Tech Time Ė Gearing

By Kev Rooney

Note Ė This was originally published in the UKKís Gazette and is reproduced here with their permission.

Now this one may not make your car look cooler, but will almost certainly help your pocket. Thought that would get your attention!

Final drive gearing is often overlooked when changing wheels and tyres or engine swaps, but can make all the difference on economy or making the most of your new found power. For example, one of the local lowriders removed the tall 15Ē wheels from his Caddy and fitted 5.20x13 tyres and wheels. I thought he drove on the motorway at 40mph because the car leapt all over the show with no shocks. But apparently the engine was screaming and drinking fuel! Read on.

Many people think that the lower your engine revs at cruising speed, the more economical it will be. WRONG! This could be well below the engines torque curve and that means it just wonít be comfortable loafing along at a speed thatís just above idle.

Youíll need to check out the details for your particular engine, but on my old 2000 Pinto powered Capri the flat torque peak was about 3500rpm so I geared the car to cruise at 70mph at those revs (3.54:1 diff). This gave me almost 40mpg, but dropping to 60mph at 3000rpm also saw a drop in economy to 30mpg.Most Yank eights in stock form produce best torque at 2500rpm but remember tuning camshafts etc. alter these figures.

Right, now itís time to get those calculators out and follow the writing on the blackboard. The following specs are comparing the effects of dropping tyre sizes from 165x13 to 145x13.

               3.9:1 Axle Ratio

               145x13 = 22ins Tyre Diameter

               165x13 = 23ins Tyre Diameter

               1.00:1 Final Drive Ratio

Next letís work out the vehicle speed at a set rpm. We need to know the loaded radius of the tyre and thatís the diameter divided by 2. Then we follow up by working out the tyre revolutions per mile, which is 10,084 divided by loaded radius of tyre (see last sentence). If youíre still with me the next formula will tell us what we want to really know, cruising speed at a specific rpm.

Formula A - Cruising Speed

Cruising speed = RPM x 60

Gear Ratio x tyre rev per mile

165x13 example

3500rpm x 60

3.9 x (10,084 / 11.5)



= 61.40mph

145x13 example

3500 x 60

3.9 x (10,084 / 11)



= 58.74mph
Quite a difference eh! So we need to work out what diff ratio we need to get back to our original figure. The formula (B) is:

(RPM x 60) / tyre revs per mile

final speed

putting our known figures into this gives:

210,000 / 916.7272


equals 3.73 axle ratio

The nearest available in the Ford range is 3.77:1. I used a 3.54:1 to get my cruising speed up to 70 mph.  

The formula can be manipulated again to help work out engine revs at a given speed if diff ratio and tyre size is known.
If we use that Caddy as the illustration for the next formula, he wants to change from 15" to 13" wheels without losing the mix of economy and speed he already has. First we need to know his revs at say 70mph. We know his tyre size is 15" (approx. 25" diameter) and current final drive is 3.9.

This formula (C) is:

RPM = Known Speed x diff ratio x tyre revs


RPM = 70 x 3.9 x (10,084 x 12.5)


RPM = 3670

Now use Formula B to work out diff ratio on 13inch wheels

(3670 x 60) / 876.8695


= 3.58

And the nearest available ratio is 3.54:1. Problem sorted.

Get the ratio right and youíll have better economy, more power and the ability to hold a conversation without the engine note drowning you out.
Stick in your engine torque figures and see if youíre losing out. Donít forget though, too tall gearing will also slow you down and youíll still lose on economy; strange eh!

Okay boys and girls, thatís all there is for this period. Class dismissed!

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