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Old 04-02-2009, 01:49 PM   #1 (permalink)
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stock tire sizing question

I have an '97 Isuzu Rodeo and that year there was 2 stock tire sizes I have the bigger "upgrade tires". They are size 245/70r16 and the smaller stock size is 225/75r16. I need to get new tires and I want to downsize to the smaller stock size, my question is will the smaller tires fit on the rims I currently have? Any help would be great I dont know everything there is to know about tire/rim sizing.

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Old 04-02-2009, 05:31 PM   #2 (permalink)
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They'll be just fine. The rim width will be a little wide for the 225's but certainly within the acceptable range, assuming stock size rims.
You may notice a skosh more accurate handling with the skinnier tires on the relatively wider rims than the 245's, because the tire sidewall is already stretched across the rim effectively pre-loading the tire for cornering. Maximum grip won't be affected much and is really a function of tire compound and tread design. On a truck you have nothing to worry about.

The lighter tire will increase acceleration due to decreased rotational mass and improve the ability of your suspension to keep the tire on the road.

Happy trails!

Last edited by beatr911; 04-02-2009 at 05:39 PM..
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Old 04-02-2009, 07:15 PM   #3 (permalink)
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Thanks a ton Beatr that sounds great.
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Old 04-03-2009, 01:21 AM   #4 (permalink)
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Don't assume that the narrower tire is lighter. 75 series tires are often rated for a higher load capacity than 70 series, in which case they'll have heavier sidewall construction.

I'll bet you could do better by actually going up to 255/70R16 for the taller effective gearing.
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Old 04-03-2009, 01:33 AM   #5 (permalink)
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I was going for the narrower tires not because of weight but because of decreased frontal area. The wider 245's stick out past the side of my wheel wells into the air flow. Also the larger tire are a bit more expensive and I am really low on funds at the moment.
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Old 04-03-2009, 10:02 AM   #6 (permalink)
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To save some bucks see if you have a used tire dealer or call some new tire dealers for thier used tire stock. We have a used tire dealer here that charges $40 for all 16" tires and they have well more than half tread left. Work out the cost per mile and it's quite cheap. We had one tire fail after several months due to a previously damaged carcass but we still came out ahead.

Even if they are not LRR tires the cost per mile is still less than any new tire.
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Old 04-03-2009, 11:17 AM   #7 (permalink)
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According to Tire Guides a 1997 Isuzu Rodeo came with 2 tire sizes - P225/75R16 and P245/70R16 on either a 6" or a 7" rim.

The minimum rim width for a P245/70R16 is 6", so I think Tire Guides has an error and that ALL P245/70R16's came on 7" rims.

The allowable rim width for a P225/75R16 is 6" to 7".

Also, the overall diameter making the switch would drop the effective gear ratio by 3/4% - not a lot, but I don't think the improvment in areo would be greater then the drop in gear ratio.

Plus - everything else being equal - larger tires are slightly more efficient in RR - meaning for the same load and inflation pressure, the RR is lower.

And on top of all of this: The particular tire you put on is going to have WAY more effect than anything discussed above. RR values vary greatly.

And on a last note: If you are in Southern California - DO NOT buy used tires. This is one of the hottest areas of the US. Rubber deteriorates over time and is largely driven by temperature. There are some efforts underway to require California tire retailers - both new and used - to disclose how old the tires are at the time of purchase - AND to outlaw sale of tires older than 6 years.
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Old 04-03-2009, 05:45 PM   #8 (permalink)
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Good lookup on the rim width data.

Interesting idea on the heat deterioration issue. Does this mean that the tires on your california vehicles should be changed part way through thier treadlife due to the possibility of heat related failure? When I lived in Barstow and driving in 117 degree heat in the desert the tire temperatures were way less than when I autocrossed on a pleasant summer day in Washington state. In iether case I experienced no heat related failures.

I'm not saying used tires is a good or bad idea, even in California. Like buying anything used, you may or may not have problems and there typically is no warranty. Most people shy away from used tires because they don't know what to look for when inspecting and get a little irrational on the safety issue (gee, what if I have a flat?). Unless the tire is ozone cracked, has cuts, bulges, etc that would cause it to be changed out anyway on the vehicle on which it occurred, it's probably OK.

