Rocktman Posted December 16, 2019 Share Posted December 16, 2019 When I first got my 17 Legacy back in 2016, I was pleasantly surprised to see that my speedo, gps & cruise control all showed the exact same speed (as opposed to my 14 Legacy where it was always slightly off). Tires were the oem Goodyear LS2s. About a month ago I replaced the Goodyears with Conti TrueContact Tours, same size. The car is no longer a DD & can go days w/o being driven. This past w/e we went to Philly & I noticed that the speedo no longer coincides with the gps & cruise control. It now appears to be about 1 mph faster. Could the Conti's circumfrence be slightly different from the Goodyears? Link to comment Share on other sites More sharing options...
whitetiger Posted December 16, 2019 Share Posted December 16, 2019 all tires vary slightly in circumference, and change as they wear. a 1 mph difference is not worth being concerned over. the fact that it was as precise as it was with your OE tires was probably luck of the draw. Link to comment Share on other sites More sharing options...
waldguy Posted December 17, 2019 Share Posted December 17, 2019 I had to smile at your problem after reading your post title (conundrum =an intricate and difficult problem). Intricacy - what whitetiger said. Difficult - not really! A tolerance of 2% to 8% is considered industry acceptable. In my experience, most speedos read a little fast -- maybe to keep you from getting as many speeding tickets. Mine reads about 1-2 mph faster at 60 mph than actual. My tires are also at about 5/32. So expect some variance. If you think 1 mph is too much read this (caution, uses km/h!): https://www.ctvnews.ca/autos/speed-check-how-fast-are-you-really-driving-1.3445497 Link to comment Share on other sites More sharing options...
george-in-georgia Posted December 17, 2019 Share Posted December 17, 2019 Turning the conversation 90 degrees - What are your impressions of the Conti TrueContact Tours ? Particularly in the areas of ride quality and road noise . Thanks George Link to comment Share on other sites More sharing options...
YeuEmMaiMai Posted December 17, 2019 Share Posted December 17, 2019 the speedo is never going to tell you the real speed, most under report by about 1-2 mph...this is by design. Even GPS will be in error when you are not on level ground. Link to comment Share on other sites More sharing options...
tumbleweed 49 Posted December 17, 2019 Share Posted December 17, 2019 I have noticed the LS2s on my Legacy are as you describe, right on according to the speedometer and the GPS. I suspect the cars may be designed with that in mind. I hope I don't get too much variation when I put new tires on it. Link to comment Share on other sites More sharing options...
whitetiger Posted December 17, 2019 Share Posted December 17, 2019 the next thread will be concern over the HVAC set at 75 degrees while blowing 74.5 degrees. Link to comment Share on other sites More sharing options...
Bill G. Posted December 20, 2019 Share Posted December 20, 2019 Air pressure will change the tire diameter too. Link to comment Share on other sites More sharing options...
Rocktman Posted December 30, 2019 Author Share Posted December 30, 2019 Turning the conversation 90 degrees - What are your impressions of the Conti TrueContact Tours ? Particularly in the areas of ride quality and road noise . Thanks George I've been pleased with them. Ride quality has been good & road noise isn't a problem so far. Link to comment Share on other sites More sharing options...
ncted Posted December 31, 2019 Share Posted December 31, 2019 According to tirerack.com, the Continentals start out at 11/32 compared to the LS2s at 10/32, but I doubt that is enough to account for the discrepancy. The total diameter is the same, but the LS2s get 777 revs per mile compared to 773 on the continentals. Again, not enough to explain the difference. If anything, I'd expect the new tire to result in a slower speed than the worn one, assuming I am thinking about it correctly. Link to comment Share on other sites More sharing options...
Bill G. Posted January 1, 2020 Share Posted January 1, 2020 According to tirerack.com, the Continentals start out at 11/32 compared to the LS2s at 10/32, but I doubt that is enough to account for the discrepancy. The total diameter is the same, but the LS2s get 777 revs per mile compared to 773 on the continentals. Again, not enough to explain the difference. If anything, I'd expect the new tire to result in a slower speed than the worn one, assuming I am thinking about it correctly. 773 vs 777 is 1/2 of 1%. Or 0.30 mph at 60 mph. With the precision of today's speedometers, and even GPS, that is virtually zero difference. Link to comment Share on other sites More sharing options...
Saltdet Posted January 11, 2020 Share Posted January 11, 2020 Turning the conversation 90 degrees - What are your impressions of the Conti TrueContact Tours ? Particularly in the areas of ride quality and road noise . Thanks George Put them on my 2015 3.6. Incredible upgrade quieter great in the snow too. Link to comment Share on other sites More sharing options...
NNJLegacy Posted January 12, 2020 Share Posted January 12, 2020 all tires vary slightly in circumference, and change as they wear. a 1 mph difference is not worth being concerned over. the fact that it was as precise as it was with your OE tires was probably luck of the draw. https://tiresize.com/wheel-offset-calculator/ I recently purchase a wheel and tire package from Tire Rack. I used the above calculator to check the size of the new rims and tires when compared to oem. My original Oems are 235/50R18 40mm offset. I ordered 235/55r17 45mm offset package. The speed as calculated above came back as Speedometer Error - (Set Original Setup to OEM Size) Speed | Actual 20 mph | 19.9 MPH 30 mph | 29.9 MPH 40 mph | 39.9 MPH 50 mph | 49.8 MPH 60 mph | 59.8 MPH 70 mph | 69.7 MPH 80 mph | 79.7 MPH 90 mph | 89.7 MPH Like someone else here said, if you stay with OEM make and model's the speedo will probably not change. I for one plan on getting rid of the OEM's as I have terrible tread life compared to the advertised mileage. Link to comment Share on other sites More sharing options...
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