Redneck Truck

Now I've just gotta figure out what tires to run and what kinda spacers I'm gonna need to run them. They are 19x8 ET35, so a 255 30 zr19 tire would be roughly the same outer dimension as the 255 40 zr17 I'm running now. I can't really go any wider or taller than that on the tire without hitting my uprights. Talking with the guy at NTB (tire retail chain), he indicated I'd have trouble fitting a 255 tire on a wheel that narrow. I figured 255 was the width to run for an 8" rim, but apparently not. If I can't run at least a 255, I'll probably let the wheels go to someone who is worried more about tucking than width, but for now, I'd like to make them work.

2.7Lturbo

nice **** dude

monkeytronic

How 'bout a 245/35-19 instead? It'll fit an 8-9.5" wide wheel, there's a decent selection of tires to choose from in this size and it's overall diameter is only 3/4" larger than a 255/40-17.

Of course, since you've already done the 6MT swap with your 4.2, the 245/35-19 size is really only about 1/2" taller than the 235/45-17 size that's factory spec'd for a C5 2.7T with a 6MT.


Just for grins what's your current cruise RPM at 60 MPH?

Doing some quick and dirty number crunching, figure at that speed a 4.2 with a 5AT, 2.727:1 final drive, 255/40-17s and in its top (5th = .803:1) gear should be turning right about 1,760 RPM.

OTOH at the same speed a 2.7T with a 6MT, 4.111:1 FD, 235/45-17s and in its top (6th = .683:1) gear should be turning about 2,240 RPM. The same combo with 245/35-19s would be turning about 2,200 RPM.

Unfortunately, there naturally will be an impact on gas mileage with the 4.2 + 6MT spinning 25% faster at speed and the ever so slightly larger tires won't do anything appreciable to improve gas mileage when out on the open highway--you'd need huuuuuuuuuuuge 32.3" overall diameter tires (read: donked out 4.2) to pull that off.

Redneck Truck

It's surprising you mention sizes and their impact on gas mileage. A larger, heavier tire and wheel will reduce fuel economy in all areas where mass (both rotating and static) are a concern. These wheels weigh about 25 lbs a piece, and the tires will probably weigh the same, so I've just added 5-10 lbs per corner, provided I don't end up running a second set of lug bolts with wheel spacers.

I'd prefer a wider tire to a narrower one, because I've heard the owners of much lighter, better balanced cars complain about having *ONLY* a 255 in the front. I've already got the big sway bars, so I would really prefer to go wider, even though it will have a negative impact on mileage. I've got a TDI, a 1000RR, and a nissan pickup for gas mileage, anyway.

I was hoping someone would tell me they had already run a 255 30 (or 35) on a 19" wheel with success, because the guy at NTB seems to think I'll have a lot of trouble even mounting the tire on the rim. I've thought about widening the rears 2 inches, getting 1-inch spacers, and running a staggered setup (295/30/19 and 255/35/19), but for a front-biased all wheel drive car that understeers badly, it would do nothing to improve front traction and would increase the already plenty adequate traction in the rear. With that in mind, a staggered setup like that would clearly be more for form than for function.

Any opinions? Do y'all like them? I thought they were really pretty, and I really like the idea of a cast wheel over a forged one for both rigidity and durability.

Redneck Truck

Also, the adverse impact the larger diameter tires would have on the power and longevity of the brakes and on the ride quality would likely outweigh any benefit I'd reap in the fuel economy department. Cruising RPM seemed surprisingly high at 90 the one or two times I drove it (like 3-4000). IIRC, I think I calculated a gear-limited top speed of like 180 at 7k. Still, I've never seen any empirical evidence that higher cruising rpm is worse for fuel economy. RPM may be higher, but load is the same, so injector duty cycle would be proportionally less, right?

PPGOAL

I would be worried that staggered tires/rims would rip up the differential. The quattro on these seems to demand the same rubber all around.

Johann2.7t

imho most 235s are perfect on a 8" wheel

Redneck Truck

Same diameter, yes. 255 35 19 and 295 30 19 are the same outer diameter. Same goes for 275 30 19 and 235 35 19.

monkeytronic

FWIW my calculations only take into consideration the effects of gear ratios, speed, RPM and overall tire diameter--not wheel or tire weight. Besides, it's actually possible for one brand/model of wheel and one brand/model of tire to weigh more (or less) than another brand/model of wheel and another brand/model of tire in the exact same respective wheel and tire sizes.

Naturally a 32.3" overall diameter tire/wheel would be one fuggin' heavy-arsed **** but that's the overall diameter that would be required to effectively "re-gear" a 4.2 with a 6MT to its 5AT equivalent with both transaxles in their respective top gears without actually physically changing the front and rear axle final drive ratios--which is a fairly spendy proposition in and of itself. IOW that's how relatively extreme the difference is between the 6MT's top gear x FD ratios vs. the 5AT's top gear x FD ratios.

Of course, among other factors, engine RPM at a steady cruise speed will affect fuel mileage and what works great for the 2.7T won't necessarily work all that great for a 4.2 and vice versa.

Redneck Truck

I'm aware of gearing and its potential effects on mileage; I'm talking about their actual effects in this case. It's been addressed extensively on Motorgeek - a taller 6th gear is something like a $1000 job and when asked, at least one person cited absolutely no change in fuel economy following the swap despite cruising at a much lower rpm, because the fuel economy effects of the drop in rpm and the increase in load canceled each other out. My Jeep and my Nissan truck both have tires 20-25% taller than stock, and the change in tire sizes alone did nothing for highway fuel economy and actually reduced city fuel economy once you converted from odometer miles to miles actually traveled.

As for tire weights and sizes, I couldn't find one tire in my usable range that weighed less than the tires on my car now (they are all at least a pound heavier, surprisingly enough), and the wheels I bought weigh more than the wheels on my car now. That said, I know lightweight alternatives may be available ceteris paribus, but in this case, it's really not an option.