O/t pick up truck tires

I have/drive this little Isuzu pick-up it has 195RX14 tires on it right now but they are getting bad. I have on hand a real good set of 215-75R-15 tires on a Geo tracker that has a bad engine. I am thinking about putting the 15s on this pick up and yes I have the rims I need to do so. So will it help or hurt my MPG?? I'm think it should help and sort of make 4th gear over drive going that much bigger and yes it has the room for them under the wheel wells
Hobby farm

Yes you should go farther with each revolution better milage less power. Don't forget speedometer will not be correct!

In theory it should help your mileage, but you'll never know unless you recalibrate the speedometer.

In practice, however, you may wish that you'd stuck with the smaller tires. The Isuzu was originally geared for 14 inch tires and may seem sluggish with the bigger ones. According to the chart on the link below, the 195R14 tires have a 25.5 inch diameter, while the 215R15 tires are 27.5 inches in diameter. That's about eight percent.
Light truck tire diameters

As below, your speedometer will be off. Even worse, you will probably be underpowered and will notice it uphills. Your milage may or not actually be better depending upon how much the motor lugs with the higher ratio resulting from larger tires. Tom

Not only will 4th gear be overdrive 1st gear will be like second is now. This will effect your clutch/clutches over time.

Try it and find out. Make sure the rims don"t catch on the calipers or something ?

There may be some drawbacks, but considering what NEW tires cost, I think the advantages of using what you already have will outweigh any potential disadvantages.

That big you will loose so much power you will have trouble making it up your hills not to mention the speeding tickets you will get if you actually make it to the highway.

BT/DT!
Being rather short on cash in some years, and having different sizes of tires on hand, i used them! Did make a difference in speedo readings, but put me under the 5,000 mile cap for non-emission test!
I always stayed 5 mph slower than the speedo reading and never got a ticket. On my Class C motorhome,i had to put larger tires on the back, as the original size was no longer made/ oravailable. 800/16.5's. I drove 5 mph slower than before and no problems. Was going same speed as before, but clock read differently!

To find out how much difference there is mark the side of the tire with tire chalk,paint or something,roll the tire until the mark is touching the ground,mark the ground,floor,whatever,roll the tire until the mark touches the ground again,mark it there,measure how far it rolled,do the same with the other tire,that will tell you exactly how much difference there is.There might not be all that much difference depending on what kind of tires they are.It seems like Wal Mart tires and maybe some others arent as tall as everybody elses.

I've done this for years. It will affect your speedo readings, but it really won't hurt anything. It will increase your mpg, a little bit. I have done this on every small car and truck I have bought, makes em ride a little better on Michigan roads.

Your truck has a 3.41 to 1 axle ratio and spins at 2950 RPM at 65 MPH with the stock tires.
If you had the five speed trans it would be spinning 2250 RPM at 65 MPH. Five speed versions use a different axle ratio along with the overdrive. Axles are 3.7 instead of 3.4.

Putting those Tracker tires on there increases the circumference of the tires by 7" and ground height by over an 1". It will spin 2690 RPM at 65 MPH instead of 2950 RPM. Wind resistance and ground friction is increased, but not by a great amount.

In the end, you'll probably lose a little fuel mileage, not gain - unless all your driving is on flat roads. I suspect your truck has just barely enough power as it is.

My 85 Isuzu truck is 4WD with a 2.2 diesel, and is geared the same as a gas version with a four speed. But, I've got low range to use since mine is 4WD.
I know that my truck would be useless with bigger tires in normal (high) range since it's gear to fast in 1st gear already.

By the way, I wish those Trackers had smaller wheels; they'd do much better in ice and snow that way. I've got six 4WD Trackers, and they all skid around like roller skates with those 15" tires.

Should have also said it has a 4 speed not a 5 speed and in first gear it is very easy to make the tires spin. Its a 1986 pup by the way

The way it is set up now it is very easy to spin the tires in first gear and also easy to take off in 2nd on flat ground so I figure it should not hurt any thing as for excess clutch wear etc. Guess I find out in a few days if it ever stops raining long enough for me to get them mounted

Diesel is useless with the bigger tires. Gas version not so bad because 1st gear is more geared down.

Yes, you did say your's is a four speed. 1984 and 1985 PUPs have the same axle gearing. I only mentioned the 5 speed for comparison purposes.

Gas engines have different four speed transmissions than diesels, so if your's is gas, you've got a better 1st and 2nd gear for pulling.

Gas engine 4 speed ratios - 1st - 4.12, 2nd - 2.49, 3rd - 1.5, 4th - 1 Rev - 3.82

Diesel engine 4 speed ratios - 1st - 3.79, 2nd - 2.17, 3rd - 1.41, 4th - 1 Rev - 3.82

A five speed trans can have a 1st with either 3.39 or 4.



All four speeds, gas and diesel have 3.41 axles.

All five speeds have 3.72 axles.

If you feel your's is geared plenty low in 1st, either you've got lousy traction, or lots of flat land.

I've got several PUPs and all are diesel. All useless in 1st on any hills. In fact, my 4WDs just barely are geared low enough in low range on very steep inclines.

Seems only you know what works and doesn't work where you live. I've owned enough, and driven enough PUPs and LUVs to know what gearing and tire combos works around here.

Its a gas 1986 and it has 192,000 miles on it. Still has lots of power for its size and since I have the tires just sitting on the tracker I figure use them before they dry rot away on me

I had a 75 LUV for years. It came with 700-14's from the factory. Being a Kid, I needed to put bigger tires on it. I put H78-15 tires on it, and put about 150,000 miles on it with the bigger tires on it. It was hard on the clutch, and the transmision. Of course the transmision part probably could have been prevented if I hadn't been a kid, playing in the mud, and bumper deep snow in the coast range. I put 3 reverse idlers in that trans. and 1 reverse gear (that trans. had a weak reverse, and I could only modify a certain amount). I put a clutch in every time I pulled the transmision. I would do it again if I felt the need, I just wouldn't have to hammer it like I did in the 70's and 80's.
Tim in OR