Spare tire size

ducktail · **Posted:** Fri Oct 22, 2010 7:52 am

I have a 2006 Renegade. When I first got it the original tires were Goodyears and I had problems on wet grass. I went to Michelins 235 70 R 16's. That was not the standard size but what Michelin offered in what I wanted at the time. The current AT's just make to darn much noise and my other 4x4 has rough off road covered.

Question is: Can I go back to the standard or even larger and still maintain a smaller spare? The vehicle is not lifted.

tommudd · **Posted:** Fri Oct 22, 2010 8:32 am

Not quite understanding
235-70s were a standard or I should say an optional size
What size came on yours

LibertyCRD · **Posted:** Fri Oct 22, 2010 8:52 am

I'll bet he had the 225/75-16 Goodyear STs on his Jeep from the factory. They are the worst tires ever made.

If so, when he went to 235/70-16, he actually went to a shorter tire. So his spare (assuming it's still factory) is now slightly taller than the rest of his tires.

You can go 245/70-16 and that will match up perfectly to a 225/75-16 spare tire. Those sizes are interchangeable because they are the same diameter...so you can run the spare tire without worrying about damaging your differential.

If you go any larger than 245/70-16, you WILL need to get a spare tire to match however. In that case you could still use your existing spare, but ONLY for SHORT distances to get off the road. I wouldn't drive any length like that. Best thing would be to just get a matching diameter spare.

ducktail · **Posted:** Fri Oct 22, 2010 9:50 am

At the time I switched to the 235's from the 225's I went with Michelins on all five tires. I have 4 AT's on right now. All 5 are 235 70's right now.

Can I go to the 245 70's and not worry about differential damage if I have to use the 235 70 spare? I guess that is the question.

There has been some question as to whether my speedometer would be off. So, that could be a question here too.

It is a manual transmission so I do not have to worry about an auto shifting problem.

And those Goodyear's mentioned above are what I had. Seriously, they would spinn on wet grass.

LibertyCRD · **Posted:** Fri Oct 22, 2010 10:56 am

245/70 is the same diameter as 225/75...so your speedometer will read exactly the way it did when your Jeep was brand new.

Your current 235/70s are a tad shorter, so your speedometer is currently reading just a tad faster than it did when your Jeep was brand new.

So no, your speedometer won't be negatively affected. If anything, the 245s will make it read more accurate.

Using the 235 as a spare will be fine...again, to get you off the road and to a place where you can get help. However, you shouldn't run the 235 for long distances with the 245s because that tire will be spinning faster than the other side and it could possibly heat up your differential and cause problems.

bgcarl01 · **Posted:** Fri Oct 22, 2010 3:19 pm

Know this post is ancient, but using the following equation for tire size LT 245 70R16 would measure out to 29.5 by 9.64. and the LT 225 75R16 would be 29.28 by 8.86 roughly. Aspect ratio (.70) multiplied by width in MM (245) divided by 25.4 to convert to inches multiplied by 2 because there's two sides to figure in plus your rim diameter of 16. I figure those tires would be off by almost 1/4 inch. Not much I know, but he's off by .33 right now with the 235 70 16 from the 225 75 16's, so he'd still be screwed up speedo wise with 245 70 16's.

JeepinJarhead03 · **Posted:** Fri Oct 22, 2010 3:49 pm

speedo wise , those differences in tire dia, you're talking A mph'ish at 60mph

a 30.5 ish dia tire causes only about 3 mph off at 60 with no automatic transmission issues below that range

ducktail · **Posted:** Sat Oct 23, 2010 9:46 am

I put on 245 70's. When doing 60 miles per hour as constant as I can keep at 60 on the interstate, a mile takes 3 seconds longer. A 60 mile trip is 180 seconds longer at 60 miles per hour. This is based on the speedometer being accurate with the 225 75 originals. That is an assumption.

I put on Dueller Revo 2's from the local Firestone or something like that. THEY ARE MUCH MUCH quieter that the BFG A/T's they are replacing and I would say they are quieter than the Michelins I replaced the original street tires with. I have not been in any appreciable mud or sand just yet. I would be pleasantly surprised if they can out perform the traction of the AT's.

But back to my original question. The tire dealer said I would be ok using the 235 spare. He would not suggest 1,000 miles on it and I would not do that anyway. If I had to do that I would switch the 235 to the front I believe.

LibertyCRD · **Posted:** Sat Oct 23, 2010 11:24 am

As far as the speedo, you really need to check it with GPS. My Jeep bone stock with factory tires was reading too fast. With my current 255/70-16s, I am only 1 MPH slow at 60 MPH....hardly noticeable at all. Seems like users have reported many different things with the speedo reading coming out of the factory.

Spare tire size

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