THE SHORTER ANSWER
On the whole, the lend-lease accounted for only a small portion of Soviet combat armament (tanks, trucks, planes, ships, artillery pieces, etc.). That much is a fact, and Soviet-era propaganda never let that fact sleep in peace. However, these armaments, precisely the aid from Britain that arrived in late-1941, made a powerful impact in 1941–1942 (especially during the Battle of Moscow). And if the Soviets had lost at Moscow, it would have taken a much longer time for Germany to be defeated, if at all. In addition, the lend-lease accounted for a sizable portion of Soviet war amenities and materiel, like gasoline, aluminum, explosive agents (this in particular was VERY significant), steel (this in particular was decisive), non-perishable food, etc.
THE LONGER ANSWER
Tanks:
At the outbreak of war in June 1941, the USSR had roughly 22,600–23,100 tanks and self-propelled guns [MK][BVS] (the largest tank force in the world at the time) against Germany's invading force of 3,350 [DG1]. This opened the Eastern Front of WWII. In total, Germany had 5,639 tanks and assault guns at the time, but did not deploy all of them for the invasion [PB].
In several classic battles of encirclement, the Germans wiped out the Soviet prewar armada of tanks – an estimated 20,500 were lost by the end of December 1941[K]. The Germans, on the other hand, were in a very bad shape as well. They were left with only 35% of their initial tank strength (~1,100 tanks) in the Eastern Front by the end of October [DG2], many of which would be lost to attrition and winter-induced defects in the following two months. The two titans were at their limit, and this is where the British Lend-Lease Aid of 1941 (which began in October) helped to turn the tide. In an interesting paper published in the official peer-reviewed journal of the Society for Military History, Alexander Hill convincingly showed that:
British-supplied tanks made up in the region of 30 to 40 percent of the heavy and medium tank strength of Soviet forces before Moscow at the beginning of December 1941, and that they made up a significant proportion of such vehicles available as reinforcements at this critical juncture [AH].
But this important detail is lost in the big picture where the USSR produced 102,500 tanks and self-propelled guns for the whole war (according to the official Soviet history of the Great Patriotic War, completed in 1965) [AH], whereas the US, Britain and Canada altogether supplied only 12,537 tanks and self-propelled guns [BVS]. This means the Lend-Lease aid accounts for only 11% of tanks. But the same Soviet account declared that Lend-Lease aid only amounted to 7% of tanks; and unfortunately you'll see this figure alongside, 1.9% for all artillery, 13% for aircraft and 4% for total military production, floating around on the internet (all are from the same official Soviet account).
In 2007, the renowned Russian historian Boris Vadimovich Sokolov published a paper in a highly reputed peer-reviewed journal that thoroughly trashed this Soviet-era propaganda tract. Using production reports and declassified documents, he also thoroughly showed that the figure of 100,000+ tanks is likely a serious inflation, and that the actual proportion from Lend-Lease aid is around 24% [BVS]. If you can get your hands on Sokolov's paper (requires subscription to the online database Taylor & Francis), it will completely answer all your questions. I'm so tempted to upload and share it here, but that would be a copyright violation.
^Unfortunately ^I'm ^running ^late, ^so ^I'll ^keep ^the ^rest ^brief. ^Sorry!
Aircraft
Official public Soviet-era figures all place the proportion of Lend-Lease aid at 13–15%. Sokolov's analysis places it at ~30%.
Fuel
The US, Britain and Canada supplied the USSR with 2.586 million tonne of aviation fuel and light-fraction gasoline, 1.4 times greater than the Soviet production during the war [BVS]. That was a very big boost to the Soviet Air Force.
Explosives
Sokolov stated the following:
Allied supplies of powder and explosive materials also played an important role. We assess the production of explosive materials in the USSR during the period from mid-1941 to mid-1945 as approximately 600,000 tons. No less than 295,600 tonne of explosive materials were supplied by the US. In addition, 22,300 tons of powder were supplied by Great Britain and Canada. Thus, Western deliveries of explosive materials reached 53 per cent of the total volume of Soviet production.
Tires
Sokolov stated the following:
We must also note that American deliveries played an essential role in the supply of the USSR with tire covers (outer tires) and separate types of food-stuffs. Lend-Lease supplied the Soviet Union with 3,606,000 tire covers, at the same time that the Soviet industry of 1941–45 delivered 8,368,000 tire covers (which included only 2,884,000 of the large tire covers named "Gigant"), while in 1945 vehicle tire cover production constituted 1,370,000 in comparison with 3,389,000 in 1941. American deliveries amounted to 43.1 per cent of Soviet production.
References
[MK] Martin Kahn. Russia Will Assuredly Be Defeated: Anglo-American Government Assessments of Soviet War Potential before Operation Barbarossa. (Peer-reviewed journal article). Link: http://www.tandfonline.com/doi/full/10.1080/13518046.2012.676498.
[PB] Pier Battistelli (2008). Panzer Divisions: The Eastern Front 1941-43. (Book). Page 65.
[K] Grigory Krivosheev, Soviet Casualties and Combat Losses in the Twentieth Century. Page 252
[DG1] David Glantz (2010). Barbarossa Derailed: The Battle for Smolensk – Volume 1. (Book). Page 20.
[DG2] David Glantz (2012). Operation Barbarossa: Hitler’s Invasion of Russia 1941. (Book). Page 183.
[AH] Alexander Hill (2007). British Lend-Lease Aid and the Soviet War Effort, June 1941–June 1942. (Peer-reviewed journal). The Journal of Military History: Vol. 71, issue 3.
