285/75-16 observations

MountaineerTom · July 7, 2002

In the last week, I've went past 3 different radar guns that show what your spped is. Two on the interstate in construction zones, and one here on base as your going through the front gate. All three times the radar showed exactly the same speed my speedometer was showing. I havn't reprogrammed with a HPPIII or similar product either. The two on the interstate were speeds of 55 and 65 MPH, with the crusie control set. The speed on base was 30 MPH (cruise control not used).

So...maybe recalibrating for just the speedo is not really needed. I've also not had any problems or concerns with the ABS after going with bigger tires. The brakes actually seem a little better.

Just passing my experiences. Anyone else with 285's who's not using a HPPIII notice the same?

ClipperSkipper · July 7, 2002

Tom:

I didn't see the OEM tire size however if your speeds

are consistent with radar and it looks correct, no worries.

Where is 'on base' btw?

Have a great one!

MountaineerTom · July 8, 2002

Opps, the original tire size was 265/75-16. Not to big a difference, but I've heard it could be off by 5 mph or so when switching from 265's to 285's.

"On base" is on Shaw Air Force Base, SC. Where I work, and live.

sincityZ71 · July 8, 2002

I noticed the same exact thing when I switched to 285s too. I even broke out the GPS and check my speedo to it and it came out just fine, I think I only saw about a .2mph difference, so no real difference. And I agree that I think the braking is actually better with the bigger tires.

gandolphxx · July 8, 2002

what is the possibility that you just corrected the notoriously inaccurate Chevy speedo.

MarkD · July 8, 2002

Now that you mention it, it seems like I adjusted the stock wheel size DOWN with my Hypertech on my '99 with the stock tires.

MountaineerTom · July 8, 2002

what is the possibility that you just corrected the notoriously inaccurate Chevy speedo.

gmcjoe · July 8, 2002

Hey Tom.

Interesting news about your speedo. Did you have the 265 tires before the 285s?

I haven't programmed mine yet as I was out of town, but maybe I won't??

Also, I'm thinking I can feel the different tread pattern in the steering wheel. Is this normal with a more agressive tread pattern at highway speeds? I'm wondering if I should visit the tire dealer this week unless it is normal.

Thanks!

Joe

MountaineerTom · July 8, 2002

Hey Tom.
Interesting news about your speedo. Did you have the 265 tires before the 285s?

I haven't programmed mine yet as I was out of town, but maybe I won't??

Also, I'm thinking I can feel the different tread pattern in the steering wheel. Is this normal with a more agressive tread pattern at highway speeds? I'm wondering if I should visit the tire dealer this week unless it is normal.

Thanks!

Joe

I did have 265/75-16 Firestones, and then changed to 285/75-16 BFG AT's, without doing any kind of reprogramming. The radar thingy at the front gate was not turned on this morning when I went off base, so I didn't get to check it again.

I didn't notice any different feel in the steering wheel. ??? You may have one just slightly out of balance. Maybe the tire shop could re-balance them for you. I like the ride better with the stiffer tires.

CMNTMXR81 · July 8, 2002

Some points to ponder;

1) as already mentioned, our speedo's aren't something I'd bank the house on. Your best bet for an accurate speed reading would be to have a TechII or AutoTap program hooked up and taking a direct reading off the VSS (Vehicle Speed Sensor).

2) At what speed were you doing this comparing? If you're doing 30mph, the difference isn't all that much especially between the two sizes you had. Conversely, if you're doing 100mph, that difference is much greater.

As an example, your speedo is 3.731807438736158% too slow with the difference in size between 265's and 285's. Presuming that your speedo is GM certified (I.E. 100% accurate), at speed, the speedo difference's are;

@ 30mph = 1.119542... mph slower than actual

@ 100mph = 3.731807438736158 mph slower than actual

So as can be seen, if at 30mph (or similar speed), factor in speedo calibration, the radar's calibration and the window of 1.119542mph, the difference could be negligible.

CMNTMXR81 · July 8, 2002

DOH!!! Forgot to add the site I use to compute the % difference for tire size.

http://www.secret-secret.com/turbo/DIY/tir.../turbotire.html

MountaineerTom · July 8, 2002

30, 55, and 65 MPH were the three speeds I showed on my speedo, and the same speeds showed on the radar thingy. Cruise control was set on the 55 and 65 readings, on level ground. The 30 MPH reading was on level ground also, but the cruise was not set, jsut me driving at 30 MPH. I know speedo's are not the most acurate peices of equipment around, but I was suprised to see 3 different readings, on 3 different days, on 3 different radar stations, at 3 different speeds, all show the same on my speedo and the radar.

CMNTMXR81 · July 8, 2002

The difference between 30 and 100mph is only 2.6122265mph so whether you were doing 55, 65 or whatever, that window for error is rather small. A window that could be buffered by poorly calibrated equipment on both ends.

Keep in mind, the radar unit in that speed notifier isn't routinely sent out for calibration (if at all) like a normal radar unit in a squad is "required" to be. So it too isn't entirely accurate.