Speedometer 4mph off after tire/wheel size change

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Chrismb5.7

Chrismb5.7

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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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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?
 
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Chrismb5.7

Chrismb5.7

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What about 4.10s? I've heard they make for great performance. I'll be honest with you, I'm a stranger to rear end gears lol...
 

corneileous

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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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corneileous

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What about 4.10s? I've heard they make for great performance. I'll be honest with you, I'm a stranger to rear end gears lol...

You could go with 4:10’s but your mileage will suffer a little. But the thing of it is, you would have to go with a 4:30 to bring it up to a 4:10 if that’s what you had stock but I’m not sure if that’s right. But one thing’s for certain; just to get your stock 3:55 gear ratio back for those 245/75’s that your sticker says you had stock, you’d need a 3:72 just to get your stock 3:55 back.
 
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Chrismb5.7

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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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Okay, thanks a lot for the advice and info. I've ordered a programmer to recalibrate the speedometer. Should have it tomorrow. I'll decide afterwards if I wanna do the gear swap. Again, thanks.
 
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Chrismb5.7

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Check out this chart for gear to tire size. Guesstimation with transmission ratio of 1:1.

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If I'm reading this correctly, which I'm not sure I am... 4.10 gears would be good to daily with my 33s? Thanks for the chart btw, for some reason I couldn't find one.
 

Mike Murphy

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If I'm reading this correctly, which I'm not sure I am... 4.10 gears would be good to daily with my 33s? Thanks for the chart btw, for some reason I couldn't find one.

As the chart is a guesstimation, it would be somewhere between highway and city driving in the transmission 1:1 ratio.
 
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Chrismb5.7

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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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Just read this again and went back to my door sticker, stock is indeed 245/70R17. Not 75s. Incase that was still on your mind lol.
 

Malveaux0089

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I can't remember the exact name of the gear itself but I was looking into that gear for the speedometer cable when I had my Wrangler. If you install bigger tires then you will be going faster than your speedometer states. Sorry I can't be of more help with the name of that one gear but I know it can be done without spending a bunch of money on a super chip and tuner.
 

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A lot of assumptions being made here, like that taller gears results in better fuel economy. That may be true with an indoor EPA mileage test at 55 mph but that does not reflect real world driving which is why people seldom see fuel economy with a vehicle that comes close to the EPA or the manufacturers numbers.

Truck manufactures need to meet the federal CAFE numbers and so are using very tall gearing on their trucks. Going to larger than stock tires only compounds the problem if someone is actually going to use their truck to haul or tow heavy loads. What I like about the Ram Rebel is that it was engineered for its large 33" tires with its suspension and 3.92 gears.
 

Marshall

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I expected the change... I came here for help to fix the issue. These forums are not for people who feel they should say things like that to somebody asking for help. I do not need your help. Thank you.
O.K. I had a comment about the computer, but I'm gone.
 

corneileous

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A lot of assumptions being made here, like that taller gears results in better fuel economy. That may be true with an indoor EPA mileage test at 55 mph but that does not reflect real world driving which is why people seldom see fuel economy with a vehicle that comes close to the EPA or the manufacturers numbers.

Truck manufactures need to meet the federal CAFE numbers and so are using very tall gearing on their trucks. Going to larger than stock tires only compounds the problem if someone is actually going to use their truck to haul or tow heavy loads. What I like about the Ram Rebel is that it was engineered for its large 33" tires with its suspension and 3.92 gears.

Taller gears do help mileage to an extent. My 18 Hemi 1/2-ton with 3:92’s gets an impressive 18, 18.5 on the highway but those with the taller 3:21’s do get a little better. As far as who would get better mileage towing between the taller 3:21’s and the shorter 3:92’s- that I don’t know. I would imagine the shorter-geared truck would get better mileage towing because although it’s motor is turning faster at the same speed, its motor however isn’t having to work as hard to tow the same load.

But hey... my limited with 275/60/20’s are 33” tires as well. 33’s aren’t just exclusive to the Rebel. Practically every 20 for the 1500 is a 33.


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dhay13

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Actually an internal combustion engine is most efficient when the manifold vacuum is at it's highest. So if you want to maximize fuel efficiency hook up a vacuum gauge and monitor it. I do have to say that this was with older carbureted non-computered motors but would think it would still apply.
 

corneileous

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Actually an internal combustion engine is most efficient when the manifold vacuum is at it's highest. So if you want to maximize fuel efficiency hook up a vacuum gauge and monitor it. I do have to say that this was with older carbureted non-computered motors but would think it would still apply.

I think that only applies to the difference between lugging the motor and not.




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