So for the thrifty that can inspect a tire or have access to reputable used tire dealer used tires can reduce your cost per mile.

The failure we had I beleive was a hard curb hit. After several thousand miles the damage finally showed itsself (unless the wifey caused it). Changed the pair out before it was a problem. Still money ahead.

4WD.
One thing I didn't think of before was the 4WD system. Often used tires are not available in matching sets of 4. The '97 Isuzu might not, but more modern 4WD systems may require a full matching set.
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Old 04-03-2009, 07:34 PM   #9 (permalink)
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wow thanks for all the info everybody,

There is a used tire place in my area and I will most definitly check them out before I purchase anything.

Also my Rodeo isnt a 4wd it is the base level 2 wheel drive 2.6L manual
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Old 04-04-2009, 07:37 AM   #10 (permalink)
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Quote:
Originally Posted by beatr911 View Post
........

Does this mean that the tires on your California vehicles should be changed part way through their treadlife due to the possibility of heat related failure?

..........

Perhaps. This is an age vs heat history kind of thing. Clearly, tires that have been operated at high speeds have generated more heat than tires that have been used where the speeds are confined to 35 mph.

5 states have been identified as having the most heat related tire failures - something on the order of 90% - more or less in this order: AZ, CA, NV, TX, and FL. I should point out that this statistic is "number of tires" and is partially driven by population.

FL seems the odd man out here, but if you look at the temperature history of FL, you'll notice that it never gets cold in FL - it stays warm year round. This is probably due to FL being basically a peninsula, and almost completely surrounded by water. The maximum temperature is reduced, but the low temperatures are also increased.

Quote:
Originally Posted by beatr911 View Post
.......

When I lived in Barstow and driving in 117 degree heat in the desert the tire temperatures were way less than when I autocrossed on a pleasant summer day in Washington state. In either case I experienced no heat related failures.

..........
Needless to say, autocrossing generates a lot of heat, but it also wears tires out quickly - ergo, the tires are worn out before their heat history catches up with them.

Not to mention that autocrossing is typically a short duration event and the heat that is generated is basically on the surface and doesn't have much of a chance to affect the inner structure of the tire where heat damage could cause potential problems.

Plus, autocross tires are typically high speed rated. (H and higher) and aren't as affected by temperature as S and T rated tires are.

Heat related tire failures are fairly rare. Even the infamous Firestone Wilderness AT only had failure rates in fractions of a percent. A person in AZ, for example, could go through life thoroughly abusing tires and statistically it's possible that he would never see one. The problem is that a tire failure is potentially a life threatening event! Which is why you will hear lots of warnings that seem overly dramatic.

Quote:
Originally Posted by beatr911 View Post
.........

I'm not saying used tires is a good or bad idea, even in California.

....
Well, I'm saying it: If you live in AZ, CA, NV, TX, and FL, DO NOT buy used tires.

Quote:
Originally Posted by beatr911 View Post
........

Like buying anything used, you may or may not have problems and there typically is no warranty. Most people shy away from used tires because they don't know what to look for when inspecting and get a little irrational on the safety issue (gee, what if I have a flat?). Unless the tire is ozone cracked, has cuts, bulges, etc that would cause it to be changed out anyway on the vehicle on which it occurred, it's probably OK.

..........
The problem here is that most people not only do not know WHAT to look for, they don't even know to look. Plus many purveyors of used tires do not inspect the tires they sell. Potentially dangerous tires ARE NOT screened out. It's these unscrupulous dealers that ruin it for everyone.

In order to put some numbers to this, a private investigator went out and purchased over 100 used tires. No criteria, he just purchased what was for sale at the corner used tire lot. Then the tires were inspected by experts. 25% of these were judged to be unfit for sale. There were unrepaired punctures (with the nails still in them!), tires worn below the legal limit, - and the worst part was there were obviously separated tires in the mix. This is a very disturbing statistic!

Don't get me wrong - I'm a bit of a cheapskate - but I know what to look for in a tire, and I will never, ever buy a used tire. The risk is just too high for my taste.

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