Tires

[beaconlight]

I am going to the Jan Cubfest George is sponsering so I put 4 new tires on the PU. They were due and this was a good reason to do it now. The 2003 Dodge 2 wheel came with 240 17. I had a fit trying to find same so I replaced with 265 17 as the 4WD have. They are a little bigger abd I suppose will knock the Speedometer off. I am curious if any one knows by how much and can the computer be reset for the different tire.

Bill

[John *.?-!.* cub owner]

The dealer can change it in the computer pretty easily, but I don't know what they charge.

[magnumpi]

Bill: Don't bother going to the dealer - the change is much less than you might expect. I just did this same exercise on my daily car and discovered the higher profile tires I put on caused a 5% error in the reading so at 60, I'm off by 3 MPH !! Not worth messing with the dealer or the truck's computer. Here's the site to use - pretty neat ! Craig

http://www.miata.net/garage/tirecalc.html

[Jim Becker]

The calculators like the Miata site are OK for getting the general idea of how a different size will look on a car or truck and whether there is likely to be interference, but are not usually close enough for looking at speedometer error. The maufacturers publish revolutions per mile figures that can be quite different than the calculated numbers. Too late in this case, but measuring 10 miles down the road both before and after is another way to check how much things changed.

By the way, if your truck has ABS, the change in rotation rate is a major factor in ABS operation. Changing tire size can have a bigger effect on the ABS than would appear from the speedometer error. If the change in published revolutions per mile is more than a couple percent, checking with the dealer (if you can find someone that knows anything) is probably a good idea.

[magnumpi]

Good points, Jim. A guy can also go to http://www.tirerack.com and get the revs/mile of every size of every tire they sell. Craig

[beaconlight]

Thanks guys. You have me thinking. I wil;l check the sites and make a decision.

Bill

[John *.?-!.* cub owner]

or if you know someone who has a GPS that will give MPH, borrow it. they are pretty accurate.

[magnumpi]

Bill: I posed the ABS function question to a guy who's a guru when it comes to Saabs (we both drive them - no Saab story jokes are necessary at this point !! hehehe). His answer to the different profile question affecting ABS function is as follows: "As long as all of the tires are the same size there will be no issues at all.

That 3% change might simply make the speedo read true for the first time."
Now Saab is 100% GM-owned so the Delco-Remy function may differ from that of Daimler-Chrysler, Ford etc. Maybe asking one of the techs at the local dealer might be useful. I'm sure side-to-side profile differential would have some adverse affect, but here you're changing all the tires at once. Good luck, I'm sure you'll experience no problems. Craig

[Jim Becker]

I just repeated what I was told (quite emphatically) by a friend at a Chevrolet Zone office. This was in the context of a '91 pickup I had. The Shop Manual for that truck has a paragraph titled "Speedometer Calibration" that says: "The speedometer must be recalibrated when rear axle ratio or tire size is changed. Refer to INSTRUMENT PANEL AND GAGES." Interestingly enough, this paragraph is in the ABS section of the manual. This truck had rear wheel only ABS controlled by the speed sensor on the drive shaft, so it was oblivious to size differences of individual tires.

[magnumpi]

Jim: I'm not doubting you for a minute, but my experience with car companies/dealerships has shown me they are a wee bit less than honorable. As an example, Saab used to have a check engine light that came on automatically every 30,000 miles so unsuspecting owners would have to go to the dealer for "service" ! Time and time again I see stories on the Saab site similar to this one where customers are told lies by the dealerships and have parts replaced that never needed replacement !! BUNK BS stuff IMO. They have done their best over the years they've owned Saab to reduce the quality of the brand; hence, the quality of the dealers as well. I've owned my last GM product - EVER !!It's all about making money to keep shareholders happy - customer satisfaction is a distant memory.......thus the budding success of Toyota, Honda and the other imports' which just run and run and ......... Happy Holiday. Craig