Tricky story (Time for a tire size change)

[nerij]

Hello pals

Here is my story… more than 3 year ago I moved Tires size from 235/60R15 to a all terrain 31x10.5R15, then I run into speed issues due the tire size change, I asked for some advice and it was that I had to swap the Axle, so after a hard time looking for the axle I swap it from a 7.5 to a 8.8 Ring Gear diameter and problem was partially solve although not at it should be but I have living with that for the last 3.5 years... Now is time to replace the tires but this time I will like to have a smaller tire size but keeping the same rims size 10.5xR15 so I went to a tire store and they told me that the smallest size it could fit it’s a 275/60R15….

Now after the entire tragedy novel I have described, My doubt is as follows; if I move to a smaller tire size, do I might run into speed problems again due this time now I have a 8.8 axle? I’m assuming that I must not but better have first a good advice, and the other it’s really the size 275/60R15 the smallest size I can fit in a rim 10.5XR15? question is because the guy in the store told me that they do not usually have that tire in stock but of course he was just trying to make the sale but just want to confirm

Well hope you can give me a hand on this instance

Thanks in Advance

 

[snomaker321]

What do you mean by speed issues? Is it just that the speedometers off or you physically cant go fast enough? Switching the axle size wont do much(although the 8.8 is stronger) but changing the gears inside will. For 31" tires, you probably want 4.10s if you didnt already have them to get your acceleration feeling more stock

 

[alwaysFlOoReD]

Original tire diameter=26.1"
tire size now=31"
new tire size=28"
I'd say your probably o.k. with your rear axle, the truck will feel quicker off the line and your speedo will probably still be out of whack, depending on the gears that you have in the rear end. You can do some research in the technical section of the Ranger Station, top of the page, third blue button from the left; tech library>calculators>combined tire and speed calculator.
Good luck,
Richard
Edit; I got to thinking about the sizes; the tire calculator I used may be off, I remember 235/75 15 as being ~ 29". Can some one confirm?

 

[fx2prerunner]

I belive there is also something in the tech section about changing the spedo gear in the trans to correct the reading

 

[gcfishguy]

It's not the axle model itself that determines the gear ratio...it's the...well...gear ratio.
If you swapped out a complete axle assembly of the same rear ratio, you didn't change anything as far as how fast things are turning, and power band, etc.


Chances are when he says 'the smallest', he's actually meaning 'the narrowest'.
In say, a 235/75/15, the 235 is the section width. (235mm, or 23.5cm)
The 75 is the aspect ratio (a measurement of how much width makes up the height) (a 75 series tire means that 75% of the 235mm is applied to the height, OR 75% as tall as it is wide)
And the 15 is the rim size. (d'uhh)

A 'normal' size rim for a 235 or slightly bigger (31x10.50x15 for example) would be around 8 inches.


He's likely considering the fact that you have a 10" wide rim, so you'll need to match it to a wide tire.

If that's the case and you want to run a smaller (less overall height) tire, in a metric size, you'll need to go with less sidewall (lower aspect ratio) and maintain your overall height (OD).

Meaning, a 265/55/15 (26.5" OD) would be about the same overall height as your original 235/60/15's (within .4") and the extra 30mm of section width would suit your 10" rims.

The 275/60 he's suggesting would be good for your rim width, but apples to apples, would be a full 2" taller than your original 235/60's....so I wouldn't recommend that....but with the axle changes and so on, I don't know what end result you're looking for.


Nothing's written in stone, but chances are the tire shop has fitment guides and they would be opening themselves up to possible liability by installing a tire suited to an 8" wide rim on a 10" rim and sending you down the road.


@ alwaysfloored, yep, a 235/75/15 is a 29" tire...but the op said his originals were 235/60/15... 26.1" Your used the right numbers but likely didn't even catch that they were 60's.

My favorite is the miata tire size calculator. (I know, Miata..never even sat in one, but their site has an awesome calculator.)

 

[nerij]

Thanks all for the replays Highly appreciated

snomaker321
What I mean with speed problem was that my Ranger could not go faster than 65mph in flat road, after the axle change that was solve an it runs good… up to 80mph with no big issues but again only in “flat” road, but if I have to go up in a climb my truck will not go faster than 70mph...

gcfishguy
Thanks for all the info provided, also what I’m looking is to improve “the low speed” when going up in a climb and as you mention that what makes the change effective is the rear ratio in the axle and as I did have some improve when it was changed I might be concluding that could not have big issues going to a tire 2” bigger that the original size.

By te way checking the site of miata tire size calculator I find the following page which I find it helpful as well due you can compare metric with inch size and also will give you the speedometer difference(just in case you are interested)

http://www.1010tires.com/tiresizecalculator.asp

Once again thanks all for you time to reply this post…

 

[alwaysFlOoReD]

I bookmarked both calculators,
thanks,
Richard