Wheel size upgrade.

PappyChris wrote:
Wasn't my intention to debate tire sizes.
A simple vacuum gauge can make my point.

I will be using a tire in diameter close to what was originally recommended on a 16" wheel and spend my time working on the VE of the engine to increase fuel economy of my flying brick.

Well Chris,

In our group there is probably more engineering development going on than there is at most of the rest of the RV industry. The bad news for you is that in spite of changes of engine, going to both port and throttle body, rear end ratios, the standard line is that these coaches will get 8~10 mpg depending on how you drive and if you tow.....
Engines very from the Olds 403 (the 455 was a common OE) to the Cadillac 500 and GMC 8.1. Rear end (actually a final drive) can be anyplace between 3.07 (OE) and 3.77 and performance may improve, but mpg doesn't.
Most are still open loop carburetor engines with both points and HEI, many now have knock controlled TBI, and they can do 10+ a lot of the time, but it can't be counted on.
Several have installed diesels. Those do a little better, but not enough to pay for the conversion.

My advise? Tune it up and enjoy it. The more you drive it to interesting places and events, the less the fuel cost will bother you.

Matt

Hi pnichols,

Yes it is the E450 from 2004 with a V10. I purchased Toyo tires this time round. They are not 285 but rather 235.

I also found a "sweet spot" at 78 kph (49 mph). If I drive there I get my best mileage. Going to 80 kph (50 mph) costs me nearly 1 mpg.

pianotuna wrote:
In the 8000 miles I've driven since the change I've averaged about 10.5 mpg. Before it was 10 or less.

Don ... is your 28.5 foot Class C on the Ford E450 V10 chassis?

If it is, as you're well aware that 10.5 miles per gallon after going to larger diameter tires is outstanding for that engine/chassis combination and you've made my point on what making your gearing a bit taller can do if you take it easy on the gas pedal after you do it.

Good point on you're reduced rolling friction also being a contributor. Are your 235 tires Michelin's newer M&S2 line? Michelin mentions their M&S2 tires (NOT their old M&S line) as being "Green X" rated due to lower rolling friction.

FWIW, I've ran 3 diameters over stock on my 4X4 offroad GMC pickup for years for excellent gas mileage (if I kept my foot out of it) and terrific ground clearance. The pickup's RPM was around 1700-1800 when cruising on the highway - right down there where diesel cruise RPMs run.

pnichols wrote:
When going down the road, the work to overcome wind resistance is not related to the diameter of the tires and/or what mechanical gear the engine is in.

A vacuum gage will read the same for the same RV on the same stretch of road, at the same speed, in the same mechanical gear, and under otherwise identical conditions - one time with a smaller diameter tire, the second time with a larger diameter tire. The only difference should be slightly better gas mileage with the larger diameter tire due to less friction and (maybe) less heat loss in the engine at lower RPMs.

Pnichols: My formal education in this area and 40+ years engine tuning experience would disagree with most of what you've written so there is no sense in us hashing this over anymore between us. I'm not a good debater...
Only thing I wanted to find out is what size wheel people were using (other than 16.5).

Don: I live on the eastern side of the Flint Hills here in Kansas. We have a lot of long straight road but quite a bit of hill climbing too. Since most of my future destinations are going to be mountain climbing in the east I am going to stick with a smaller diameter tire to help with mechanical advantage on the climbs. The minor loss in fuel economy (if any) will be acceptable when it comes to climbing those hills.

When going down the road, the work to overcome wind resistance is not related to the diameter of the tires and/or what mechanical gear the engine is in.

A vacuum gage will read the same for the same RV on the same stretch of road, at the same speed, in the same mechanical gear, and under otherwise identical conditions - one time with a smaller diameter tire, the second time with a larger diameter tire. The only difference should be slightly better gas mileage with the larger diameter tire due to less friction and (maybe) less heat loss in the engine at lower RPMs.

Hi PappyChris,

Rolling resistance goes down on a wider tire, and effective gear ratio goes up. In the 8000 miles I've driven since the change I've averaged about 10.5 mpg. Before it was 10 or less.

I'm not saying it would always work out that way--and I believe it helps that I live on the prairies where it is relatively flat.

Wasn't my intention to debate tire sizes.
A simple vacuum gauge can make my point.

I will be using a tire in diameter close to what was originally recommended on a 16" wheel and spend my time working on the VE of the engine to increase fuel economy of my flying brick.

Hmmm ... increasing tire diameter has the identical effect to using a gear, or gears, with a taller ratio. For cruising, overdrive does this very thing. When cruising a larger diameter tire merely makes your "overdrive" more "over".

However under non-cruising conditions - where you're accelerating or pulling up a grade - a larger diameter tire could sometimes make your transmission have to stay in one gear lower longer, or make your transmission have to torque-convert ("slip") more.

Since most of the total road hours in an RV are spent under cruising conditions, larger diameter tires should result in overall slightly improved gas mileage.

I would only add that increasing tire size can adversely effect fuel economy. Increasing tire size in effect loads the engine more at a given speed. It has the same effect as reducing the axle ratio which the engine sees as a higher load. All of this of course is dependent upon road and driving conditions and loading of the vehicle. I do agree that they do need higher ground clearance though. My dump pipe is crazy low. In fact it had broken the gray water tank fitting. Discovered this after I'd bought it. New water tank installed but wish I'd rerouted. I have a set of Firestone air bags in it that help a lot.

pianotuna wrote:
Hi,

I went from 225/75R16 to 235/85R16. This required a wider offset for the rear dual wheels. I bought all new wheels. It increased the weight allowance for the tires which was a nice side benefit--and mileage is a little better (perhaps 5%)

It did give me more ground clearance by about 1 1/8 inches.

Outstanding! You did the right thing to accomplish three improvements at the same time. You also accomplished a 4th thing - you reduced tread wear somewhat - less revolutions per mile means that each molecule of tread rubber contacts the road surface less times per mile ;) .

I did almost the same thing so as to improve mileage, ground clearance, tire wear, and rear dual cooling while at the same time not having to buy new rims. I went to a 215/85/16R tire but stayed with the stock 16" rims. A 215/85/16R tire is actually taller (a larger diameter) than the stock E350/E450 225/75/16R tire. The 215/85 tire is a bit narrower so spacing between the rear duals is actually a little wider for better sidewall cooling when traveling in high summer temperatures. Both the stock tires and the new 215 tires were Load Range E, so I did not improve weight allowance.

IMHO, stock Ford (and Chevy, too) based Class C motorhomes are too low to the ground for the widest range of camping situations because these chassis were probably designed as a delivery van for easy access when loading and unloading on hard surface roads. Their fender well and front suspension steering spacings are plenty generous enough for 16 inch tires of a larger diameter than that of 225/75/16R tires.