383 on 37s Gear ratio

[Super_chevy]

Should I swap 4.88s or 5.13s? wheels are heavy 37x13.5 with 24x14 rims. 383 spins tires with stock ratio 3.73 don’t plan on going up in tire and want something I can still drive around town.

 

[Nad_Yvalhosert]

That depends on your OE tire size and gear ratio.
Which is...?

 

[Nad_Yvalhosert]

Here's my scenario. 1995 TBI with 180 mildly upset horse pressures. Originally 3.73 and 245 tires.
Went to 315 tires, so mathematically it would need 4.25 gears. 4.56 would be decently accurate, but I went to 4.88 figuring the extra weight of the tires.

NTD÷OTD
×OGR = NGR

New Tire Diameter ÷ Old Tire Diameter
Multiply that by the Old Gear Ratio
That's your New Gear Ratio

 

[GoToGuy]

If that's correct on your tire diameter being 37 inch thats affecting your 3.73 to simulate a numerically lower number. As say 3.43 or less. Would have to do the math to be exact. Good luck.

 

[1998_K1500_Sub]

I would shoot for a gearing that gives me 1900-2200 RPM at 70MPH (or at whatever speed you most often drive) in "top gear".

ALTERNATIVELY, change the rear gears appropriately to make the actual speed (GPS measured) match your indicated speed (speedometer).

Download a GPS app to your phone and measure the speed, then check the tach for the corresponding RPM, for your truck as it is now in high gear (assuming you have the 37s on it now).

Change your rear gears appropriately to raise or lower the engine RPM to match the target range.

If you don't know what gears you have now, check your RPOs codes in the glovebox for one of the codes below, assuming you have factory gears. If not "factory gears", determine the gear ratio by hand (not hard, see threads on GMT400 which describe process).

- GT4 (3.73 final gear)
- GT5 (4.10 final gear)
- GU4 (3.08 final gear)
- GU6 (3.42 final gear)


Check out this thread:

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[RichLo]

Speed/Tire/RPM calculator

Custom tubular suspension systems with coil overs and rack and pinion steering. Complete systems front and rear, custom axle housings, big brakes and more for your pro-touring Mopar. Custom designed parts for street rod and other makes.

www.reillymotorsports.com

Tire Size, RPM, Speed, and Differential Ratio Calculator

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