Yeah, the dude should have typed '14 instead of 14', which would be 14 feet lol. But he means they're off a 2014 jeep

 

Ha ha, ok I get it. Thanks. I thought the law of physics or arithmetic had been repealed.
Christine, can you give me the executive summary?

 

LOL Did you see my "geek" come through??

The computers in the truck use the speed of the output shaft of the transmission to calculate what the speed over ground is assuming you have stock tire sizes. To do this it must be programmed with the rear axle ratio and it assumes the size of the tires you are running are stock.

With larger tires, the computers will report to the speedometer what speed it thinks the truck is going based on the rpms of the transmission output shaft. With the rear-end ratio unchanged, the number of revolutions of the driveshaft to make the wheel rotate once is unchanged, say 3.73 rotations of the driveshaft to one rotation of the wheel. The distance the truck travels for each wheel rotation is the circumference of the tire. If you have a larger tire, the circumference is larger.

For my 30x9.5s, they are about 3% further around than the stock ones.

I have found the following gadget pretty useful to monitor the engine and transmission temperature along with the actual speed: UltraGauge OBDII Scan tool & Information Center

I originally got to for the transmission temperature feature but it also has a way of reporting a calibrated speed. The later helps remind me of my actual speed.

 

The problem I had with my Suburban was that I changed the gears from 3.73 to 4.10 so the VSS was off (as you said). I didn't care that much about the speedometer reading, but the ABS was confused because the front wheel speed is read directly from the wheel speed sensors, so it didn't change. The computer interpreted that as a mismatch between front wheel and rear wheel speed and turned on the Check Engine Light. I had to correct the reading from the VSS to match the readings from the front wheel sensors so the ABS would work.

BTW, that '99 Suburban was the worst POS I've ever owned. Almost made me give up Chevys altogether.

 

So the larger tires means less jump off the line but it will stay in 1st gear longer (i have a/t)? And the top end will be 7% higher?
The speedo will be off but otherwise there will be no harm to the truck?

 

Yep. That would be my understanding.

 

With my 31's on my non ZR2, I just keep my Garmin powered up in the bottom left of my windshield and bring the speed and mileage trip on it to full screen, works like a champ lol

Around town when my speedo shows 45, I'm actually doing about 48 per the Garmin. The faster you go, the worse it gets. On the highway when the speedo says 70, it's actually closer to 76-77ish

 

GPS app might be a good solution for me too. In which case I could forget about getting someth9ng with only a small speedometer error, I could go with a much bigger tire, like 30.5"
Now, I have a visual dislike for the big gap above the tire. Now the tire is 27", if I go with a 30", how much will that gap close, if any?

 

Going from a 27 to a 30 is three inches, so the radius will increase by 1.5". So that's how much the gap will close up

 

So those sizes, 27" and 30" would be the nominal diameter of those tires with the radii being half that. If you go from 27" to 30" tires, the gap will decrease by about 1.5".

Here is a useful link for comparing all sorts of tire sizes: https://tiresize.com/calculator/

For starters, you if you compare the stock 2wd tires, 235/70R15, to 30/9.5R15 you'll discover that the 235/70R15s have a diameter of about 28.0". Of course this does not allow for the compression at the bottom of the tire where it contacts the road. For those two sizes, there is about a 7% error in the speedometer (the speedometer would under-report the speed when using the larger tires).