not load a tune with an SCT too.

http://www.harborfreight.com/automot...agnostics.html
http://www.summitracing.com/search/d...ration-devices

 

Yeah, I'm not much interested in tuning until warranty is a goner. I don't feel like crossing that bridge. No worries with warranty if I recalibrate for tire size I suppose?

 

Not according to MGD, I have no idea. But if you have a Ford dealer do it, I can't imagine it affecting your powertrain warranty. I'd run it by them if you get your own device, just to be safe.