The lable on my door says stock tire size is 245/75R17. Ive had the truck for about 10 months now, the 285s were on it when I got it. And I have never driven it with the stock wheel/tire size. I'm not sure if it was recalibrated for the tires but before I changed them it was spot on, so I'd assume so. Stock 20s came with 3.92s out of the factory right? Would it be better to just go with the 3.92s?
So you had the smallest of the 17’s.... I guess tire rack could’ve been wrong or maybe it’s because I wasn’t sure if your truck was a 2WD or four-wheel-drive, or if it was single cab quad cab or what but when I looked your truck up on tirerack.com it said that the four available 17” tire sizes for your truck was a 245/70, a 265/70 and a 275/70/.
Either or, if your stock tire size is indeed a 245/75, that’s only just shy of an inch and a half taller than those 285’s that were on it when you bought it. At 70 miles an hour speedo speed, your road speed shoulda been a little over 73 actual so maybe it was recalibrated, maybe it wasn’t. But if your stock size is in fact the 245/70 then you’re only talking a decimal point shy of a half inch taller than those 285’s. Speed difference would have been very close at that point. 70mph gauge speed but 70.9 road speed.
But that I can’t answer that part about 20’s back in them days automatically coming with 3.92 gears... maybe someone else knows that but coincidence or not, my 2008 Ram came with the stock 20’s and it had 3:92’s... coulda just been because I had the tow package too but again, I don’t know for sure. My 18 also has 3:92’s but to my understanding, with that 8-speed, you could opt for either or- the 3:92’s or the 3:21’s. Only difference is, you’ll get better economy with the shorter option but you won’t be able to tow as much. 10,000 max trailer weight vs 8,000.
As far as your last question- that would be entirely up to you. If your stock size is a 245/75 then the difference in height between that and your new 20’s is 1.5 inches taller than stock. With a 3:55 gear ratio, those new 20’s brought your final gear ratio down to a 3.39.
If they are a 245/70 then your new ratio would be a 3:28. Height would be 2.5 inches taller than stock.
If you did a lot of towing then a gear swap would be a good idea but if you don’t tow that much then just recalibrating your speedo is probably all you’d have to do.
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