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Car has 245/55/20 rather than the stock 235/45/20 tires , so around 8% larger.
Curious on how far out it has made my gauges out of spec.
Short trip was reported to be on Google maps ( not that accurate ) 39 KM, car indicated I travelled 35.9 KM and my old Garmin gave a final distance of 36.8KM. So with the larger tires I did not get 8% difference only over 2%. a few steep grades on the trip so probably the Garmin is a little low.
Meanwhile on several fairly level stretches ( a few dips and humps) at 60 KPH on the GPS, the car read 61 - 62 KPH so very accurate, at 80 KPH I had the same difference of 1 - 2 KPH or about 1 MPH high. Very Happy.
So with larger tires the readings are fairly accurate, but if running with factory tires the odometer would reading higher and the speedo would be showing much lower.
Have any one else tested their car to see what reading they received?
Speedometers are usually optimistic and overstate the speed by up to 10% on stock tires. This also accounts for tire wear. The wheel circumference gets smaller as the tire wears over time. Speedometers are basically never allowed to understate the actual speed. Even GPS devices may overstate the speed such as radar detectors to minimize the risk of a speeding ticket. To know your true speed, you have to use a known calibrated GPS device that doesn't overstate the speed.
There's a good tire size comparison calculator at the link below that calculates the speedometer errors at various speeds. So depending on how much yours was overstating with the stock tires, you may now be closer to actual or it may understate and actual speeds are higher.
My stock 235/45/20 tires resulted in very close to perfectly accurate speed / speedo correlation (with essentially new tires).
I went to 255/50/19 tires (yes, different wheels) and my error is around 3.5% (going faster than indicated on the speedometer). The tires had around 65% tread (yes, tread wear does change the numbers).
It's common for cars to run a few % slower than the speedo, but on my GLK I didn't see that.
Time yourself over a measured mile or km. Unless your watch is off, it'll give you the actual speed..
Tire size change from 235/45R20 to 245/55R20 gives actual speed of 108 kph at speedometer of 100 kph. That’s probably enough to get a ticket. Maybe drop your speed by 5 kph.
If you really want to know your real speed, many controlled access highways in the U.S. & Canada have mile or km markers. The Canadian markers were pointed out to me when I was tagged to drive a company van from Calgary to Edmonton. Use a stopwatch held by a passenger and set cruise control set to 100 kph for ease & accuracy.
I just switched to 245/45/20. My speedo is still low by almost 2%. If speedo shows 110kph, actual speed is 108. Instead of calculating, just get a speedometer app for your phone.
With the larger tires the speedometer is almost perfect and the odometer is out 2% so no calculations needed in Canada. In the USA they cling to the old British system, so a little mental calculation is needed at each speed sign.
I was just curious to how far out everything would be with the original size. Probably would never get a ticket LOL
Last time I drove in Montana I met a new friend and helped pay his wages. Yet plan to drive from home down to Seattle in June and follow Highway #2 to North Dakota and then drive back North to Canada.
Last time I drove in Montana I met a new friend and helped pay his wages. Yet plan to drive from home down to Seattle in June and follow Highway #2 to North Dakota and then drive back North to Canada.
Aside from an 8% speedo/odo change (disregarding the actual accuracy), is there any change in the transmission shifting points, or do they relearn?
Transmission shift points are based on Torque and RPM, so I'd be surprised if there was any change other than the 8% increase. Of course, since you have a higher effective gear ratio, you're going to be pressing harder on the go pedal to get the same acceleration, so that changes things...
Car drives so smoothly that unless I stare at the tach I never feel the transmission shift. But with the larger tires it has to be a difference and the wider tires probably reduce fuel economy.
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Speedometer and Odometer Error with Larger tires
