Ford Class C with 6 speed transmission

I have a e350 with a v10 a 5speed trans. at 60mph the rpm are 2000 it runs very well at 2200 rpm that give me about 65mph and 6-7 mpg.

In my fleet I have the exact same motorhome (Redhawk 31XL) same model year but one with the older chassis and one with the newer one. In 60K miles on each, we never noticed any difference.

Gjac wrote:
So you just change to a higher aspect ratio tire, sounds like a cheap solution to get greater mpg. I guess if you go too high on the same size rim the MH might have more lean when taking a corner. Do any of the small C's come with larger rim size say 17 or 18 in? I know a lot of the TC guys put larger rim tires in the rear mostly to increase their payload.

I might add that any side-sway (lean) issues from wheels with small(er) rims - relative to tire sidwall height - can be minimized by using higher tire pressures. I run 80 lbs. of pressure in the larger-diameter rear tires of our small Class C ... that does not require 80 lbs. for weight carrying reasons. This provides stiff tires that don't move much - either leaning back & forth, or compressing up & down.

As a good example, note the large big-rig tires (sometimes supporting an 80,000 lb. or more total rig load) that have large tire sidewall heights relative to their rims. These trucks are able to safely do this without tire-side-sway issues merely because they're running very high tire pressues.

Yes, 17" wheels are very feasible. The front & rear outers are widely available aftermarket. The inner steel (and spare) was a little trickier. Ford never made one in our bolt pattern. But Chevy did, the first half of this decade. The center bore was smaller, so I had them machined out to the 4.88" bore we need.

I'm running 235/80r17 in the back, which is good for 11,340 pounds. I run wider 265/70r17 up front for a larger footprint, for off-road travel. But the 235s would be fine up front as well if you stay on pavement.

So you just change to a higher aspect ratio tire, sounds like a cheap solution to get greater mpg. I guess if you go too high on the same size rim the MH might have more lean when taking a corner. Do any of the small C's come with larger rim size say 17 or 18 in? I know a lot of the TC guys put larger rim tires in the rear mostly to increase their payload.

Gjac wrote:
Don and Phil when you talk about taller tires are you talking about going to a higher aspect ratio tire or going to a larger rim size like from 16 in rims to 19.5 in to produce a larger dia tire?

As far as I know, the Ford E350 and E450 cutaway van chassis that Class C builders start with usually come from Ford fitted with 225/75R16 Load Range E tires.

A few years ago I changed the tires on my E450 based Class C to 215/85R16 Load Range E tires. These tires have the same load rating, are slightly narrower, and have a diameter 1.2 inches larger - which increases my Class C's ground clearance by 0.6 inches. This isn't much, but every little bit helps ... and I didn't want to raise by too much the distance that my DW has to climb up into the cab (she has a bad back).

The Ford E350 and E450 cutaway van chassis come with 16 inch steel rims. Using these same rims, I believe (but am not absolutely certain) that these two chassis can also be fitted with at least 235/75R16 or 245/75R16 tires to make for even taller overall gearing and greater ground clearance using the stock steel 16 inch rims. However, using these larger sizes should be thoroughly researched further by anyone thinking about doing it and should probably only be done on an E450 chassis with it's differential ratio of 4:56.

FWIW, on my 4X4 PU with 16 inch rims I've in the past had 265 tires on it using it's stock rims and gearing ... for great off-road ground clearance without any chassis lifting required.

Hi,

There is NO WAY a 19.5 inch would fit.

My increase is a ratio of 1 to 1.08.

That is pretty much identical with Phil's.

Gjac wrote:
Don and Phil when you talk about taller tires are you talking about going to a higher aspect ratio tire or going to a larger rim size like from 16 in rims to 19.5 in to produce a larger dia tire?

Don and Phil when you talk about taller tires are you talking about going to a higher aspect ratio tire or going to a larger rim size like from 16 in rims to 19.5 in to produce a larger dia tire?

Gjac wrote:

pnichols wrote:
I think that I've improved my motorhome's V10 fuel economy over what is possible with it's 5-speed 5R110W transmission by retrofitting it with non-stock tires that are larger in diameter. Going to a larger diameter tire makes all gears "taller" in any vehicle for improved fuel economy when cruising.

Since our Class C motorhome is a relatively small one for it's E450 chassis, I don't need the extra pulling power that the E450's 4:56 ratio differential provides, so I may as well get some improved mileage through use of larger diameter tires.

An E350 chassis has a 4:12 rear differential ratio, so larger diameter tires on it under a motorhome might make it lug down on hills a bit.

Hi Phil, what was you MPG before and after the tire change?

I don't know exactly, Gary, as I don't track gas mileage precisely. I'm assuming that in the long run taller overall gearing (as larger diameter tires provide) will improve cruise gas mileage somewhat - it only makes sense that that would be the case.

My speedometer reads about 1.5 MPH lower due to the non-stock tire diameter, but that is of no concern to me. We cruise very conservatively at only 57-60 MPH and in overdrive this results in around 2000-2100 RPM engine spin speeds.

My taller tires are just a bit narrower, but since the stock Ford rims are rated for a range of tire widths we have not experienced any problems with bead leaks. The slightly narrower tires also cause improved air flow cooling between the tires in the rear dual tire sets when traveling in high ambient temperatures. Tire tread wear and sidewall heat stress are also improved a bit because larger diameter tires turn less rotations per mile.

However, the main reason we went to a larger diameter was to improve ground clearance for all undercarriage components during occasional travel on rough roads. RV trip flexibility is very important to us because we cannot predict what situations we may be presented with.

Johnny Hurryup wrote:
My old'99 Ford F53 Class A chassis with the V 10 and 4 speed trans was doing about 2600 RPM at 60 mph in overdrive. What can I expect from a Class C ,E 450 6 or 5 speed at 60?

Here's at least one data point for ya Johnny-

My rig: 2007 31ft WGO Class C on a 2006 Ford E450 chassis w/5-speed
At 60 MPH: Approx 2200 RPM