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WHAT GEAR RATIO IS BEST?

When you choose bigger tires for your vehicle, there‘s more to consider than just who they‘ll fit your wheel wells. An increase in tire diameter will affect overall gearing. At any give road speed, engine RPM will be reduced and, as a consequence, both performance and fuel economy may suffer.

As a rule of thumb, an automobile or truck engine is most efficient at 60 to 80 percent of the RPM at which peak horse power is developed. The accompanying chart shows engine RPM at 55 mph for various combinations of final gear ration and tire diameter. If you want to determine RPM at a speed other than 55 mph, or for a combination of ratio and tire size not indicated here, the formula is:

M.P.H. x Gear Ratiox 336 = RPM_______
Tire Diameter

If you don‘t know your vehicle‘s final gear ratio but it‘s a recent model, look on the inside of the glove compartment door. There, you‘ll find a specification panel which should include the proper figures.

If it‘s an older model, you may have to crawl underneath and look for the ratio on a tag bolted to the differential. On a four-wheel drive, it may be on the front differential instead of the rear one.

 

About This Chart

Recommendation Based on Average Power Curve of Most Engine in Direct Drive Gear of Standard Transmission at 55 MPH:
OPTIMUM = 2,300 - 2,600 RPM To Improve Milage
Choose a Ratio Listed ABOVE the Optimum Range RPM as listed on this chart.
ACCEPTABLE = 2,000 - 3,000 RPM To Improve Power
Choose a Ratio Listed BELOW the Optimum Range RPM as listed on this chart.

Overdrive Transmission Owners may select a gear ratio lower on the chart for more power.

NOTE: the information given above will help you read the chart, which shows both tire size and gear ratios. Find the tire size you wish to use listed on the top row of the chart then look down the row for the numbers in the red background, then reading across the row to the left find the gear ratio that is best suited to your vehicle.
THIS CHART SHOWS ENGINE RPM @ 55 MPH
 
Gear
Ratio

Tire Diameter
24"25"26"27"28" 29"30"31"32"33" 34"35"36"38"40" 42"44"
2.5619721892182017521690163115771526147814341391135213141245118311261075
2.7321022018194018691802174016821627157715291484144114011328126112011147
2.9422642173209020121942187418111753169816461599155215091429135812941235
3.0823722277218921082033196318971836177917251674162615811498142313551294
3.2124722373228221972119204619771914185417981745169516481561148314121348
3.3125492447235322662185211020391973191218541799174816991610152914561390
3.4226332528230323412257217921072039197519151859180617561663158015051436
3.5427262617251624232336225621812110204419821924189618171772163515581487
3.7328722757265125532462237722982224215420892027196919151814172316411567
3.9030032883277226692574248524022325225221842120205920021897180217161638
4.1031573031291428062706261325262444236822962228216521051994189418041722
4.2732883156303529232818272126302545246623912321225521922077197318791793
4.5635113371329131213010290628092718263325542478240823412218210720061915
4.8837583607346933403221311030062909281827332652257725052373225521472050
5.2940733910376036213491337132593154305529622875279327162573244423282222
5.3841433977382436823551342833143207310730132924284127622616246823672260
5.7143974221405839083769363935173404329831983104301529312777263825122398
6.1747514561438542234072393238013678356334553354325831673001285127152591
7.1755215300509649074732456944174274414140153897378636813487331331553011

 

The preceding data was taken from the 1997 Dick CEPEK master catalog.

 


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