do I need speedo calibration for going 20 inch ?

[melvinakshay]

Ive got an LS with stock 18 inch, I just upgraded the wheels to 20 inch (20*9 and 275/40/20 all round).
The car does handle a lot better and I definitely feel the difference. Do I need any kind of speedometer calibration ? I think my speedo might be a bit off. The reason is now the gears feel taller. What I mean by that is earlier 40mph would be something like 4k rpm @ 3rd. Now 40 is barely 2-2.5k rpm @ 3rd. had to get used to the sudden change. I kinda change gears based on the sound though.

[Sterling]

No change needed... the diameter of the tires is the same. Not sure why you seem to be noticing a difference.

[formare]

Is 275 40 20 the factory tire height?

[n116ebo]

Originally Posted by melvinakshay

Ive got an LS with stock 18 inch, I just upgraded the wheels to 20 inch (20*9 and 275/40/20 all round).
The car does handle a lot better and I definitely feel the difference. Do I need any kind of speedometer calibration ? I think my speedo might be a bit off. The reason is now the gears feel taller. What I mean by that is earlier 40mph would be something like 4k rpm @ 3rd. Now 40 is barely 2-2.5k rpm @ 3rd. had to get used to the sudden change. I kinda change gears based on the sound though.

If you have any doubts about the correct speed take your GPS and go for a cruise and see if speedo and GPS match.

[The Stig]

Originally Posted by formare

Is 275 40 20 the factory tire height?

For the rears yes

[Sir Nuke]

Originally Posted by melvinakshay

Ive got an LS with stock 18 inch, I just upgraded the wheels to 20 inch (20*9 and 275/40/20 all round).
The car does handle a lot better and I definitely feel the difference. Do I need any kind of speedometer calibration ? I think my speedo might be a bit off. The reason is now the gears feel taller. What I mean by that is earlier 40mph would be something like 4k rpm @ 3rd. Now 40 is barely 2-2.5k rpm @ 3rd. had to get used to the sudden change. I kinda change gears based on the sound though.

melvin, your LS came with 18's....with P245/55R18 all-season on them.....When you put the 20's with 275/40's (stock tire size for RS and SS equipted cars) the ROLLING DIAMETER of your wheel/tire combo didn't change.....so there should be NO DIFFERENCE due to changing the wheels/tires.

[melvinakshay]

Originally Posted by Sterling

No change needed... the diameter of the tires is the same. Not sure why you seem to be noticing a difference.

does having 275/40/20 on the front make a difference ? I know the recommended size for the front is 245/45/20 and the rims would be 20*8 on the front. I've got 20*9 and 275/40/20 all round ?

Originally Posted by formare

Is 275 40 20 the factory tire height?

20X8 and 245/45/20 front
20X9 and 275/40/20 rear (I got these all round )

Originally Posted by Sir Nuke

melvin, your LS came with 18's....with P245/55R18 all-season on them.....When you put the 20's with 275/40's (stock tire size for RS and SS equipted cars) the ROLLING DIAMETER of your wheel/tire combo didn't change.....so there should be NO DIFFERENCE due to changing the wheels/tires.

hmm, I guess the rolling diameter might not have changed... but I definitely feel a difference while gear shifting. I mean now the RPM is way off compared to the speed. I think the 3rd gear now behaves like the 4th gear on the 18s. Its hard to explain but it feels different. I did miss-shift a few times cause of the change. I'm not talking about the acceleration, just the speed/gear difference.
99% of my travel is in the city so I'm usually @ 3rd or 4th gear. I rarely hit the 5th or 6th

[The Stig]

Originally Posted by melvinakshay

does having 275/40/20 on the front make a difference ? I know the recommended size for the front is 245/45/20 and the rims would be 20*8 on the front. I've got 20*9 and 275/40/20 all round ?

no it doesn't make a difference. It is the same tire diameter overall.

[melvinakshay]

Originally Posted by stieger

no it doesn't make a difference. It is the same tire diameter overall.

:( any guesses as to why the gears feel different now ? could it be the extra weight ?

[The Stig]

That or it's all in your head

[nancygrl]

245/55R18 = 28.6" tall....275/40R20 = 28.7" tall....So, no real difference in your overall diameter.

Your stock18" wheels = 32.0lbs each.....Your 20" 20x9 wheels = 32.0lbs each
Your stock 18" tires = 30 lbs each.......Your 275/40R20 tires = 36 lbs each


So the only measurable difference is that the 275's add 6 lbs on each corner.

[melvinakshay]

Originally Posted by stieger

That or it's all in your head

:( well... ha ha idk but I could swear it feels different.

Originally Posted by nancygrl

245/55R18 = 28.6" tall....275/40R20 = 28.7" tall....So, no real difference in your overall diameter.

Your stock18" wheels = 32.0lbs each.....Your 20" 20x9 wheels = 32.0lbs each
Your stock 18" tires = 30 lbs each.......Your 275/40R20 tires = 36 lbs each


So the only measurable difference is that the 275's add 6 lbs on each corner.

That is VERY helpful information. I was a bit bothered about that extra weight ! but I guess 6 pounds of rotational mass on a 3750 pound car wont be much of a difference

[DraggnFrameGMC]

If you change your tire sizes and it is bigger than factory does it throw off just the speedo or does it mess with the mileage, mpg and fuel trip #'s as well? I know I should have lost some mpg with a bigger tire but 5mpg for a 1.5" taller tire. That didnt even affect my duramax that much when i lifted it and went with a 10" taller tire.

[Sir Nuke]

YES IT WILL....it will effect them all..........If you put on TALLER tires (1.5" larger diameter) you speedo will be registering 60 MPH.....when you are ACTUALLY running 63 mph......and for every hour you drive at 60....you would have actually gone 63 miles.

so you heve two different effects going on. Although you will be thinking your going slower than you really are......your also showing fewer miles on the odometer than you have actually put on your car.