I know that the boss bought a new Dodge truck and put bigger tires on it and then took it right back to the dealer to have the speedo recalibrated. A major Dodge, Toyota, Scion dealer, not some shady outfit.
I am with a Trans Am/Camaro group and the boys do this sort of thing all the time, although I prefer to leave mine stock.
If the factory offers a range of different wheel sizes as options and ships them with the speedometer reading incorrectly, I imagine that someone might raise quite a fit about it. But, no, I am not certain. But if the dealer can reset them, I don’t see why the factory wouldn’t just give them the right calibration to begin with.
Much of the discussion of changes in tire diameter, especially increases in diameter, has centered around the recalibration of the odometer (which automatically addresses the speedometer issue).
When a manufacturer offers different wheel/tire options the tire diameter, in almost all cases, will stay the same. The concern isn’t reprogramming the odometer, it’s the operating RPM of the engine. Every engine is designed to operate efficiently based upon the weight of the vehicle, transmission gear ratios, rear end ratios, intended use of the vehicle, etc. An increase of 2" in diameter will dramatically effect the operating RPM of the engine. It will operate at a lower than optimum RPM (lugging). This will put undo strain on the engine, similar to starting out in 2nd gear instead of 1st or locking a transmission on overdrive at too low of a speed. It will also, as previously noted, reduce fuel mileage.
When you see these 4WD vehicles with oversize/tall (31", 32", 33", etc) tires, the gear front/rear differential gear ratios have been changed. From the factory they may have come with 3.73:1 gears, but to compensate for the larger tires they might lower the ratio to 4.56:1.
There was a class action suit brought against Honda a few years back because it was shipping out Accords and one or two other models with speedometer/odometer readings 2-4% on the high side. As a consequence, they bumped up the warrantied mileage by 5% and gave lessees 5% more allowed mileage.
The main focus there, of course, is that it was essentially shortening the warranty period by a few percent.
It does make me wonder, though, about the oft-heard claim that +/- ten percent is acceptable error on speedometers. I always thought that sounded high, and I really doubt it now that I see a manufacturer getting hammered over a far smaller bias. The engineer quoted in the article states that he considers two percent unacceptably high.
Ten percent does seem a bit high, but was probably not quite so unreasonable in the old days of purely mechanical systems. But it would seem to me that going much below 2% would get you into the area where tire inflation could have an impact.
Does anyone know how much over- or under-inflating a tire can effect its circumference?
It makes me wonder about tire wear, too. Most tires start out with a tread depth of, what, 10/32 of an inch? Replacement is mandatory at 2/32, which gives a total radius change of a quarter of an inch between brand-new and ready-for-the-scrap-heap.
Radius of, say, a P215/70R16 is, ummm… (8 in) + 0.7*(215 mm)/(25.4 mm/in) = 13.9 inches.
0.25" / 13.9" = 1.8%, and since circumference scales with radius you’d get the same percentage change in circumference.
Borderline, I guess, if we’re treating 2% as a threshold.
I love this place! Everyone can put in a peice of the puzzle and we get a fuller picture.
I think UncleFred, (the OP), probably just went back and asked the dealer about the Rav4 he was interested in.
UncleFred here. Actually we have been focusing on shopping by price for the desired configuration. But with the very helpful knowledge gained here I will ask the salesman about this topic.
It is not unusual to find MrsUncleFred and I know more about a car than the salesmen do.
I am a tire shop owner and I can’t agree more with this.
Here’s a true case: My car (a Citroen C4) can be factory equipped with three options: a) Steel 16" rims and 205/55/16 tires, b) Alloy 16" rims and 205/55/16 tires, c) Alloy 17" rims and 205/50/17 tires. Mine came with option c.
Now I don’t have the exact numbers handy, but I remember that the 16" alloy rim is actually about 1.5 kilos HEAVIER than the 16" steel rim and the 17" alloy rim is another 1.5 kilo heavier. The 205/55/16 and 205/50/17 tire weights on the other hand are almost identical.
Just for the heck of it I removed the stock 17" wheels and bought a set of 16" steel wheels but instead of the stock 205/55/16 I installed 205/45/16 semi slick tires. Even with the much stiffer sidewall of the semislicks, I couldn’t believe the improvement in ride quality, especially when driving over potholes and uneven surfaces. It was like driving a different car.
What ghardester said. Thetire size will be in the vehicle’s computer.
I bought some larger tires for my Suburban. I yanked out the ECU and sent it to a ‘dude’ who reprogrammed it for me (he actually did some other things to it but tire size was one of them).