O/T--speed of truck when changing tire sizes

How much do you think the actual speed of a truck changes when you go from stock 245/75/16's to 285/75/16's? Just wondering how fast I was really going when the speedo said 75mph on the interstate the other day with these 285's (truck originally had 245's from factory). Thanks.

Tape the circumference. Wouldn"t percentage increase in size equal percentage increase in speed?

I can't under stand why any one would change the size tire on a truck or car. They are made for that size and any thing you will take away from performance of that truck or car.

You would actually be going 80.8 mph if your speedometer was reading 75.

There are a bunch of calculators on the web that will show you the speeds and changes when going from different tire sizes.

BTW, that new tire is 2.3 inches larger in diameter.

JMS is correct. The truth is in the details, and the details in this case include the fact that the tire manufacturers produce tires with statistically different rolling diameters for the same marked size. To actually determine the real world difference, it is necessary to put the old tire on the ground in a parking lot (inflated to the correct pressure, and marked near the tread sholder) starting with the mark at the pavement, roll it forward 10 revolutions. Measure.
Mount the new set pressure, and roll it 10 revolutions measure. the % difference will be very accurate. Jim

If I understand the tire numbers correctly, your old tires were 773.9mm tall (ground to top of tread), and new ones are 833.9mm tall. Since the distance is related to ratio of the circumferences and the rotations per time, if we hold the rotations per time constant it reduces down to the the ratio of the diameters. 833.9/773.9 = 1.0775
1.0775*75mph = 80.8 mph

Please note that I am assuming the tires are perfectly rigidly round (no load on them) AND that you speedometer is accurate for the original tire size. With these considerations the increase in speed shown above is an upper bound limit.

I believe the optimum size was chosen from the manufacturer.
But was it selected for the average use, or the maximum load intended?
Was it best for soft field use on a farm truck, or the highway?
was there an option to use perfectly good tires from a different vehicle that was going to be scrapped or sold?
Was there an attampt to obtain a little extra mileage from effective gear ratio?
Unless there is a danger of instability, tire rubbing fender or chassis, failure from overloading if smaller tires were used, or powertrain wear from mismatched rubber, I see no issue in doing tire swapping. No offense intended, just my opinion. Jim

Don't know what make of truck you have them on, but I know with the Dodge's you can go to a dealer and have the computer recalibrated to the tire size. The speedometer reads correct after that. Cost is $40-60 depending your dealer.

The 285s are 7.5" more circumference and give a 7.8% drop in overall gear-ratio. When the speedo reads 60 MPH you'll actually be going 65 MPH.

There are slight variables, depending the brand and model of tire, but they don't very very much.

Thanks everyone. The reason I have these on was I needed to buy tires for a crew truck and I found a decent deal on these 285 load range E's. I always wanted to try them, so I moved the tires that were on my truck to the crew truck and bought new ones for me! A little selfish I know. I knew the gear ratio would get higher, but this Duramax and Alison don't really care, they pull really good anyway.

Don't ya know...you will GOING just as Fast, only the Wheels will be turning slower....!!!

All you needed to do was measure the TIME between 10 Mile-Posts on a fairly level stretch of freeway and you can figure your speed..
I use 10 mile-posts, to get a good average..

Ron.

Being your truck is a '01 or newer(since you have a Duramax)you can recalibrate the new size into the computer.Take it to you GM dealer or find a shop that has a Tech2 scanner.

Balderdash. The tires shipped with the vehicle from the factory were chosen to be the CHEAPEST they could buy. Not the greatest for performance. Nor even the ones they set the speedometer gears for in some cases. My dad had an '86 F150 that the speedometer read about 10% fast. He got great mileage, he thought, because it also registered 10% more miles than mile posts and he drove it gently. When I bought it the next set of tires I picked to calibrate the speedometer and there were a couple sizes taller to achieve that.

The '98 F150 I have now has manual transmission and came with 3.08 gears. Wouldn't pull the hat off your head nor any trailer over 2200 pounds. Had to start in first gear on level ground unloaded because of a close ratio transmission. One way to get more thrust for trailer starting would have been to change to 24" diameter tires in place of the about 30" it came with. I changed the rear axle gears to 4.10 and kept the stock tire size for ground clearance. I changed the speedometer sensor driven gear to correct the calibration which became computer adjustable later in that model year. Didn't change the gas mileage, unless I run it up in the low 80s per hour and it pulls good. Has great throttle response. It was geared wrong for the engine that is one that peaks out at high rpms. I rated it at 4700 pounds trailer and I have weighed truck and wagon at 11,000 pounds over the scale.

Gerald J.

You must have some sort of portable GPS around. Or you have a friend who has one. Go for a little spin with the GPS in the speed mode and make yourself a little chart.

Gordo

I went from a 245-16 to a 265-16 and it changed my 07 chev. about 2.5 mph. Put the truck at 60 and clock it between mile post on the x-way with you watch. 58 sec your going 62, 62 sec, your going 58, and so forth.

http://www.net-comber.com/tirecalc.html

There are about a thousand similar pages out there if this one does not figure out what you need

GM will no longer do that.

Perhaps I'm stating the obvious, but why don't you simply check your speedometer against the mile markers? It takes less time than it did for you to write your post here. All interstate highways have mile markers. Time the number of seconds it takes for you to cover one mile, and divide that into 3600. Result is miles per hour.

I was going to. On the phone about work almost the whole time. Only on the interstate for a short while. Just was wondering.

Calculations are not presice. There are too many variables. It always comes down to loaded radius. That is the distance between the axle centerline and the pavement. That radius depends on such variables as ambient temperature, tire temperature, tire pressure, speed, load over driving axle, and probably a few other unknown factors like slippage. Ask any NASCAR tire tech but be sure to bring some Excedrine.

Speed is interesting because tires will grow due to centrifical forces. Ask any drag racer.

The best answer to your question is found by putting a revolution counter on the tire or axle and running the vehicle at a constant speed over a measured course (many miles). It would be nice if you could automatically start/stop the counter at speed while going through the traps.

But if you only need an approximate measurement, start with the published loaded radius values offered by the tire mfgs. Calculate the percent difference between sizes.