^ Plus the speedo issue.
You might be able to have your tranny reprogrammed or revalved for the tires. Then there’s the torque converter lockup rpm.
Yes, a couple sizes will cause the speedometer to read about 3 mph low at 70 mph (so it’s 73).
It’s better to think of the gearing affect as moving the shift range and RPM ceiling rather than in terms of low gearing (talking automatic transmissions here), so long as the engine can still pull at lower RPM in overdrive. The across the range RPM reduction may mean a gear you never downshift into is now viable.
A concrete example. My 2004 Sequoia has a classic Toyota 4 speed transmission with the 4.7L V8. Overdrive is fairly useless going up I-70 (RPM too low in the power band) so I usually lock it out and drive in 3rd, especially with the added weight of passengers and gear.
With stock tires, 3,500 RPM will hit in 3rd gear at around 65 mph. I don’t like to flog it over longer distances, so that’s about the ceiling. 2nd gear then for steep passes is only useful to about 45 mph before the flogging begins.
I put 285/75/17 AT3W, which is a full size 34” tire, on the Sequoia pretty soon after I bought it. If I regeared the differentials to say 4:88 from factory 4:10 ratio, I would have lower gearing than stock, so more effective gear power at the wheels, but now my 3rd gear ceiling would be 60 mph and 2nd would be reduced as well. And the lower gears still wouldn’t make overdrive low enough to stay out of 3rd.
But by leaving the differential gearing stock, I can cruise in 2nd at 55 mph and 75 in 3rd. Overdrive (4th) is still great downhill, although I’ve lost a bit of compression braking. If they fit, I would run 35’s because both 2nd and 3rd are so much better, and I need second to not bog down on passes.
This point is further illustrated by my Land Cruiser on 37” tires, also a four speed. I have 5.29 diff gears replacing factory 4:10, which is 18% lower than stock in combination. I know this, because I have a Yellow Box that corrects the speedo at the 18% overgeared setting. And RPM is about 300 over stock at 70 mph.
So second gear here is a mess, it’s revving at 40 mph and 3rd is lowered as well. It’s awesome offroad and on anything steep and slow, but the net effect of those low gears is going slower because the headroom is gone, even though it cranks an effective “big” gear.
None of this matters when considering the power of a 2014 Grand Cherokee. You could generate as much effect of a single metric size by varying your tire pressure. Tires don’t make as much of a difference as charts say they do, because of radial tire rolling radius, unless maybe you like max PSI. Gears are exact, because metal.
This is the Yellow Box I bought to correct the Sequoia speedo. I’ve had for over two years. No reason to bother, the shift points and speed ranges are perfect with the larger tires. I’ll probably do it at some point, but that’s really a matter of not getting tickets because your speedo reads low if the shift points are responsive.
I’m not saying anybody should or shouldn’t change tire size, simply that tires are also a gear and you can make everything from single size changes for looks that won’t show up at all except in grins to shifting the entire range to using a low gear on the highway.
Modern vehicles are so powerful with so many gears that sticking to factory size doesn’t need to be an absolute rule. I mean, people are lifting Crosstreks and putting 31” tires on them. And we know how finicky Sube owners are
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. I have experienced very little of the downside of such tread on my truck. They are relatively quiet and surprisingly stable at 80+ on the highway despite being able to confidently slog through the worst winter conditions or even rock crawl if you so desire. They are also relatively inexpensive compared to their competitors
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