How does +2 tire size affect towing?

[Poppy]

This may be considered a dumb question by some. I was taught the only dumb question was the one that's not asked, so here it is.
I bought a 2006 Chevy 2500 4x4, then found out the tires are 285/75r16, instead of the oem size of 245/75/r16. I found out the speedometer reads about 5mph slow at 60pmh, and the odometer is off, too. It also lowers my fuel economy by about .7 %. What I'm trying to find out is, how does it affect my towing ability? Do I need to start thinking about going back to original size? They are Michelins with maybe 85% tread on them.

[SkyBound]

It depends how much weight you are towing, how many gears you have in the tranny, what your differential gear ratio is, and how much power you have from the engine.

As a general statement, it will reduce your available torque to some extent because of the greater rotational mass and diameter of the larger tire. Depending on the mentioned factors above, it may be so minimal that you don't notice it, or it could bog you down to the point that your tranny will hunt for gears or won't stay locked, overheating issues, terrible fuel mileage, etc.

Since your truck is a 2500, it probably has good gears and a beefy tranny so unless you are towing at the upper limits of the vehicle you should be just fine. I think 245/75/16 look dumb on those full size trucks anyways.

I'm towing with larger tires on mine with no problems at all. I run 285/55/20 which is 33.5in overall diameter and my 6500lb 28BHS it perfectly obedient behind it.

[mike837go]

Originally Posted by Poppy

This may be considered a dumb question by some. I was taught the only dumb question was the one that's not asked, so here it is.
I bought a 2006 Chevy 2500 4x4, then found out the tires are 285/75r16, instead of the oem size of 245/75/r16. I found out the speedometer reads about 5mph slow at 60pmh, and the odometer is off, too. It also lowers my fuel economy by about .7 %. What I'm trying to find out is, how does it affect my towing ability? Do I need to start thinking about going back to original size? They are Michelins with maybe 85% tread on them.

Since SkyBound dealt quite well with the drivability issues, I won't comment further about that.

However, since you know the larger tires have thrown your speedometer and odometer off, how can you reliably calculate fuel mileage? Especially, down to 7/10 of a percent? MPG will vary by up to 10% just because the tank was filled differently (topped off vs. short-filled because it kicked off at $$.90).

The diameter change from just the normal amount of wear will throw the speedometer off by about 3 MPH @ 60.

75% of 2 CM X 2 (the difference in diameter of the tires) is 3 CM. It was mentioned that the tires are 33" tall. So the difference when new is 3.5% larger on the 285's. Not a heckofalot.

Actually, the gear ratio of the larger tire to the ground will give you slightly better highway fuel mileage that is impossible to measure since your odometer was calibrated for the smaller tires.

[Poppy]

Thanks, SkyBound. My truck has the gas 6.0, 4 sp. automatic with a 410 gear. My last trip, I was running a speedo reading of 60 to 65, not realizing the +2" tire size. I ended up getting 7.5 mpg. I didn't think that was very good, so I will be slowing it down on the next trip.

[Seann45]

With +2's your 4.10 rear end now becomes a 3.73. (approximate)

[SkyBound]

Originally Posted by Seann45

With +2's your 4.10 rear end now becomes a 3.73. (approximate)

Which is just about perfect if you ask me. You are lucky to have those 4.10s! They are a perfect gear if you are looking to go with bigger rubber. How much does your trailer weigh? And yes, definitely keep the speed down a bit... You should be able to pull off 8-9 MPG without much else different.

I get 9.6-9.8 MPG with my set up doing about 55-58mph.

[Poppy]

Mike, When I found out the difference in the size between the 2 tires, I knew I couldn't get an accurate mpg figure. I hadn't thought about the wear of the tires making a difference in the speedo readings, either. Somewhere on the WWW, I read that larger tires would lower the fuel economy. (They can't put it on the web if it isn't true, right? LOL!)

Sean, I wondered if the 3.73 gearing would be close to it.

