After installing taller wheels and tires, how to correct speedo.

mkleymann89 · May 5, 2015

I put a taller wheel/tire combo on my 2004 sierra 2500 SLE 6.0. Of course, this throws off the speedometer, as well as the odometers. What is the best way to get this fixed?

Chris · May 5, 2015

Custom tune. Will correct, and reapond better to your driving

Doug_Scott · May 5, 2015

It depends on what size you went to. If you went to a tire size that was an option on your truck then the dealer can change a setting in the PCM. If they are not comparable to any factory option size, then a custom tune is the best way. On my 2010, I went with 4.10 gears, and since they were not an option or even available from factory, my only solution was BlackBear. Changing tire size from the 245-70-17 to the optional 265-70-17 was able to be fixed by the dealer.

mkleymann89 · May 7, 2015

I went from a 245/75/16 to a 265/70/17. If I order a CAI from blackbear will the tune that comes with that be able to fix it?

Doug_Scott · May 7, 2015

Don't think the CAI comes with a tune. You will have to talk to Justin at BlackBear about cost to correct. You could also check the biggest custom wheel shop in your area to see what they suggest.

pcf · May 8, 2015

I went from a 245/75/16 to a 265/70/17. If I order a CAI from blackbear will the tune that comes with that be able to fix it?

Your speedo is fine, it's close enough for government work.

A 245/75r16 has a nominal diameter of 30.4 inches.

A 265/70r17 has a nominal diameter of 31.6 inches.

Your new tires are 3.3% larger than the old tires. At 20mph you'll really be doing 20.7mph and at 60mph you really be doing 62mph. If it's important to you, in 100,000 miles on the odometer you will have driven 96,200 miles. Of course none of this account tire wear, etc, etc.

mkleymann89 · May 8, 2015

Let me put it this way, I made a 310 mile trip and the odometer was off by 10 miles. At 60 mph on the speedometer, I'm actually going 64 mph. That's gps and radar tested which I believe is as accurate as it's going to get. I would like to be able to set my cruise control and not have to worry about it. Plus I want to be able to accurately track fuel mileage, and 10 miles on a 300 mile trip makes a big difference in fuel mileage. I know, short term, it's probably not a big deal. But it eats at my ocd. I think I'm going to call Justin and see what he says. I was told that all blackbear CAIs came with a tune. I appreciate all the help gents.

Doug_Scott · May 8, 2015

Stock speedometers are not always 100% accurate. Typically the odometer is a lot closer to being 100%. Nice thing about calculating speed differences due to tire change is that it is pure mathematics, and is easily applied to distance as well.

It is widely known among owners of VFR800 Honda motorcycles that the speedo is 8% off from factory, but odometer is close enough to call it 100% accurate. Used to ride with a buddy who had a Yamaha FJ1200, and while riding along, we would use his speedo to set the cruise speed(cops have a lower "allowed over" number for sport/tour bikes) due to mine being off so much. On a 500km trip, the odometers were out by 1km from each other, and that was likely due to the tenths not being recorded.

Anything under 4% is going to be hard to correct. If you look at tread depth on the new tire it may be as much as 10/32 to 11/32 inch deep. By the time the tire is worn out, it will be as much as 1/2 inch smaller in diameter, and at that point the speedo will read 61 at 60mph. After doing some checking, it appears that your original tires were the same diameter as my original 245-70-17 tires on my 2010. I switch my wheels and tires out for summer, and I am using the 265-70-17 tire size. Speedo difference is very minor. Not sure why you are hung up on calculated mileage, it is not like the mileage is actually less in real life. Unless you are trying to calculate if going to a taller tire will give better mileage due to engine speed being lower at 60mph?

mkleymann89 · May 8, 2015

Fuel mileage has increased, dramatically in fact. I calculated it the same way (miles traveled according to gps) with the old set up and new set up, almost 2 mpg better. I expected a couple tenths at best. And I know y'all will probably tell me I'm an idiot and I shouldn't worry about it but 4-5 mph difference in tires that probably have 40%-50% tread bothers me and when I get new tires I know it will be worse. +/-1 mph would be acceptable.

pcf · May 8, 2015

Just to mess with your OCD, your rates of error don't come close to adding up. 60/64 = .9375 or 6.25% difference. 300/310 = .9677 or 3.23% difference. Your speedometer error is almost a 100% greater than your odometer error. That 3% would require some pretty extreme up and down hill driving to account for the difference.

If you're running GPS use that for your calculations and don't worry about what the odometer and speedometer say, they'll never be accurate enough to make you happy.