magnum

Hey guys-

Don't mean to be a dumb ars, but where are you getting your percentages on the tire size changes? The reason I ask, I went with the 285's, they turn 631 rev per mile compared to my 265's which would turn 655 rev per mile. Now--my math skills are a bit to be desired, but I see a difference of 96%. So I would calculate that at 60mph on my speedo(which was really acurate b4 the tire change) my new speed would be 60x1.04 or 62.4 mph. Am I really off with this calculation? ???

AlpineRAM

"but I see a difference of 96%."
Nope, you see a difference of 4%.(Actually 3.8%)

AlpineRAM

magnum

Alpine,

Thanks, I see what I was doing. Just a bit bass ackwards,
My old tires are 96% as large comparing them to the new 285's......Right? Or I could state that the new ones are only 4% larger than the 265's. Is this right, or am I still having a rectal/cranial inversion ??? ;D

AlpineRAM

Yep, they are 4% larger. ;D

Over here your speedo may show up to 6% more than you actually do and must not show less than you do. If the regulations are similar in the US the mfg will make a speedo that's dead on at the "luxury" model with the biggest tires and use the tolerance for the base model with the smaller tires.
Something else to be considered is that different tire mfgs have different revs per mile for the same dimensions. The BFG MT I use will also get significantly smaller through wear.

The difference in flange heigth between the 265 and the 285 should be roughly 15mm, so in my case a worn down MT 285 would be dead on with a new 265.

AlpineRAM

wood_n_soldier

I went from 265 to 285 75r16's and noticed no difference in my trucks performance. I do a lot of freeway driving (it's a 2001 truck and I have 85000 miles on it) I also have the 4.10 rear - -so do the bigger tires help or hinder when it comes to mileage?

dingo

I agree with phox your tires will start to look smaller.. Ive also got the 315's with 355 rear end and i add about 7 miles to the speedo ...not too hard. If your truck has a lift at all it will look better with the bigger tires, they look akward raised with smaller tires. I have also got the d25's they handle pretty good and no tire rub.
oh ya it is always nice to have the bigger tires there when you need them, they dont get stuck as easy in holes out in the hills just my thoughts....

phox_mulder

[quote author=wood_n_soldier link=board=5;threadid=23375;start=15#msg222237 date=1071184154]
I also have the 4.10 rear - -so do the bigger tires help or hinder when it comes to mileage?
[/quote]

With the 4.10 they'll help a little.
Your final ratio will be closer to the stock tires with 3.55 rear.

With the 3.55 they most likely won't.
Unless they are tall and skinny,
tall and fat creates too much resistance, both wind and road contact.


phox

Sea Bass

I also have been pondering moving up to 285's but have been hesitant with my 3.55's. Just out of curiousity, what kind of rpm's are you guys running with 315's and 3.55's at 70 mph?

dingo

with the 315's if I remember correctly at 70 mph the tach is at about 1900 or 2000 rpm with the auto -355 rear end

phox_mulder

Mines a bit lower than that.

1700 give or take at 70mph.


phox

blackjack

I also run the 285's my orig. equip was 265's but the door said 245. anyhow I quess the factory calculated the correct speedo when the put the 265's on. I also have the 4.10 gears so I really like the 285's. according to the DML tire calcutator there is a 3.6% difference. My tranny seems to be fine but I have no experience with 3.55 gear so may be a factor. anyhow I am glad you put the tire that makes you comortable on , the BFG T/A's are a good tire for our trucks in my opinon.

JR2

Couple of things to note.

The dealer can only adjust tire size up to 265-76 or smaller. So if you have bigger you are out of luck.

The most inportant reason for adjusting your speedo is for your anit-lock brakes. The speed is used to help calculate the braking force. Having your speedo off a large amount can make a big difference in how your brakes perform.

I have 3.54 gears and 256's, I really want bigger tires but I am not going to do it because it will make me run ever lower RPM and lug the engine even more...

wood_n_soldier

I think when I'm going 70ish my tach is closer to 2200 rpms. Is that to high? you all seem to be lower. ?