SkyBound, I really wasn't looking to get larger tires. I was just concerned that these 285's were too much for the long hauls.
The MPG's is really what lead to me starting this thread. My first tank, I figured I got 8.4 mpg, but the second tank is when I ran the truck at 65 (speedo reading) and got 7.2 mpg. That lead me to some serious looking for a problem, until I seen the sticker on the door with the recommended tire size of 245. I know that is 2.36" difference.
That's when I found out I was actually doing 70 mph. I don't knowingly tow that fast.
BTW, my TT is a JayFlight 24FBS. I think GVW is 7200 lbs.

[sailor]

A decent GPS will give you fairly accurate speed and distance numbers. You could then reference those numbers to your speedometer and odometer to give you reasonable data without the GPS.

[Edd505]

Originally Posted by sailor

A decent GPS will give you fairly accurate speed and distance numbers. You could then reference those numbers to your speedometer and odometer to give you reasonable data without the GPS.

Works really well. Got a Mustang GT in a swap that came with 15's and had low profile 18's when I got it, speedo wasn't even close. I used a cheap GPS that went with car when sold.

[mike837go]

Originally Posted by Poppy

Mike, When I found out the difference in the size between the 2 tires, I knew I couldn't get an accurate mpg figure. I hadn't thought about the wear of the tires making a difference in the speedo readings, either. Somewhere on the WWW, I read that larger tires would lower the fuel economy. (They can't put it on the web if it isn't true, right? LOL!)

From a straight, gear ratio standpoint, larger rear tires (bigger diameter) will turn fewer times over the same distance of road than smaller tires. Therefore, all other factors (too many to realistically consider) being equal, your actual MPG will be higher.

But, now you have to spend extra fuel in order to spin up the additional mass of the tires.

There was a huge hoopla in NYC back a bunch of years (1980's?) where taxicabs were deliberately installing much smaller than specified tires. The fare was charged based on subjective distance covered. The smaller tires amounted to 10-15% overcharges. Lots of fines levied!

[Scoutr2]

Originally Posted by mike837go

Actually, the gear ratio of the larger tire to the ground will give you slightly better highway fuel mileage that is impossible to measure since your odometer was calibrated for the smaller tires.

That's what I was thinking. If the tire is taller, the circumference is greater, so every revolution of the wheel will take you just a little further down the road, which would improve your fuel economy. But that may be offset by the decrease in torque, making your engine work a little harder during acceleration and on hills.

Automotive engineers discovered long ago that increasing tire diameter helped them achieve greater fuel economy. My 1989 truck had 15" wheels. My 2007 truck has 16" wheels. And the newer ones have 17" and even 18" wheels. That is by design so that pickup truck manufacturers can meet the CAFE regulations.

[mike837go]

Originally Posted by Scoutr2

That's what I was thinking. If the tire is taller, the circumference is greater, so every revolution of the wheel will take you just a little further down the road, which would improve your fuel economy. But that may be offset by the decrease in torque, making your engine work a little harder during acceleration and on hills.

Automotive engineers discovered long ago that increasing tire diameter helped them achieve greater fuel economy. My 1989 truck had 15" wheels. My 2007 truck has 16" wheels. And the newer ones have 17" and even 18" wheels. That is by design so that pickup truck manufacturers can meet the CAFE regulations.

Buuuuuut, if the engineers were really after better fuel economy, they would be tweaking the transmission and differential gear ratios to get fewer engine revs per mile. Which only helps gasoline fueled vehicles.

Because Diesel engines don't use a constant mixture, small changes in engine revs per mile don't impact fuel usage. Gears that allow a Diesel to run with clear exhaust use the least fuel per mile.

(Should I add 50 MPG VW Jetta TDi to my signature?)

[Scoutr2]

Originally Posted by mike837go

Buuuuuut, if the engineers were really after better fuel economy, they would be tweaking the transmission and differential gear ratios to get fewer engine revs per mile. Which only helps gasoline fueled vehicles.

Because Diesel engines don't use a constant mixture, small changes in engine revs per mile don't impact fuel usage. Gears that allow a Diesel to run with clear exhaust use the least fuel per mile.

(Should I add 50 MPG VW Jetta TDi to my signature?)

