What fuel mileage will I get if I lower my diff ratios

Hi there. Do you have a 5 speed manual. If so why not try driving around in 4th gear and see if the economy improves first. Bob

LOL, you're driving around in a 3ton truck and you're worried about fuel economy.. LOL.

Leave some of the "fair bit of extra weight" at home for starters. Do you NEED to carry it all around the burbs?

I would keep it as is, your economy at this point is pretty good for the vehicle....

You will regret it on the highway if you do change ratios...

Hi Bucky

Your fuel usage will change in the direction of your revs and as a guide only at about 1/2 the change rate.

If changing from 4 : 1 to 4.5 : 1 then fuel use goes up 4.5/4 * 0.5 = 6% +/- 50%

I would try and change just transfer case ratios.

yep you will get better mileage if you only change diff ratios and not the speedo cogs as your speedo will tell you that you are going a lot further but ya puttin in less fuel ....... and it gets a lot of people also ....
My advise......... do it, you seem like you have already put a lot of thought into it and i agree on most points also, worth a try and i dont thin k you will be worse off, most probally better off ....
Keep us posted as to the results, i will be interested ...

Bucky00,
I've got a patrol which weighs 3.3T, Originally it had 3.9 diff ratios after fitting 33" tyres I found off-road down hill engine braking was useless so fitted 4.3 diff ratios which brought it back better than standard, I found the the revs on the Hwy at 100 changed from 2200 to 2400 but it didn't struggle on hills any more and seems to have a lot better pickup. The fuel usage will be a bit better but will depend how you drive it with the increase power. If you go on LCOOL and tell them what tyres & diff ratios your running and what you want to do they should be able to give you all the info you need.
Cheers Dave..

Correct me if I'm wrong, but a larger tyre has a larger circumference. For every rev of the wheel the speedo thinks you have gone so far when in fact you have gone a bit further. Therefore the only real way to measure how far you really travelled is to use a gps. Then you can get an accurate consumption figure. Using the speedo to measure how fast you are going or how far you have gone is wrong with different tyre sizes. With 31 in tyres 100 kms on the speedo might be 100 kms but with 33 in tyres 100 kms on the speedo would be closer to 107 actual kms. Lowering your gear ratio will not improve economy otherwise we would all drive around in 3rd gear all day. Overdrive was invented to get better economy. Putting larger tyres on acts just like overdrive. Mike

Changing diff ratios won't help anything.
For an 80series Cruiser with the 1HD-T motor, the first improvement will be to change to a 3" exhaust as the standard exhaust is restrictive, particularly over the rear diff; the second will be to add an intercooler.

If it was me, I'd do the exhaust first, and see how you go. If you want a bit more then go the intercooler.

The old fashioned auto boxes sap a lot of power from the engine. A 5 sp conversion might work out better.

I drive a HZJ75 series non turbo with a 500kg camper on the back(about 2700kg all up) and get 13-14 litres/100 everywhere I go.
I dont really have any complaints about power on the open road.
Most hills can be tackled in 5th gear if I have enough speed,otherwise ,a quick flip back to 4th keeps it moving at a steady 95-100.
I reguarly do shark bay to Perth in 8 1/2-9 hours with 3 breaks,not bad for 800klms.
Im running 235x85x16 which are relatively narrow and are about 32 in

Maybe a dyno test to see how much HP you are getting would be good place to start.
I use to drive my bosses 93 80 series turbo ,5sp,and tow a 1 ton excavator and it had more than enough power for the hills

The old fashioned auto boxes sap a lot of power from the engine. A 5 sp conversion might work out better.

I drive a HZJ75 series non turbo with a 500kg camper on the back(about 2700kg all up) and get 13-14 litres/100 everywhere I go.
I dont really have any complaints about power on the open road.
Most hills can be tackled in 5th gear if I have enough speed,otherwise ,a quick flip back to 4th keeps it moving at a steady 95-100.
I reguarly do shark bay to Perth in 8 1/2-9 hours with 3 breaks,not bad for 800klms.
Im running 235x85x16 which are relatively narrow and are about 32 in

Maybe a dyno test to see how much HP you are getting would be good place to start.
I use to drive my bosses 93 80 series turbo ,5sp,and tow a 1 ton excavator and it had more than enough power for the hills

The old fashioned auto boxes sap a lot of power from the engine. A 5 sp conversion might work out better.

I drive a HZJ75 series non turbo with a 500kg camper on the back(about 2700kg all up) and get 13-14 litres/100 everywhere I go.
I dont really have any complaints about power on the open road.
Most hills can be tackled in 5th gear if I have enough speed,otherwise ,a quick flip back to 4th keeps it moving at a steady 95-100.
I reguarly do shark bay to Perth in 8 1/2-9 hours with 3 breaks,not bad for 800klms.
Im running 235x85x16 which are relatively narrow and are about 32 in

Maybe a dyno test to see how much HP you are getting would be good place to start.
I use to drive my bosses 93 80 series turbo ,5sp,and tow a 1 ton excavator and it had more than enough power for the hills

I've spoken to 4wd specialist mechanics today.

They seem to disagree with a lot of things being said.

Prime example, they've told me that it will help the fuel economy and this is how one described it:

You're riding a push bike and you're in a high gear and when you get to a hill, you're using up a huge amount of energy trying to get up, sure it requires less rotations of the pedal, but you're still using more energy and it's heaps slower.

Let's say you're on a push bike taking off in a high gear, sure it's less rotations of the pedal per metre, but you're still using more energy and you're heart will be beating harder and it's slower as well, worst of both worlds.

They also told me exactly what I said about the highway.

Lowering the diff ratios back to standard size tyres helps it on the highway just as much as makes it worse because let's say you've got high diff ratios / large tyres and you get to a hill, you'll slow down and have to change back a gear, with smaller tyres, you'll cruise up without slowing down much and at the end of the day, you're fuel mileage will be just as good if not better.

These mechanics have owned cars with altered diff ratios and said that that's what happened to them, so it's not just cars they've modified, it's cars they've owned and set up in their own workshops.

Out of interest, have you done the maths on the $$$$ if you were to go that way?

Assume you change from 4.1 ratio to 4.3 (about 4.5%).
Best case scenario would save 0.63 L/100k. (optimistic in my opinion)
That would save 82cents per 100k
Cost of chnaging diff ratios I've guessed at $4000.
It would take you 487,804km to recoup that cost.

Am I missing something??

"If I lowered the diff ratios one size taking it back to 31 inch tyres, approximately what fuel mileage would I get? "

the easy way would be to put a set of 31's on for a week and try them to see what the difference is.