better mpg's

[Schrade]

I put 225's on the Venture. I'm gettin' almost 2 MPG's (1.8+) better.

I was expecting more. I went from 245's to 265's on the Express one ton, and MPG's went from 14.2 to 16.1.

You WILL lose acceleration. Keep the light foot.

[534BC]

What were the before and after tire OD's on them?

How did you calibrate speedo if it was nessecary? Thanks.

[Schrade]

Originally Posted by 534BC

What were the before and after tire OD's on them?

How did you calibrate speedo if it was nessecary? Thanks.

For speedo calibration, a measured distance, in my case was 167 miles with the old tires, and 163.7 with the taller tires. The difference was about 1.02 %.

If your speedo says 50, or 70, or whatever, just add about 1.02 %:

50 = 51, 70 = 71.4.

I couldn't measure outside diameter; they weren't mounted at the same time. All I could do was look at the difference of a measured distance.

[534BC]

Why do you suppose you got such a drmatic increase in mpg?

[Schrade]

Originally Posted by 534BC

Why do you suppose you got such a drmatic increase in mpg?

I don't know if I'd call 2 mpg's dramatic...

Taller tires has an effect similar to changing the gearing of the drivetrain. With taller tires, your engine will be turning slower RPM's, at the same speed (not as indicated by the speedo - remember, it, and the odometer, will show less than what is actual).

Another way of explaining is like a car with a 5 speed tranny. In 4rth, at 50 mph, the rev's are 2500, for example. Go to 5th, stay at 50 mph, and the rev's drop to 2100, give or take a hundred.

Brake wear will be proportionately greater too.

[534BC]

I was wondering how you got 10% better mpg with gearing the vehicle up 1-2 %. What do you suppose another increase in gearing would do? What about a smaller tire that geared down about 2 %?

[Schrade]

Originally Posted by 534BC

I was wondering how you got 10% better mpg with gearing the vehicle up 1-2 %. What do you suppose another increase in gearing would do? What about a smaller tire that geared down about 2 %?

I imagine a smaller tire would give slightly better acceleration, and slightly LESS mpg's.

On a 2000 Venture you can't fit anything larger under the coil spring base. Mine now has about 3/4 inch clearance.


Check some four-wheel drive forums. I've seen formulas (formulae?) that actually calculate the necessary change of your ring and pinion, for when you put 35", 38", or 44" tires on your Jeep/off road vehicle!

[SimGeek]

Originally Posted by cbec1999

I imagine a smaller tire would give slightly better acceleration, and slightly LESS mpg's.

On a 2000 Venture you can't fit anything larger under the coil spring base. Mine now has about 3/4 inch clearance.

My '97 has 235/60-15s on it with no issues. I do doubt you could put 235/75-15s on it though. The 235's are as wide as I could go on the stock rims. Can't comment on MPG change since the van had 225/60-15s (!) on it when I bought it - too small a diameter in my opinion, but the 235's are just a bit smaller than stock diameter.

[ventureman]

Funny this topic of tire size has come up. For reasons other than fuel economy, I am comtemplating 205/75-15's on my '98 extended. The reason is I don't plan on keeping the van more than a year or so and my mechanic has these tires on "clearance". So my question...is there any real harm in altering tire size from OEM (215/70-15)as long as I change all four?

Thanks!

[SimGeek]

Those 215/75s would only be about 1/4-inch taller overall than the stock size, so there should be no problem at all (mine are about 1/2-inch smaller diameter). Do make sure the tires are approved for your rim width (I would guess you are OK) - every tire (not tire size, but brand/model/size) has a minimum and maximum rim width specified. Yes, you should change all 4 tires - though you could probably get away with changing them in pairs (front vs. rear), especially if they are that close in diameter. As far as I know, when you're this close to stock size the only affect is the speedometer will be slightly off - in your case it will probably read about 1/2 mph lower than actual at 60 mph so it's not worth thinking about.

[Schrade]

Originally Posted by ventureman

Funny this topic of tire size has come up. For reasons other than fuel economy, I am comtemplating 205/75-15's on my '98 extended. The reason is I don't plan on keeping the van more than a year or so and my mechanic has these tires on "clearance". So my question...is there any real harm in altering tire size from OEM (215/70-15)as long as I change all four?

Thanks!

Sim is right on the speedo - it will show faster than you're actually going, with 205's.

MPG's would go down, but I'm guessin' only 1, or so...