[BVS] Boris V. Sokolov (2007). The role of lend‐lease in Soviet military efforts, 1941–1945. (Peer-reviewed journal). The Journal of Slavic Military Studies: Vol. 7, issue 3, pages 567-586. (Originally published in Russian in 1994).
"Saved" is probably taking it far, but I'd say Lend-Lease was an important part of the Soviet victory in WW2. A few things worth reading (Peter Samsonov can be divisive to say the least, but he at least backs up what he can with primary sources):
http://tankarchives.blogspot.com/2014/06/lend-lease-statistics.html
http://tankarchives.blogspot.com/2013/10/lend-lease-shopping-trip.html
http://tankarchives.blogspot.com/2015/06/lend-lease-shipping.html
Numbers are being put together these days with more archive access and translation of Soviet history books. You can probably get more detailed numbers in the next few years or sooner (depending if you can read Russian or not). The general opinion is that things like food, planes, and trucks were the most important parts of Lend-Lease, with radios, artillery, and tanks next down on the least in order of importance. Things like small arms weren't particularly valued by the USSR.
Planes were incredibly important because the Luftwaffe hit the Red air forces hard in Barbarossa, and those planes and pilots that managed to survive were being ground down. Nearly half of all losses from 1941-1944 were replaced by imported planes. The Soviets were especially fond of the P-39, and put it to good use. American trucks (most famously the Studebaker) were an indispensable part of the Soviet war effort for transporting supplies and personnel. It was one of the most numerous pieces of machinery we sent, to the point where supply rate eclipsed their ability to assemble them.
Food was another obvious one. The US could send loads of preserved, packaged foods for frontline duty at a time when the USSR was just trying to get their army together and in proper fighting order. Anecdotally, SPAM was a favorite.
Artillery was fairly obvious. they weren't going to drag every piece back to Moscow during the initial push if it meant an encirclement. So there was a lot that needed to be replaced, and eventually was.
Tanks were a mixed bag. The Reds begrudgingly accepted our M3 lights and M3 mediums (which were both wildly outdated designs for the Eastern Front), while gladly accepting the M4. They were especially fond of the M4A2 (Dimitry Loza's memoirs are entertaining if you're curious about how they were used). These became an important front-line asset, to the point where they were even issued to their better-prepared Guard amored units. The USSR was ahead of the curve on tank technology, so they took what they could to plug holes where they couldn't supply. They generally tended to look down on German and British tanks, but usually sang the praises of the mechanical reliability and innovation present in US designs.
Half-tracks and Jeeps made appearances in good numbers, but I don't think they were quite as indispensable. The Red Army was fine on small arms, and adding new ammo to the supply chain would have been too much of a hassle.
Britain contributed what they could, but it was nothing terribly substantial, and the Soviets weren't particularly enamored with the British supplies they could receive. It's more than forgivable, though, considering the state Britain was in at the time.
I didn't really answer your question, but it's still something I'm researching, myself.
The Soviets were capable of producing everything they got by lend-lease. However, lend-lease allowed them to conscript more men and focus production in the areas of armoured vehicles and artillery, as trucks, canned and preserved food, locomotives, rolling stock, boots and much else was provided by the Americans and the British.
However, barring lend-lease, the Soviets had a large stockpile of gold and some of the most productive gold mines in the world.
The Soviets held about $2 800 000 000 in gold in 1941 - this number includes the $573 877 500 Spanish gold reserve they had taken charge of as payment for delivieries to the Spanish republic and for safekeeping. Production was about $182 000 000 yearly.
Considering US lend-lease was worth some $11 300 000 000, the Soviets would have been able to purchase about 1/3 of that by cash-carry with their gold reserves.
2 write ups by the god mod Zhukov on the subject.
A bit of a necro, but there are problems with Alexander Hills figures for British lend lease tanks.
He classifies Matildas and Valentines as heavy and medium tanks respectively.
They were not. They were medium and light respectively, not only in tonnage, but in capability and armament.
Russian sources also indicate that only a couple of dozen of these vehicles were active during the Moscow struggle. Suggesting that light and medium tanks made up as much as 40% of all medium and heavy tanks in the area is disingenuous. Especially when we consider that the British tanks were largely immobile, unable to traverse through the deep snow.
A very quick note from me. The other answers talk about the scale of Lend Lease but it's also worth considering the timing of it. The impact of Lend-Lease surely comes not only from how many tanks/trucks/planes that were provided and the when they arrived.
The importance of this can perhaps be best illustrated in the table below, which charts US Lend-Lease aid to the USSR. As can be seen, it took time for Lend-Lease to ramp up, only really coming into its own in the second half of 1943. Contributions to the Soviet war effort in 1941 were negligible and still relatively minor in 1942 (some 5% of Soviet GNP).
| Year | US Aid ($ mil) |
|---|---|
| 1941-42 | 1396 |
| 1943 | 2436 |
| 1944 | 4074 |
| 1945 | 2764 |
| Total | 10670 |
| (US Lend-Lease aid to the USSR in money value) |
So by the time Lend-Lease really began to make a difference the Soviets had stopped the Germans at Moscow and Stalingrad and were about to defeat them at Kursk. These were victories that were achieved largely, but not entirely, with Soviet weaponry.
This is not to downplay the importance of Lend-Lease: it certainly made the latter Soviet advances possible and helped prevent the over-heated economy from crashing. But in terms of 'saving' the USSR, I'd argue that the key battles had already been fought (and won) by the USSR before Lend-Lease shipments really began to tell.
Source is Harrison's Accounting for War