Dr Fildo Report post Posted February 10, 2014 I recently had a flat tire. First flat with this car. I have a 2008 AWD edge with 20" rims. I like that the spare is in the cargo area, but I was surprised to see that spare tire/donut is 17". I was under the impression that with AWD it was important to run all 4 wheels with the same tire size. The spare tire area is big enough to store the full size tire in there when the spare is on the car, so why wouldn't they supply a 20" spare on all vehicles with the 20"rim/tire? I bought the car used, so maybe the dealer switched the spare?? Has anyone else run into this issue?? Is the 17" spare standard on an AWD with 20" rims? Or is it no big deal to run a smaller spare tire? Share this post Link to post Share on other sites
akirby Report post Posted February 10, 2014 It's not smaller - the tire is the same diameter. The wheel diameter is irrelevant - it's the tire diameter that's important. Share this post Link to post Share on other sites
rossid Report post Posted May 29, 2014 Wondered about this when looking at spare in the Edge we just bought. Share this post Link to post Share on other sites
Dr Fildo Report post Posted July 17, 2014 I don't think it's the same diameter. With the donut on the car, that side is WAY lower than the other side. Share this post Link to post Share on other sites
Dr Fildo Report post Posted July 17, 2014 I'm having the new tire installed today. I'm going to compare the size and post back... Share this post Link to post Share on other sites
Dr Fildo Report post Posted December 1, 2014 That's the donut in front. The 20' rim with tire is behind it. Clearly not the same diameter. I'm wondering how harmful to the differential it is to run on the donut. It can't be good. Share this post Link to post Share on other sites
akirby Report post Posted December 1, 2014 Not clear at all. You can't tell from that angle. What are the posted sizes of each tire? Share this post Link to post Share on other sites
Dr Fildo Report post Posted December 1, 2014 My wife has the car right now, but the camera is level to the top of the donut. There is approximately a 2-3 inch difference is size. I will post the actual tire and donut specs later. Share this post Link to post Share on other sites
WWWPerfA_ZN0W Report post Posted December 1, 2014 A 3% allowance is built into the AWD system, so if you are outside that, time to get the tire fixed/replaced pronto. Share this post Link to post Share on other sites
akirby Report post Posted December 1, 2014 If that's the case then I suspect someone put on larger aftermarket tires. The spare should match the factory tire diameter. That's why we need the exact sizes of both the 20" tires and the spare. Share this post Link to post Share on other sites
Waldo Report post Posted December 1, 2014 Ford's AWD systems are designed to recognize when you put a smaller spare tire on and they will disable the AWD system. As long as the tire is between I think 3% and 5% difference, the system will recognize it and no damage will be done. Share this post Link to post Share on other sites
akirby Report post Posted December 1, 2014 Interesting.......so the factory spare could be up to 5% smaller? Share this post Link to post Share on other sites
ls973800 Report post Posted December 2, 2014 You got me curious enough that I just checked the spare tire in my 2013 Limited. I purchased it new with the 20" wheel and tire option, and know that the spare has never been exchanged or even taken out of the vehicle. BIG DIFFERENCE between the two sizes used as shown! NOTE- I have a FWD model, not an AWD. Don't know if those spare sizes would be different. Factory tire size is 245 50 20 Spare tire size is 165 80 17 The first number is the factory 20 inch tire, the second number is the factory 17 inch spare tire as shown on a tire size calculator site. Tire diameter 29.6 inch 27.4 inch Sidewall height 4.8 inch 5.2 inch Rolling circum. 93.1 inch 86.1 inch Revs/Mile 680 736 Ride height difference -1.13 inch Rolling circumference difference -7.6 % Speedometer difference +8.2 % Speedometer reading at 60 mp/h 64.9 mp/h 2 Share this post Link to post Share on other sites
akirby Report post Posted December 2, 2014 Interesting. Must not be a problem for 50 miles or Ford wouldn't have done it that way. Share this post Link to post Share on other sites
WWWPerfA_ZN0W Report post Posted December 2, 2014 If AWD is disabled during this time, or there is enough tolerance built into the PTU to handle it, I can see that happening. If AWD is NOT disengaged, however, and the PTU is marginal, I wonder if it would still be safe. Share this post Link to post Share on other sites
akirby Report post Posted December 2, 2014 If AWD is disabled during this time, or there is enough tolerance built into the PTU to handle it, I can see that happening. If AWD is NOT disengaged, however, and the PTU is marginal, I wonder if it would still be safe. Why would Ford sell it that way if it wasn't safe? That's my point. It came that way from the factory. Share this post Link to post Share on other sites
