I came across a video from a transmission shop online. He mentioned that changing the tire size on a vehicle can adversely affect the transmission longevity. I have changed the width of my tires from 265 to 275 and the sidewall has gone up from 75 to 80. Obviously the width should not affect the gear ratio, but the sidewall might affect the gear ratio. I would like to get everyone’s opinion, if this small change in tire size will have an affect on transmission longevity and performance?

 

Lots of things place extra stress on your transmission, including driving style and whether you tow and/or offroad. Bigger tires add more weight at each corner which would also adds more stress to the transmission. How much and how does that affect the longevity of the transmission? Who knows?! Your increase in tire size looks minimal so I wouldn't be too concerned. Just service the transmission fluid when needed (every 5-6 years or 60k miles is a general rule) and enjoy the ride!

 

Taller, heavier, higher rolling resistance tires increase transmission temperatures.
With proper temperature management larger tires should have a minimal impact on transmission life in normal scenarios.

 

... approximately a 5% difference ... will have an influence, but probably mostly lost given other considerations mentioned in earlier posts.

 

That "sidewall" number is actually called the Aspect Ratio, and is the ratio of the sidewall height to the tire's width, so for a given aspect ratio increasing the width of the tire does indeed increase the height (diameter) of the tire. You've increased both, which compounds the effect. From a mathematical perspective, the overall diameter equals width multiplied by the aspect ratio (as a percent) multiplied by 2 (because there are two sidewalls) divided by 25.4 (conversion from millimeters to inches) plus the wheel diameter. I'm not sure why they have made it so complicated with a mixture of metric, imperial, and percentages, but here we are.

For your two stated tire sizes, assuming a 17" wheel:

[(265 x 0.75 x 2)/25.4] +17 = 32.65"

[(275 x 0.80 x 2)/25.4] +17 = 34.32"

The difference between those two tires is 5%, which some might argue isn't a super big number, but the kicker is that the tire size from the factory is 265/65R17 (or 265/60R18) which is only 30.56" (or 30.52") diameter, therefore your actual size increase compared with factory is 12%, which IS a big difference.

Yes the weight of the tires has an influence on the load and it is rotational inertia (because they spin) which has roughly twice the effect on acceleration as translational inertia (weight that doesn't spin), but it is the change of effective gear ratio which really adversely affects the transmission. What you've done is effectively changed the factory 3.91 axle ration to 3.48 (if such a gear existed). To "correct" for the increased tire size, you'd need to re-gear to 4.39 (if such a gear existed).

Anyone who doesn't think a 12% increase in tire size adversely affects the longevity of a 6-speed automatic transmission with a very high 0.586 double overdrive ratio is either ignorant or foolish.

 

Agreed that a significant change in rotating diameter would have an effect on transmission. Just want to add, that the numbers murderman has posted are theoretical calculations. That is, he is using theoretical values for diameters and aspect ratios. In order to really understand, you should take actual measurements of the width and diameter of the tires. After all, these are molded layers of varying materials: belts and rubber and variation exists.

 

Absolutely, and the effective ratio is actually based on the rolling loaded radius of the tire which is a lower number due to the weight of the vehicle. But here's an interesting aspect of that consideration. If one takes the theoretical radius and subtracts nominal 1/2" due to weight loading:

275/80R17: (34.32/2) - 0.5 = 16.66"
265/65R17: (30.56/2) - 0.5 = 14.78"

The difference is now 13%; the theoretical values actually underestimate the difference because the deflection due to weight loading is a greater percentage of the smaller tire's radius.

I've got a ton of real world data that I've collected with various differential ratios (I do my own gears so it's not very expensive) and tire sizes using a range of GPS speeds and corresponding digital tachometer RPMs for my Jeep to back up all this theory, but the same principles apply for a GX.

 

If you want to quantify the additional "real world" stress on the transmission, monitor before/after transmission fluid temperatures.
I believe a truck running 34s averaging 180 degree fluid temps will most likely outlast another truck running 32s averaging 200 degree fluid temps, assuming the same maintenance schedule.

Average fluid temperatures will outweigh tire diameter in transmission longevity, IMO, yet just to be clear larger tire diameters do tend to increase transmission fluid temps.

 

With all that's stated above, consider a trans cooler to offset the higher temps perhaps ?

 

... question to the OP ... why R U running 80-series tires ? ... commercial tires ?

 

You're running a 34" tire......that's NOT a small increase in tire size. Get a CSF all aluminum radiator (they have a built-in transmission cooler) or get a Hayden....either way you're putting some strain on your transmission. Better yet, get an OBD tool to monitor your temps....that'll be the cheapest way to know what's going on.

 

This right there - Temps. Both of my 100's have trans coolers, my 06 A750F is very similar to the A760F. The rigs are very similar in weight, and the drivetrain is similar. Both of my 100's have over 250,000 on them. The trans almost never gets over 160 with the cooler. That and the transmissions have been serviced. I've had my 22 GX flushed at 46,000 miles and I will have my 23 flushed at ~50,000 miles. I will treat them just like the A750F in my 06 100. Coolant at 50,000 mile intervals as well. Diffs/TC at 36,000 mile intervals, just like the 4runner, which runs the A750E, which also gets serviced at 50,000.

I believe that if you treat these like they are under "extreme" service, that they will easily go 300,000 +.