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Poor mpg

98singlecab

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I've been running 235 75r15 on my truck for the past 6 months and since I put the 205 70r14 back on and I'm quicker yes but mpg is worse.
I have a 98 single cab with 2.5l 5 spd with a 373 rear end (will be putting in a 373 lsd)

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alwaysFlOoReD

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Did you take into account the difference in tire size? It will throw off your mileage.
 

tomw

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Smaller diameter tires will tend to show more miles traveled than actual... they turn more times for each mile traveled. MPG should show an increased number of miles for a tank of fuel. By math, anyway, but in the real world, gearing might make more of a difference if shifting at different speeds/rpms.
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alwaysFlOoReD

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^^^ ya got me! It was past my bedtime...
 

RonD

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Yes RPMs would be higher at a specific speed with smaller diameter tires.

At 60mph in 5th with larger tires you might see 2,000rpm
At 60mph in 5th with smaller tires you might see 2,400rpm

So similar to running a 3.08 rear axle vs 3.73 for lower RPM at speed and better MPG
Larger tires, in effect, lower the rear axle ratio for better MPG, although weight of the tires and wind resistance of higher stance can make any gain minimal.

It is all a trade off for what driver wants as end result, 5th(OD) with ratio of .75 was added for that reason, lower RPMs at speed.
But the trade off is if RPMs drop too low you loose torque/HP curve, so you can't maintain that speed so have to shift back to a lower gear.
This is specific to the engines torque and HP power curve
 
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98singlecab

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It's say the stock size for my truck is in between 185 and 215 r 14.
I'm doing a prerunner clone. I like the look with the 235's but don't need a speeding ticket.

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