WWWPerfA_ZN0W Report post Posted December 2, 2014 I agree with your general line of thinking, but Ford's maintenance intervals esp regarding the transmission leave something to be desired. I am simply curious to see if the spare tire would be a non-issue even with the PTU issues that are known to happen. Any stats we can look at (for any make/model)? 1 Share this post Link to post Share on other sites
akirby Report post Posted December 2, 2014 It probably wouldn't hurt to check with Ford to make sure it wasn't a factory mistake. I suppose it could negatively impact the PTU if it engages the PTU more often and/or continuously. Share this post Link to post Share on other sites
Waldo Report post Posted December 2, 2014 It says right in the shop manual that the AWD will stop working when the spare tire is used.... Share this post Link to post Share on other sites
WWWPerfA_ZN0W Report post Posted December 2, 2014 Well, there you go! That should keep the drivetrain safe, though obviously handling & response is compromised until the problem is fixed. Share this post Link to post Share on other sites
Dr Fildo Report post Posted December 2, 2014 Good to know. Nice work Fellas. Share this post Link to post Share on other sites
akirby Report post Posted December 2, 2014 So it is safe, just for a different reason. Sounds like a smart way to save a few bucks on spare tires with a few lines of software. Share this post Link to post Share on other sites
Billy Critcher Report post Posted January 8, 2015 I have a 2009 Ford Edge with the 20" wheels. Had a flat tire due to a cracked rim. Spare was installed @ tire shop,and I was directed to a place to have my wheel repaired. I made it about a quarter of a mile down the road when my car began to violently pull, jerk, grind, and clunk. I pulled over quickly wondering if my wheel had come off the car or something.(??) At this point car was un drivable in drive or reverse without doing the same. Long story short,wheel was repaired,reinstalled on the vehicle,and it was back to normal. Although I wondered ever since how much damage was done before I was able to stop the car. I have since had some intermittent clunks, until this week all of a sudden I'm getting a whining,grinding sound coming from the all wheel drive unit (verified by a very reputable auto repair facility in my area).. the spare is about 7 inches smaller in diameter as was pointed out earlier in the comments. I checked with the dealer to make sure this was the original spare size that Ford Motor Co. provided with this vehicle since I bought it used (and It is). Bottum line, I need some expensive repairs on this vehicle because of this issue. If there is supposed to be some sort of override to disengage the AWD while using the (obviously dangerously undersized) spare tire, IT DIDN'T WORK! Who do I even begin to complain to? Share this post Link to post Share on other sites
WWWPerfA_ZN0W Report post Posted January 8, 2015 (edited) Sounds like the PTU was already on the way out when the first incident happened, probably due to overheating due to low fluid level. No resort that I know of ... so far ... but to fix at your own expense. Could be a coincidence that it happened with the spare tire (meant for limited range driving, but sounds like you were WELL under), maybe not. Try calling Ford National Customer Service, or if that doesn't work out, Lincoln Customer Service. I hope our Ford rep on here (Tricia) will see your post soon and step in with advice. BTW, the difference is about 3" in height, I believe between the stock wheel and the spare: http://www.tacomaworld.com/forum/tirecalc.php?tires=245-50r20-165-80r17 which is still way above the difference the AWD can compensate for. Interesting tidbit I was unaware of (OM): "You should replace your spare tire when you replace the road tires or after six years due to aging even if it has not been used." "If your vehicle is equipped with AWD, a spare tire of a different sizeother than the tire provided should never be used. A dissimilar spare tiresize (other than the spare tire provided) or major dissimilar tire sizesbetween the front and rear axles could cause the AWD system to stopfunctioning and default to front wheel drive and could damage thesystem." "Each tire, including the spare (if provided), should be checkedmonthly when cold and inflated to the inflation pressure recommendedby the vehicle manufacturer on the vehicle placard or tire inflation pressure label. (If your vehicle has tires of a different size than the size indicated on the vehicle placard or tire inflation pressure label, you should determine the proper tire inflation pressure for those tires.)" Edited January 8, 2015 by WWWPerfA_ZN0W Share this post Link to post Share on other sites
Billy Critcher Report post Posted January 8, 2015 So it seems to me that the intelligent and "correct" thing to do on any all wheel drive vehicle, would be to supply a spare tire with the exact same diameter as the other four tires. But then doing the right thing would cost the company a few $$'s up front. Apparently that's too much to ask for in this day and age?? Share this post Link to post Share on other sites
