Towing, Tire Size & Gearing

I have a 08 Double Cab Sport 4X4 with auto tranny. I have a 5500 lb travel trailer I tow occasionally without any problems other than getting about 9 mpg. This winter, needing new snow tires I went to a 265 70/17 (31.5” diameter) which are about 1 inch larger diameter than the stock 265 65/17 ( 30.5 “ diameter). BFG T/A’s by the way, great snow tire for the Sierra Nevada. That 1 inch taller tire made a noticeable difference in the performance of the truck. I have not pulled the trailer yet with them, but when climbing grades, the tranny will down shift when it never did before and often “hunts” for the lower gear, I have also lost about 3 mpg. I am concerned that I have affected the gearing so much that pulling my trailer will now be a problem. In effect I have lowered the tow rating of the truck by changing the gearing. As a solution I am considering purchasing a second set of tires. I am considering a 255 60/17 (29” diameter). I have an extra set of wheels so switching over would be easy. The smaller diameter tire will change my gearing lower than the stock set up allowing easier towing. Has anybody tried this or used a smaller diameter tire than the factory 265 65/17? If so what size?

 

Screw the extra tires... Regear... Costs less more than likely. or roughly the same.

 

You didn't change your gearing.. just the 1" tire made the truck work a little harder. I'm not sure what the stock tire size is.. but 265's are also wider than the 255s.. .more tread to the road = more power needed to turn 'em (as well as the size difference).

Re-gearing the truck would probably be better if you really want to keep a larger tire size and get your MPG back, probably cost you about the same as buying two sets of tires every year.. Except you only pay for it once.

I also believe that 5500lbs is pretty much the limit of the truck (according to Toyota).. so yeah, it's going to hurt MPG. This is where a V8 would be nice..

 

I'm assuming you have the tow package, if so, you're properly set-up with the 265/65R17's that the truck came with. A shorter tire will make towing easier but will cause you to be at a higher RPM at highway speed so your MPG's may not improve much. Have you thought about a narrower tire as well, that would help the cause also. That way you could keep the original tire diameter so your speedo will be accurate. 5,500 lbs isn't a light load so, even if you regear, I wouldn't expect your truck to do much better with fuel economy. You might get 10-11 MPG's if you find the optimum set-up.

 

When you change to bigger or smaller tires you effectively change the gearing. Put a white mark on your tire and on your driveshaft. While spinning the tire one revolution, count the number of times the drivehaft spins. With stock tires the driveshaft will spin 4+ times to one time for tire which equals 4.11. Shorter tires will allow you to tow the same weight just with more power.

 

Alfawuf- You're towing in drive and not OD, right? Wouldn't the larger tires keep it from revving the RPM's in Drive or is it shifting between 3rd and 4th? FYI, I only get 10 mpg towing at about 63-65 mph.