I beg to differ. I am a service training instructor for hydraulic excavators, so I teach this stuff for a living. Our engineers have devised many strategies in recent years to help maintain constant engine RPM, in an effort to increase fuel economy. The hydraulic hybrid excavator released two years ago will dig all day long, with the engine at a constant 1600 rpm. That machine, along with an accumulator to start swing movement, boasts a 20% increase in fuel economy over the standard machine of the same size, using the same engine.

Any fluctuations in engine rpm, due to load on the engine, will decrease fuel economy - gasoline or diesel. But you are correct about gearing and rear end ratios. A standard 3:73 rear end gets good gas mileage, but doesn't pull very well due to loss of torque in that gear range. My 4:10 rear end pulls great, however, but my fuel economy suffers. You can have one or the other.

A diesel engine gets better fuel economy because the peak torque band is at a much lower rpm than a gas engine. That's why your VW does so well, along with 6-speed transmissions, and is also the reason most new cars in the EU these days are diesel powered. I've often wondered why American car manufacturers don't start putting diesel powered cars out there. They can get great fuel economy with smaller engines, as well as low speed power.

[Poppy]

What you all are telling me absolutely makes sense. According to the website I came across, the larger tire make 47.63 less revolutions per mile than does the smaller one.
(285 vs. 245). And it makes sense that it takes more torque to turn those revs.

Sailor, I did think about the gps just this week and found a free one for iphone that seems to be accurate. I did compare it to my speedo, and the chart I found on the website. BTW, the website I keep refering to is, tacomaworld.com. I'm sure there are others, but this one caught my eye first.

Mike, I don't recall the mess with the NYC taxi cabs. I think I was still a little wet behind the ears then and didn't care about much of anything.

This has been an interesting thread, and I've learned much. So many thanks goes out to everyone that's responded to my inquiries.

Poppy

[Tim L]

Originally Posted by Scoutr2

I beg to differ. I am a service training instructor for hydraulic excavators, so I teach this stuff for a living. Our engineers have devised many strategies in recent years to help maintain constant engine RPM, in an effort to increase fuel economy. The hydraulic hybrid excavator released two years ago will dig all day long, with the engine at a constant 1600 rpm. That machine, along with an accumulator to start swing movement, boasts a 20% increase in fuel economy over the standard machine of the same size, using the same engine.

Any fluctuations in engine rpm, due to load on the engine, will decrease fuel economy - gasoline or diesel. But you are correct about gearing and rear end ratios. A standard 3:73 rear end gets good gas mileage, but doesn't pull very well due to loss of torque in that gear range. My 4:10 rear end pulls great, however, but my fuel economy suffers. You can have one or the other.

A diesel engine gets better fuel economy because the peak torque band is at a much lower rpm than a gas engine. That's why your VW does so well, along with 6-speed transmissions, and is also the reason most new cars in the EU these days are diesel powered. I've often wondered why American car manufacturers don't start putting diesel powered cars out there. They can get great fuel economy with smaller engines, as well as low speed power.

You are correct. That is why CVT's are coming to be used more frequently. Keep the engine in its most efficient range. Fewer gears, and you go above and drop below the efficient range. More gears are better, and CVT is like infinite gears, keeping the RPMs in a very tight band.

[Idahoboy]

Well I can't speak for the whole world but every time I go to a bigger tire I loose fuel economy. I have a 12 f250 with the stock tires I was getting 15.5 to 17 ave. I went to bigger tires and now ave around 13 just my 2 cents I have tried it on 3 pickups and a jeep and always loose 20 to 30% but boy do they look better

[Jmooney]

Doesn't the taller tire also create increased side to side flex in the sidewall, compounded by the weight of the trailer ?

[SkyBound]

Yes it does create side to side flex from the larger sidewall. A 15in rim with 35in rubber will not be a stable tow, regardless if they are LT tires. You want a bit of sidewall to protect the rims over bumps and soften the ride, but the small rim/large sidewall tires are for the rock crawlers and mud boggers... not towers!

[Jmooney]

I was pretty sure, but wanted to get this important fact into the thread

[Poppy]

IdahoBoy, I curious as to how you are figuring mpg. When you went to the larger tire, did you have the speedometer recalibrated?