Do I need a Speedometer Recalibration?

[CadillacLuke24]

Clyde, my truck, is a 2015 Chevrolet Silverado LTZ Z71 Double Cab with factory 18" wheels (RPO RD1) and Goodyear Wrangler SR-A tires, in the size of P265/65R18.

 

I got him new shoes, in the way of factory 20" Chevrolet Polished aluminum wheels (RPO RD4) wearing the Factory Goodyear Wrangler SR-A tires, in the size of P275/55R20.

 

The new wheels should be available later this week, so I'm hoping to possibly mount them next week. When I do, will I need to recalibrate my speedometer at the dealer, or does GM use the same setting for 18 and 20 inch wheels?

 

Thank you for your time, and have a great day!

[O_J_Simpson]

Here is a thought. Do you have a smart phone or GPS? If so, tell us if your speedo works or not.

[Coby7]

You will be 2% off , so when your speedometer reads 100 you will actually be going 98. A recal takes minutes but they will probably charge you an hour. P275/60R20 would have been spot on.

[Thomcat]

Most accurate. Inflate tires to spec and then go on a road Rte./IS with mile markers and zero the trip meter as you pass a mile marker. Then travel an exact distance in one to ten mi., or km, and compare the trip meter reading to the actual distance covered. More miles measured off = more accurate reading.

[CadillacLuke24]

Thanks for the tips guys, I really appreciate it. It's weird that GM didn't use the 275 60R20 size for continuity, but they do a lot of crap that makes me wonder...Like cheaping out on head bolts in the Northstar

[Coby7]

Thanks for the tips guys, I really appreciate it. It's weird that GM didn't use the 275 60R20 size for continuity, but they do a lot of crap that makes me wonder...Like cheaping out on head bolts in the Northstar

Actually some trucks come with P275/60R20, saw some for sale on here.

 

And as for the Northstar it wasn't the bolts it was the threads in the aluminium block that would rip out. Should have used helicoils from day one.

[CadillacLuke24]

Hmmm, maybe I didn't look hard enough.

 

True that, and GM wised up and changed the length of the bolts in 2000, and the thread pitch in late 2004. Virtually eliminated the problem.

 

That's ok, if my DeVille ever pops her heads, I'll stud the block

[Chris]

Thanks for the tips guys, I really appreciate it. It's weird that GM didn't use the 275 60R20 size for continuity, but they do a lot of crap that makes me wonder...Like cheaping out on head bolts in the Northstar

275/60R20 is not equivalent. Hard to find an equivalent. 275/55 is as close as you will get

 

GM doesn't run the 275/60R20 on the 1500 trucks. People put them on with a level. Ram does run that size though so they are out there and can be cheap at times

[CadillacLuke24]

Excellent! I figured go with what I've seen. My dad's CC 2015 has 275 55R20s, and that is all I could find on eBay.

 

I like the Wranglers I got on now, so I think I will be happy with my purchase.

 

I benchmarked my current wheels against my phone GPS, so now I will know if the new wheels and tires are off any.

 

Thank you Chris

[HoosierZ]

I recently went from the stock 265/65/18 to 285/65/18. I expected the speedometer to be off 2-3 mph at 70. My Garmin GPS shows 71 with my cruise set at 70. Apparently, the speedometer reads a little fast with the stock tires. Yours will likely be spot-on with the 32" 20s vs your 31.5" 18s.

[tanner709]

You will be 2% off , so when your speedometer reads 100 you will actually be going 98. A recal takes minutes but they will probably charge you an hour. P275/60R20 would have been spot on.

It would actually be the other way, speedo reading 100mph, in actuality doing 102 mph.

 

Most dealers will not agree to a re-cal for whatever reasons... blah blah blah re-flash, void warranty, etc. I think they open a can of worms and pile on loads of work if they agree to it once.

[John Puckett]

Have a crazy vibration in my 2007 chevrolet silverado 2wd at around 70 only really feel ans notice it not under torque load... Put hostile 20's and 305/55/r20. Also major rubbing issue, what would be my best brand choice for lifting, and after the lift will she need an alignment or any other parts to keesmoothriding smooth and done right price does not matter please help first truck im gping up with... Gonna be my aaphault queen

[tanner709]

Have a crazy vibration in my 2007 chevrolet silverado 2wd at around 70 only really feel ans notice it not under torque load... Put hostile 20's and 305/55/r20. Also major rubbing issue, what would be my best brand choice for lifting, and after the lift will she need an alignment or any other parts to keesmoothriding smooth and done right price does not matter please help first truck im gping up with... Gonna be my aaphault queen

 

Yes of course you need an alignment with the lift, you actually need one even after a tire size increase. You change the angles of the front end wrt the road.

 

Don't get ahead of yourself with mods. Plan them out and look at the whole package when you upgrade tires/suspension.

[Thomcat]

275/60R20 is not equivalent. Hard to find an equivalent. 275/55 is as close as you will get

 

GM doesn't run the 275/60R20 on the 1500 trucks. People put them on with a level. Ram does run that size though so they are out there and can be cheap at times

Chart looks good on paper and sizes are for mounted tires off the vehicle. Problem is with different profiles, although the overall size of the unmounted tire wheel combo is the same the sidewall thickness will vary. And once a weight is placed on the tire the sidewall thickness against the road will shrink proportional to the air pressure in the tire, weight applied and thickness of the sidewall. The most accurate method would be to measure the distance from the center of the flat spot on the tire sitting on the ground under vehicle load at the specified pressure to the center of the mounted wheel and compare the radii of the two tires for comparison.

 

.02 The actual speed of the vehicle as measured by the speedometer is based upon the rotations of the axle, the distance by the #rotations of the axle times the circumference based upon the diameter of the tire. But the actual circumference is not that of the unmounted tire, but a reduced circumference based upon a diameter that is 2x the reduced radius after the sidewall has flexed at the appropriate pressure under the weight of the vehicle. Speedometer calibrations are based upon the stock size tires with the stock profile/sidewall thickness and likely a fudge factor to account for the reduction due to tire flex. Easy to verify over inflate the tires by 10 lbs and the odometer will over report the mile marker, underinflate by 10 lbs and the odometer will under report based on the same mile markers.

 

Aside from a speedometer that is a little off, the main risk in changing tire profiles drastically is road force balancing problems. On large diameter tires the weight of the wheel/tire can cause wheel vibration problems as detailed by GMs concerns in recent literature and specialized equipment to diagnose these problems.

 

http://sandyblogs.com/techlink/wp-content/uploads/2017/03/GM_TechLink_01_January_2017.pdf