Put new and taller tires on my 04 1500 5.7 yesterday and she's reading 4 mph too slow. I'm pretty sure the 20" dodge chrome clads that are on it arent stock... My door says there are 17s on it. Rear diff gear is 3.55. I went from 285/45R20 to 275/60R20. It feels just a tad bit more sluggish in 1st. Still breaks the tires loose with no problem though. Also, will this change affect my mpg? If so, will it be significant? Any help is appreciated, thanks!
First and foremost, those 285/45’s you had were just under two inches shorter that even the largest of the stock 17’s your truck came with. What is your original stock tire size?There’s like three different OE 17’s for your truck; 245’s, 265’s and 275’s. How long have you had that truck? We’re those itty-bitty little rubber band tires on it when you bought it? Have you ever driven the truck with the stock size tires on it?
As far as guesstimating how much your mileage will be affected, we kinda need to know what your original tire size is and whether or not your truck has been recalibrated to run those 285’s or not, bein if that was what was on it when you bought it or, as far as what’s been on it since you’ve owned it.
Those 275/60/20’s you put on are the stock 20’s for our 1500’s; but compared to those 285’s you had, you went up three inches in height from those. And even compared to the largest 17’s that your truck could’ve came with factory, your new 20’s are a just a touch over 2 inches taller than those.
Another thing to consider- or at least what I found on the old 98 1500 I had back in the day; it’s stock size was a 245/75/16. When I bought it, it had already had a set of worn out BFG/AT’s on it that were 285/75/16. That’s 2.3 inches taller than the stockers. Granted, those BFG’s were in fact 8-ply LT tires, and even though they were also only an inch and a half wider than the stockers, they were still a much heavier tire just because of them being 8-ply but still, my mileage and the overall performance of the truck went up considerably after I recalibrated my speedometer to my actual road speed. I had read that having your computer think you’re going a lot slower than you actually are affects fuel delivery, timing, and air/fuel ratio. My fuel mileage came up quite a bit too because I was all the sudden racking up more miles to a tank than I was before.
And even though your gears are 3:55’s.... that was with your stock tire size. Your new 20’s are gonna make your 3:55’s feel even shorter. In other words, your new gear ratio now is a 3:46. You would need 3:64’s to bring your gear ratio back up to the stock 3:55’s with them 33” 20’s.
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