Changing TIre Sizes

[Eddie Watkins]

I am needing new tires before I go on a trip and will have to special order the right sized tires. It will take about a week to get them. I have a 2005 GMC 1/2 ton crew cab pickup with 245R70X17 tires and can find 265 R70x17 tires in stock. The tires are considered an acceptable replacemtn accrding to the tire manufacturer's tire chart. It will mean that when my speddometer says I am driving 60 I will actually be going 62.2 mph. There used to be gears at the end of the speedomater cable that could be changed to correct the error. is there any way to do that now? Are there any other pitfalls that I should watch out for?

Eddie

[Steve Leverich]

Can't answer as to availability these days, but if I remember correctly the different speedo gear sets were only made to compensate for axle gear ratio, not tire size - if it were me, I'd just put the tires on and remember NOT to "push it" - if the cops in your area will ticket you for 2.2 mph over the limit, I'd go live somewhere else

If you do the math, 62.2/60 would change a 3.55 axle ratio to 3.424 - in most vehicles I'm familiar with, the next ratio down from a 3.55 is a 3.23, then 3.08 - going up, its 3.73, 3.90, 4.11, etc - all these jumps are at least twice as wide as what you'll get with the tire change.

Hope this helps... Steve

[Wes Bischel]

Eddie,
If I remember - and did my math correctly, you're looking at a tire that's about a 1/2" taller than stock. I wouldn't sweat it myself. Typically speedometers have a bit of error and the manufacturers build it in on the low side. So unless you're a speed demon, it shouldn't matter all that much - IMHO.

As far as the speedo gears, I can't answer that on new cars - sorry not much help there.

Wes

[Eddie Watkins]

Thanks, I pretty much drive the speed limit anyway and the pollice here give you a few miles over. I would guess at least a full 5 mph before they stop anybody. I do have a towing package so I have different gears from normal. It had not occurred to me that it would change my gearing ratio. I'm not sure if it would hurt or help.

[Steve Leverich]

The slightly taller tires would give you slightly less accelleration on startup and maybe cause the transmission to shift down on a hill at 1-2 mph faster (actual speed) - I doubt that you'd even notice the difference though... Steve

[Jim O'Dell]

Two things. You can probably get the dealership to reprogram the ECM (main computer) to the tire size you put on to get the speedo correct, or as correct as it is now if you changed rear end ratios.
Other item is go to a 265/65 instead of the /70. That should get the tire height down a bit...maybe even match where you are now. We used to take pick ups and do conversions on them at a previous job. IIRC, we went from 245/70 stock tires to 275/60s, and the height of the tires was virtually the same-no adjustments needed for the speedo. Jim.

[Ken Garlock]

Hi Eddie. I recommend that you go to the Discount Tire website and walk through finding a set of tires for your truck. From what I can find, tires for your truck are EXPENSIVE, ranging from $120 to $250 each.

In general, when you go up one tread size tire, your go down one size in profile. In your case, going from 245 to a 265 would mean going from an R70 to an R60.

Remember, you get what you pay for, quality costs.

[Eddie Watkins]

I figured Jim would have the technical answer and he did. Thanks, for the help. I knew the answer(s) would be here. Boy are the 17" tires pricey. The same tire in a 16" will be $50-80 less than it will be in a 17". I agree, Ken, tires are too important to go with lower quality to save some money.

Eddie

[mark page]

Eddie,

I don't know on GM vehicles but the "pinion factor" on Mopar trucks can be changed to any level that was OEM optional for the vehicle. In other words, if the tire size was an "option" it can be changed. Also, if you jump a size in height, a drop in the cross/sectional ratio is needed to keep you close to the same "pinion factor". It should be close but maybe no cigar. If the vehicle is all wheel drive or 4WD, replace all four tires when doing so. Any questions, just shout.

[Chris Damm]

I wouldn't worry about the speedo as most read 2 mph slow from the factory. I have done that change many times and it has not been a problem. You may get a little better milage but not enough to measure. Here's a tire size calulater that might help: http://www.miata.net/garage/tirecalcold.html

[Matt Meiser]

If it really concerns you as Jim said the ECM can be reprogrammed. I had the ECM in my Colorado reprogrammed by an aftermarket company which did some things like change shift points, etc, power curve, etc. Made a big difference. I actually got a whole new computer, so it can be put back to stock if needed. But I remember tire size changes were one of the things they could accommodate.

[Lee Schierer]

I am needing new tires before I go on a trip and will have to special order the right sized tires. It will take about a week to get them. I have a 2005 GMC 1/2 ton crew cab pickup with 245R70X17 tires and can find 265 R70x17 tires in stock.
Eddie

Unless I'm completely missing something here and that is certainly possible since I'm not a tire expert, but as I read the numbers you provided the only difference is the width of the tread, which should have no effect on your speedometer. The circmference (diameter) of the tires should be unchanged or practiacally so. The width change will affect the pounds per square inch load on the road surface as there will be more contact area.

[Matt Meiser]

The way I understood, the 70 specifies the sidewall height in percent of the width. So a wider tire (the 245 or 265 portion) will be 70% of the width difference taller (x2 for the top and bottom.) So 20mm / 25.4mm/in * .7 * 2 = 1.1"

[Mike Henderson]

This is off subject but it looks like you guys have settled the original question. I have an additional question for the tire experts.

Tires are mix of metric and English measurements - the width of the tire is in mm while the rim size is in English. Do they measure tires this same way in Europe (and the rest of the world)? Or is the rim size in metric?

Just wondering...

Mike

[Chris Padilla]

Mike,

A place to start:

http://www.tirerack.com/tires/tirete....jsp?techid=38

I buy all my tires for my BMW from Tirerack....