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Discussion Starter #1
Okay, you all have me completely twisted on this issue...

4.10 people get poor MPG as the RPMs are up.

3.73 gets decent MPG

3.42? probably get insanely good MPG

Why then when people add larger tires it seems that MPG goes down...???

With the bigger tires the rear end ratio is effectively being dropped and should increase MPG. Granted making for a slower vehicle, but that is another problem.

Dunno?
 

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Discussion Starter #2
FWIW on the 99 Burb it has 3.73 rear with taller (and skinny) 235/85 R16 tires. I am not sure if MPG is any better than 245/75s, but the Burb is so slow with the taller tires and tall gears.

The 96 with 4.10s needs tires soon so I will probably swap them out from the Burb and just get regular 245/75s on the Burb.
 

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Resistance, rolling and wind (lift to fit tires), and heavier rotating mass IMO
 

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klm2500hd;1583043; said:
Resistance, rolling and wind (lift to fit tires), and heavier rotating mass IMO
Exactly.
If I am on a long trip,more or less level highway,I have gotten almost 19 mpg over an 800 mi.roundtrip to Denver.(with 35's)A good headwind will drop that by 2-4 mpg.:eek:
 

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tires & mpg

I agree, you'd think with taller tires would increase mpg. I don;t think rolling and wind resistance is going to effect it. How about leaving from a dead stop? heaver rolling mass? My tires and wheels can't be that much heaver. I noticed stop and go mpg is bad, hwy milage is great. But what do I know I bought the truck with a lift and wheels on it...
 

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klm2500hd;1583043; said:
Resistance, rolling and wind (lift to fit tires), and heavier rotating mass IMO
I agree FWIW. I just looked into this a little when buying new tires. There's about a 7# difference between the 265's and 245's I was looking at.
 

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It's not as simple as saying this will happen if you do that.

Some factors will not be the same on all vehicles and some will.

If the gearing is currently at the best ratio already for your type of driving, weight, and aerodynamics you can only make it worse by changing it.

If the gearing is currently too high or too low you can improve it or make it worse.

Adding wider and or taller tires will certainly make more drag. If this is compensated by ratio changes or made worse depends on the vehicle.

Drag is mostly what keeps us from doing 200 mph @ 50 mpg.


Wind resistance and road resistance can be improved or made worse with tires and wheels.


So lets say your gears are just about right for best mileage and you go to taller wider tires. Your gearing is worse and you have more drag, heavier, and rolling mass has increased.
 

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I guess we'll have to live with poor mpg so our trucks look tougher... Don't get me wrong I would have no problem with stock wheels, tires and height, but on a long bed truck it sure looks good with 4 extra inches... that's what she said.
 

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It takes more power to turn a taller tire from a dead stop because of rolling resistance, than a shorter tire, once you get your rpms up, and are cruising at what ever mph, your mpg will be better with the taller tires because the truck is covering more ground at the same rpms as with the shorter tires. In stop and go traffic, with taller tires, you will lose mileage without question, unless you change to lower gear ratio with the same tall tires, now your mileage in stop and go traffic will be better but you'll be turning the motor higher while trying to cruise at say 65 mph, which in turn, burns more fuel.
 

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Lower rpms could mean less power at highway speed. Which could give lower mpg if your truck is heavy and catches a lot of wind. Underpowered vehicles get lower mpg. Going faster takes more power, if the rpm your bigger tires dictate is lower power than the original RPM you will again see lower mileage at highway speed.

If your RPMs are higher than optimal you would see an improvement at hwy speed if the weight of the tires and rims plus the wind drag and road resistance did not take it all away.

RPM and fuel use are not a linear relationship. A loaded down motor can easily use more fuel than one running at higher RPM.

To test this run along at a set speed and RPM and push in on the parking brake and see that your mileage decreases even if your RPM and speed stay the same.
 

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Did you reset your VSSB to calculate the new ratio?
 

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I had 61k on my 12 ply 235/86r16 tires. My pickup has been happy running at 64 mph by odometer. Milage was reading was off by 8%. I have always taken that into concideration. Since all of the upgrades have been done over the past year, I wanted to go back to the stock 225/75 for a performance check, and I was tired of the tire rub on the rear. Well I threw on a nice set of Firestone AT's and now my pickup is happy all of the time. The old girl wants to run fast now. I have to remember to set the cruise to keep me out of trouble. I haven't run a full tank through to check milage yet, but based on my observations I really like my setup with stock tire size.
 

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Most people don't consider the taller tire when figuring speed or MPG. If your speedo or Odomometer is off by 10% then your miles traveled is also off by 10%. Just a thought.
 

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V -- s -- s -- b :)
 

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Vehicle Speed Sensor Buffer - divides the signal from the speedo sensor on the trans output shaft for proper tire dia and R&P gear ratio, splits it up for the speedo, ABS, PCM.

Module in a white plastic box behind the glovebox.

Yep - that is the correct spelling for the VSSB. :)
 

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gmctd;1583438; said:
Vehicle Speed Sensor Buffer - divides the signal from the speedo sensor on the trans output shaft for proper tire dia and R&P gear ratio, splits it up for the speedo, ABS, PCM.

Module in a white plastic box behind the glovebox.
OK ... that makes sense... how does one go about resetting the VSSB, should one be so inclined to do so?
 

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It has soldered wire jumpers to set a divide ratio - GM has one for each tire diameter and R&P ratio, or you can reset it by removing the wires and installing a 16-pin dip-switch.

It will need adjusting if you're not running on production rubber, or you got 20" wheels, or 10bolt 22" wheels, etc.

Some of the a\m trans builders sell them.
 
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